Survival and Fecundity of Adult Swallow-tailed Kites (Elanoides forficatus) Breeding in Florida
FINAL REPORT
Kenneth D. Meyer
March 2005
Florida Fish and Wildlife Conservation Commission 620 South Meridian Street Tallahassee, FL 32399-1600
Survival and Fecundity of Adult Swallow-tailed Kites (Elanoides forficatus) Breeding in Florida
Kenneth D. Meyer
Avian Research and Conservation Institute 411 N.E. 7 Street, Gainesville, Florida 32601
Submitted as final report for Florida Fish and Wildlife Conservation Commission Project NG99-016 March 2005
This report is the result of a project supported by the Florida Fish and Wildlife Conservation Commission’s Nongame Wildlife Trust Fund. It has been reviewed for clarity, style, and typographical errors, but has not received peer review. Any opinions or recommendations in this report are those of the authors and do not represent policy of the Commission.
Suggested citation: Meyer, K. D. 2005. Survival and fecundity of adult swallow-tailed kites (Elanoides forficatus) breeding in Florida. Final report. Florida Fish and Wildlife Conservation Commission, Tallahassee, Florida, USA.
Survival and Fecundity of Adult Swallow-tailed Kites (Elanoides forficatus) Breeding in Florida
Kenneth D. Meyer Avian Research and Conservation Institute, 411 N.E. 7 Street, Gainesville, Florida 32601
Abstract: Estimating annual survival and identifying sources of mortality are essential for describing population trends and conserving vulnerable species. There have been no prior attempts to estimate the survival of adult swallow-tailed kites (Elanoides forficatus). The species nested in at least 21 states prior to the early 1900s, but a sharp population decline from 1880 through 1940 resulted in the present limited distribution in just 7 states. Suggested reasons for this decline of the U.S. population include habitat loss as a result of agricultural development, logging of bottomland forests, shooting, and egg collecting. Although we now know that the U.S. population migrates over 8,000 km to spend the boreal winter in South America, threats faced by kites during this period have not been investigated. The goal of this study was to estimate the 2 critical demographic rates for which we lacked reliable estimates: adult survival and breeding effort among females, the latter to be used in conjunction with existing data to compute fecundity. We used the Kaplan-Meier estimator with satellite (n = 19) and VHF (n = 25) telemetry to estimate survival of 44 adult swallow-tailed kites from the migratory U.S. breeding population. Because mortality was rarely confirmed for birds with which we lost contact before their transmitters were due to expire, we categorized each case based on the likelihood of mortality versus radio failure (e.g., signal loss during protracted water crossing in adverse winds probably indicated mortality). Four analytical scenarios ranging from least (all loses attributed to mortality) to most (only confirmed deaths categorized as mortality) conservative produced annual survival estimates of 0.68–0.94 (CIs ± 0.150–0.20). The median of the estimates based on scenarios 2 and 3 was 0.81. Survival did not differ between the sexes or for birds with satellite (20 g) versus VHF (12 g) transmitters. Losses, however, were unevenly distributed across seasons: 2 during breeding in the U.S., 10 on the winter range in South America, and 10 on southbound or northbound migration. Breeding effort was high (92%) in our study. This result is a useful first approximation of breeding effort and probably can be taken as a likely upper limit of what occurs across a wider geographic range. Although tentative, our estimates for the likely range of annual survival and for breeding effort will inform population-modeling efforts. We suggest that, at least for now, the range of 76–86% be used as an estimate of adult survival for modeling the demographics of the U.S. population of swallow-tailed kites, with caution due to the broad confidence intervals. Depending on the population’s sensitivity to adult survival, both the high and low estimates may result in the same prediction for a population trend, whether declining, stable, or increasing. If this is the case, we will have learned much and can then decide on the potential value of managing for increased adult survival. Knowing the timing and location of mortality, furthermore, will support conservation planning for the small and vulnerable U.S. population of swallow-tailed kites.
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ACKNOWLEDGMENTS I am grateful to the many people who contributed to this project. Gina Zimmerman, Jeff Kingscott, and Stacie Schoppman provided competent and dedicated field assistance. Steve Lowrimore and Rob Hix of Plum Creek Timber Company provided access, information, and field assistance, including nest-finding. Frank Ogborn and Jim Hendrix served as pilots on our telemetry flights. I thank Rob Bennetts for running the Kaplan-Meier analyses and assisting with our interpretation of the results and Gina Zimmerman for her help in preparing this report. The Florida Fish and Wildlife Conservation Commission (FWC) provided major funding for the project; Michael Evans and Stuart Cumberbatch administered the contract. Microwave Telemetry donated 5 transmitters, and the Georgia Department of Natural Resources (GADNR) allowed us to use data from 8 radio-tagged kites, including 6 with satellite transmitters. Data processing by Service Argos was paid for by FWC, GADNR, and Avian Research and Conservation Institute.
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TABLE OF CONTENTS ABSTRACT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii ACKNOWLEDGMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 OBJECTIVES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 METHODS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Study Area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Finding and Monitoring Nests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Capturing and Radio Tagging Adults . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Monitoring Radio-tagged Adults . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Measuring Breeding Effort . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Estimating Annual Adult Survival . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 RESULTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Finding and Monitoring Nests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Capturing and Radio Tagging Adults . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Measuring Breeding Effort . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Monitoring Radio-tagged Adults . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Estimating Survival . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 DISCUSSION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 RECOMMENDATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 LITERATURE CITED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
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INTRODUCTION Survival is one of the demographic rates critical to the modeling of bird populations. Although adult birds usually have higher survival rates than juveniles (hatch year) (Loery et al. 1987, Nichols et al. 1992), population growth of long-lived species is generally thought to be more sensitive to adult rather than juvenile survival (Mertz 1971, Grier 1980, Nichols et al. 1980, Beissinger 1995). The effect, however, may be highly variable among species (Saether and Bakke 2000). Sound estimates of survival rates are essential given the importance of knowing population trends when planning the conservation of species at risk. The seasonal and geographic correlates of mortality, furthermore, are important for focusing management efforts where they will be most cost effective. Investigating survival, however, usually requires substantial time and expense, which can discourage study and preclude useful predictions of population change. No prior attempts have been made to estimate the survival of adult swallow-tailed kites (Elanoides forficatus). The species nested in at least 21 states prior to the early 1900s, including all of Florida, the coastal plains of the southeastern United States, and along drainages of the Mississippi Valley as far north as Minnesota (Cely 1979, Robertson 1988), but a sharp population decline from 1880 through 1940 resulted in the present limited distribution in just 7 states. Swallow-tailed kites now nest in association with wetland habitats throughout peninsular Florida and with major river systems of the lower coastal plains of South Carolina, Georgia, Alabama, Mississippi, Louisiana, and Texas. The U.S. population of swallow-tailed kites spends the boreal winter in South America (Meyer 2004b). Recent population estimates, based on nesting and roosting studies, range from 800 to 1,150 pairs (about 3,200–4,600 individuals, including nonbreeding adults and young of the year) at the end of the breeding season (Meyer 1995). Suggested reasons for the marked reduction of the species’ range include habitat loss as a result of agricultural development, logging of bottomland forests, shooting, and egg collecting (Cely 1979, Robertson 1988, Meyer 1995). There is no substantiated evidence of a significant increase in number or reoccupation of former range. The species is cautiously considered stable with both increases and decreases in local sub-populations. In setting priorities for conservation efforts, the Florida Fish and Wildlife Conservation Commission’s (FWC) Nongame Wildlife Program ranked the swallow-tailed kite as one of Florida’s most vulnerable and poorly understood species (Millsap et al. 1989). The species has been recommended for consideration for a management listing of Endangered at state and federal levels (Meyer and Collopy 1996).
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Most of the demographic data needed to model Florida’s population of swallow-tailed kites have been collected since 1988, including estimates of juvenile survival, nesting success, productivity, and age at first reproduction (Meyer 1995; Meyer and Collopy 1995; Meyer 2004a,b). The two critical rates for which we lack estimates are adult survival and breeding effort among females, the latter to be used in conjunction with existing data to compute fecundity. The goal of this project was to estimate annual survival and breeding effort for radio-tagged adult swallow-tailed kites breeding in Florida.
SURVIVAL AND FECUNDITY OF SWALLOW-TAILED KITES—Meyer
OBJECTIVES 1. Locate ≥30 nests per year in 2000 and 2001. This objective was amended in 2002 to cover the breeding seasons from 2000 to 2003 to expand the sample of radio-tagged birds. 2. Capture and radio tag ≥15 kites in 2000 and ≥20 in 2001 with VHF transmitters designed to operate for at least 2.3 years. This objective was amended in 2002 to incorporate birds fitted with satellite transmitters in Florida as part of a concurrent study on the stopover ecology of migrating swallow-tailed kites (Zimmerman and Meyer 2004) and to include birds marked during a concurrent study of nesting ecology in Georgia (K. Meyer, unpublished data). These additions also expanded the sample size. 3. Monitor radio-tagged adults from the ground and air in the subsequent 1–3 years to determine their fate. In 2004 objective 3 was amended to extend the monitoring effort through June 2004. 4. Determine whether or not relocated radio-tagged kites breed each year in which they are monitored. 5. Estimate adult survival and breeding effort (percentage of breeding-aged females that attempt to breed during a year).
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METHODS Study Area The project focused on the northern half of peninsular Florida, where we have studied swallow-tailed kites since 1996. Nesting success and productivity in this region were nearly the same as for southern Florida in the previous 9 nesting seasons. The area is large enough to be heterogeneous and generally representative of nesting conditions for swallow-tailed kites while allowing for manageable logistics. Our sample of radio-tagged adults was augmented with birds captured near nests in southeastern Georgia as part of a different study. Finding and Monitoring Nests Nest-finding methods were the same as for previous work in Florida (Meyer and Collopy 1995). We began searching for nests in early March, relying on our knowledge of previously used areas, new sighting reports, and our own direct observations of breeding activity. Nest searching is most successful during the courtship and nest-building stages (March and April), when kites are most conspicuous and their behavior is more likely to lead the observer to the nest. Considerably more effort is required to find nests once incubation begins (mid-April to mid-May). Nests were monitored at no greater than weekly intervals to determine timing and causes of failures and to age the young for optimal scheduling of adult trapping. Capturing and Radio Tagging Adults We used a mist net near nests to trap adult kites during May and June 2000–2003. This trap consisted of 2 2-x-12–m nets tied together along their length (making 1 4-x-12–m net) and attached at each end to a pulleyed rope apparatus suspended from the top of a 10-m extendable pole. The net was placed within 30–50 m of an active nest. A live disabled owl (usually a great horned owl [Bubo virginianus] borrowed from a wildlife rehabilitation facility) was tethered to a low perch placed below the center of the net. When a kite dove at the owl and was caught in the net, the net was quickly lowered and the kite removed. We have captured >60 adults in this way without any birds being injured. One drawback to this method is that the apparatus is cumbersome to transport and set up. It also must be erected as close as possible to the nest, in a relatively open area that gives a diving kite access to
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the lure while providing enough background vegetation to make the net inconspicuous. Many nest sites are not suitable for this trap. The responses of the nesting adults to the lure and trap varied. A few were captured within 15 minutes, some took hours, and most never came low enough to strike the net. An average of 1 in 5 trapping attempts (nets open with a lure present for 2–4 hours) resulted in a capture. The VHF transmitters (American Wildlife Enterprises, Monticello, Florida, USA) weighed 10 g, which is about 2.5% of the average mass of a swallow-tailed kite. They were battery powered with an expected life of 2.3 years and were equipped with a mortality-sensing circuit that doubled the audible pulse rate when the bird was motionless for >12 hours. The satellite transmitters (PTT-100, Microwave Telemetry, Columbia, Maryland, USA) were solar powered, with an untested expected life of 3–5 years, and weighed 18 grams, or about 4% of the average mass of a swallow-tailed kite. They were programmed to transmit for 10 hours, then turn off to recharge for 22 hours. Thus, we collected data for 10 hours out of every 32-hour period. VHF and satellite transmitters were attached to the birds with backpack harnesses. The harness, a design that we have used successfully on dozens of swallow-tailed kites and short-tailed hawks (Buteo brachyurus) over the last 12 years, was made of 6-mm-wide teflon ribbon (Bali Ribbon, Bali, Pennsylvania, USA). The ends of 4 separate strands were stitched together with cotton thread at a single point prior to capture and a 12-mm square of soft material was threaded onto the teflon to form a cushion at the junction. In the field, the junction was placed on the bird’s sternum and the free ends of the harness were brought up onto the bird’s back (2 strands anterior to the wings, 2 strands posterior to the wings and anterior to the legs), passed through holes at the front and back of the transmitter, tied, and stitched through the knot with cotton (VHF) or nylon (satellite) thread. This method has a number of advantages. Teflon, which is strong, nonabrasive, and nonphotosensitive, has been widely used with excellent results. The cotton thread at the closure of the VHF harnesses will eventually rot, allowing both the front and back harness loops to open simultaneously, thus ensuring that the transmitter and harness will fall free of the bird (we used more durable thread for the longerlived satellite transmitters). The harness is quickly attached in the field and can be accurately fitted and adjusted. We banded every captured bird with a U.S. Fish and Wildlife Service aluminum band, took standard measurements, and collected a small (0.1- to 0.3-cc) blood sample for subsequent sexing by genetic analysis (Zoogen, Davis, California, USA, and Avian Biotech, Tallahassee, Florida, USA).
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Monitoring Radio-tagged Adults VHF radio-tagged adults were monitored from the ground at 5- to 10-day intervals until they left the area and we lost contact with them. Thereafter, we monitored them during aerial searches every 14–23 days. Aerial tracking equipment consisted of a programmable scanning VHF receiver with a 164.000–165.999 MHZ range, wing-mounted 3-element yagi antennas connected to the receiver with coaxial cables, a switch-box for selecting either or both antennas, and headphones. We searched from a Cessna 172 single-engine high-wing aircraft flown along transects 100 km apart at an altitude of 500–700 m above ground level and an airspeed of about 180 km/hour. Previous experience has shown these altitudes and flight speeds to be optimal for signal detection and efficiency; combined with the specified VHF transmitters, detection ranges were consistently >50 km and often ≥100 km. The location and orientation of the transects varied depending on the last known position of the subjects of our search. The scanning receiver, previously programmed with the frequencies of all the missing birds, ran continuously, lingering for 2 seconds on each frequency. When a bird was detected, the scan was stopped and the signal localized. A GPS receiver was used to identify and maintain flight along the specified transects and to determine the coordinates of kites as they were detected. Search patterns varied depending on where we were flying (e.g., we avoided urban areas or other places where kites were unlikely to be and radio interference was high, and we intensified the search in good habitat or where kites had been detected previously). In general, we flew relatively straight transects, periodically (every 30–50 km) circling slowly several times before proceeding. Because we could expect the detection range to be >50 km, it was possible to cover most of peninsular Florida by flying 2 transects, about 100 km apart, that were aligned north-northwest by south-southeast along the length of the state. In the panhandle, a single east/west transect sufficed. Satellite transmitters cannot be detected from the ground or air at more than 1–2 km, and even then special equipment, which is costly and cumbersome, is required. Instead, we relied on a private company, Service Argos (Landover, Maryland, USA) for satellite data. Service Argos has placed receivers on publicly owned satellites to acquire the data, which they then retrieve, process, and sell to researchers. We received our satellite data in email reports from Service Argos every 2 days. On average, these reports included at least 1 useable location (i.e., of sufficient accuracy) for every 32 hours of elapsed time. We used Service Argos Location Class (LC) data to assess the accuracy of each fix (Service Argos 1996). Service Argos assigns
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1 of 7 classes to each location, but we considered only the top 4 classes for inclusion in our analyses. They estimate that 67% (1 standard deviation) of the locations per class will be accurate to within the following distances: LC 3, <150 m; LC 2, 151–350 m; LC 1, 351–1,000 m; and LC 0, >1,000 m (Service Argos 1996). In our experience, LC 1 and most LC 0 locations are of roughly the same accuracy. This is based on known locations of birds during satellite reporting intervals. Of the 4 classes, LC 0 is clearly the group most likely to contain wide-ranging errors. By scrutinizing each reported location in relation to prior and subsequent locations, however, the most blatant of these errors are readily obvious. After this process, we archived all locations of LC 0 and better for later use in movement and habitat analyses. Like the VHF radios, the satellite transmitters had a mortality-sensing circuit. A numerical activity counter advanced continuously (eventually resetting) as long as the bird (or at least the transmitter) moved once every 6 hours. The output of the counter appeared in the 48-hour reports from Service Argos. Measuring Breeding Effort We tracked and monitored all VHF and satellite radio-tagged adults relocated in Florida in the breeding seasons following the year in which they were marked to determine if they attempted to breed. If we observed any of the following behaviors, we concluded that a bird attempted to nest that year: incubation (either sex); a male feeding an incubating or brooding female; or either sex brooding, feeding, or attending to young. Marked birds observed courting or nest building were not considered to be nesting unless we confirmed 1 or more of the post egg-laying behaviors listed above (consistent with our definition of a nesting attempt as a nest in which eggs are laid). It was more difficult to conclude that a marked kite was not nesting. Males often cover large distances over many hours while their mates are incubating eggs or brooding small young. Females may do the same in the late nestling stage. If none of the designated nesting behaviors were observed for a marked bird during intermittent and relatively brief time periods, we tracked and monitored that individual for at least 2–3 entire days before concluding that it was not nesting. Estimating Annual Adult Survival Sampling Regime.—We marked and monitored only adult swallow-tailed kites for this study. Cumulative trapping effort from 1990 to 1999 produced roughly equal numbers of males and females, so we anticipated a relatively
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even sex ratio among tagged individuals. We expected that 2 breeding seasons (2000 and 2001) would be required to achieve our target sample of 35 radiotagged kites. The sampling year for the survival analysis was begun on 1 May of each year to coincide with the annual deployment of transmitters. Utility of Telemetry Data.â&#x20AC;&#x201D;The 2 types of telemetry used to assess survival and breeding effort of adult swallow-tailed kites differed substantially in how they operated and were monitored. The much finer location accuracy of VHF telemetry allowed us to determine nesting status with relative ease compared with the satellite-derived locations, which had much coarser accuracy. Location accuracy was much less important for estimating survival, but the reliability and detectability of the transmitters influenced our success in monitoring the status of the marked birds. Both VHF and satellite telemetry have advantages over the capturerecapture and mark-resight approaches to estimating survival, the main one being that telemetry provides more frequent status information. Monitoring still occurs at intervals, but it is relatively continuous compared with the nontelemetry methods. Of the 2 types of telemetry, satellite monitoring provides a more consistently spaced sampling schedule and usually a greater accumulation of data over extended periods of time. This is especially true for large, wide-ranging animals such as swallow-tailed kites. The cost of satellite tracking appears at first to be quite high, with an initial outlay of about $3,000 per transmitter (versus $150 for a VHF transmitter) and an additional yearly expense of about $2,000 per radio for data processing. Tracking VHF signals, however, requires a substantial investment in personnel, field vehicles, radio receivers and antennas, and flight time. In addition, solar-powered satellite transmitters have twice the life of VHF radios in the size suitable for swallow-tailed kites. Combined with their ability to provide remotely sensed location and status data from anywhere on the surface of the Earth, these factors defray the higher initial cost of satellite transmitters and yield a quantity and quality of data not possible with VHF telemetry. Statistical Analyses.â&#x20AC;&#x201D; Survival estimates.â&#x20AC;&#x201D;We estimated annual survival of adult swallowtailed kites during 4 years, from 1 May 2000 to 30 April 2004, using the Kaplan-Meier product limit estimator (Kaplan and Meier 1958). This estimator has 3 main advantages: it can be modified to incorporate the staggered entry of tagged individuals over time (Pollock et al.1989); it makes no assumptions about the temporal distribution of survival over the sampling
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intervals (i.e., the hazard function) (Lee 1980, White and Garrott 1990); and individuals can be censored when their fate is unknown, whether due to death or to failure or loss of the transmitter (White and Garrott 1990). In our analysis, individuals were categorized as alive, dead, or censored for each year depending on their known or inferred fate by the end of that year. We used SAS (SAS Institute 1988) to calculate the survival estimates with a program modified from that of White and Garrott (1990). Log-rank statistics (Cox and Oakes 1984) were used to test for differences in survival among years, between males and females, and between birds with satellite and VHF transmitters. Cox and Oakes (1984) describe 3 forms of the log-rank test that produce successively lower probabilities of Type I error with correspondingly lower power. We report the results for all 3 forms of the log-rank test, but our conclusions were based only on the most conservative version (lowest probability of Type I error, lowest power). Inferring mortality from tracking data.—Permanent loss of the signal of a marked bird (VHF or satellite transmitter) may have been due to radio failure, harness failure, or death of the bird. In the last 2 cases, the position of the transmitter (e.g., in vegetation, on the ground, under water) prevents it from recharging (for solar-powered satellite transmitters) or from emitting a strong enough signal to be detected at any useful range. An added complication for studies of survival is determining whether a bird’s survival was influenced by the presence of the harness and transmitter (e.g., the transmitter made it more difficult for the bird to fly or caused irritation due to weight and aerodynamic resistance). Inferring direct and indirect causes of lost radio signals is critical to telemetry studies of survival. Mistaking failed radios for mortality, or the reverse, biases survival estimates. Our satellite and VHF telemetry data differed in quality and quantity. Even during the breeding season, when we had access to VHF radio-tagged birds, detections were infrequent compared to satellite reports, which provided daily confirmation of survival for every extant bird. The only other times when we had any contact with VHF radio-tagged birds was during the winter when we searched from the air in South America (2000–2002) or during a stopover study on southbound migration in Mexico and Belize (2002–2003). Although our success was good at finding marked kites from U.S. study populations at these times, we had no expectations of locating every surviving bird because of the large wintering area, high cost of flying, and limited time available for searching, nor were we able to monitor over any appreciable time period the birds we found. As a result, our chances of determining when a VHF radio stopped providing a signal—indicating either radio failure of bird
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mortality—were relatively small. We would not know that a kite had died or that its radio had failed during southbound migration, winter, or northbound migration until the following breeding season when we searched for VHF signals in Florida. Satellite telemetry provides a large advantage in this respect. In addition to providing location coordinates, the transmitters had a motionsensing circuit that reported an advancing sequence of data as long as it was moving. If the bird died and the transmitter sensed no motion, the counter stopped advancing. If this happened late in the 10-hour on-period, it might not be discernable until the next 10-hour on-period. At most, however, the time at which the bird stopped moving could be estimated within a 24-hour period. Nonetheless, it was nearly impossible with either VHF or satellite telemetry to confirm mortality with recovery of a carcass. It was necessary, therefore, to use all available information to evaluate each loss of radio contact and decide whether it could be reasonably attributed to the death of the bird. Estimating a likely range for the survival rate.—Every kite that was alive at the beginning of a sampling year (1 May–30 April) was assigned a status code (a–f) for its fate by the end of that year, as follows: a b c d e f
A satellite- or VHF-tracked bird that was confirmed dead before the end of the yearly interval; A satellite- or VHF-tracked bird that was confirmed alive at the end of the yearly interval; A satellite-tracked bird that stopped reporting, with no prior evidence of impending transmitter failure, from 1 of 2 areas where losses appeared to be clustered in South America; A satellite-tracked bird that stopped reporting, with no prior evidence of impending radio failure, in areas with no other known losses; A VHF-tracked bird that stopped reporting, with no prior evidence of impending radio failure, in an area with no other known losses; A VHF-tracked bird that was alive when last detected in the U.S. prior to fall migration, on the Yucatan Peninsula during southbound migration (Zimmerman and Meyer 2004), or on the winter range (Meyer 2004a,b, unpublished data), but which was not found in the U.S. the following spring.
The Kaplan-Meier analysis is a known-fate model. That is, when the Kaplan-Meier survival estimator is derived from telemetry data, each animal
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in the sample is assigned 1 of 3 categories of fate per time interval: alive, dead, or censored. A bird is censored when the animal cannot be confirmed alive or dead and the radio is assumed to have failed or fallen off. When applying the Kaplan-Meier analysis to our data, the most conservative approach would be to consider kites with status code a to be dead, those with code b to be alive, and those with codes c–f to be censored. We recognize, however, that loss of a transmitter signal, whether satellite or VHF, with no physical confirmation of mortality is inherently ambiguous in the case of the swallow-tailed kite, a species that ranges widely on its breeding range and covers over 17,000 km on its round-trip migration. Chances of recovering a carcass are exceedingly small, even for a dead kite with a VHF transmitter that can be tracked on a very fine scale. In light of this, we ran the Kaplan-Meier analysis 4 times for each year. From most to least conservative, these 4 runs were based on the following assignments of status codes a–f to each fate category (dead, alive, censored). Run 1: a – dead b – alive c, d, e, and f – censored Run 2: a and c – dead b – alive d, e, and f – censored Run 3: a, c, and d – dead b – alive e and f – censored Run 4: a, c, d, and f – dead b – alive e – censored Timing and location of signal loss (satellite telemetry only).—In addition to estimating annual survival rates, we were able to identify dates and geographic locations for last reported signals. This permitted a qualitative assessment of the likelihood that lost signals indicated mortality rather than radio failure or loss and, to the extent that we could reasonably infer mortality, suggested areas that pose possible threats to swallow-tailed kites.
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FLORIDA FISH AND WILDLIFE CONSERVATION COMMISSION FINAL REPORT
RESULTS Finding and Monitoring Nests Our goals were to find 25 nesting attempts (nests in which eggs were laid) in 2000 and 30 in 2001 to provide enough trapping opportunities (after allowing for nest failures and unsuitable sites) to deploy 15 transmitters in 2000 and 20 transmitters in 2001. We found 27 nests in 2000 and 32 nests in 2001. Because we did not radio tag a total of 35 adult kites in 2000 and 2001, however, we continued to search for nests in the following 2 years, finding 31 in 2002 and 17 in 2003. Capturing and Radio Tagging Adults In 2000, we captured and placed VHF radio transmitters on 17 adult swallow-tailed kites in Florida, exceeding our objective by 2 transmitters. This was the largest number of adult kites that we had ever caught in one season. In 2001, however, we captured only 10 adults, half our targeted objective, despite greater trapping effort than in 2000. This included a female that had been radio tagged the previous year (replacing her VHF radio with a satellite transmitter). Our goal was to radio tag 8 adult kites in 2002 to complete our sample. Prior to the start of the field season, FWC approved a proposal to study stopover ecology of migrating swallow-tailed kites, including the purchase of 8 solar-powered satellite transmitters that would also provide data for our survival study. We deployed all 8 on adults in 2002 and fitted a ninth bird with a VHF transmitter, thus exceeding by 1 our proposed sample of 35 radiotagged adults in Florida. In 2002 and 2003, we also radio tagged 8 adult swallow-tailed kites in southeastern Georgia (6 satellite and 2 VHF transmitters) and applied the same monitoring protocol to these birds as we were using in Florida. As a result, we attained a total sample of 44 swallow-tailed kites for the present analysis of adult survival, consisting of 25 with VHF and 19 with satellite transmitters (Table 1). The 44 kites consisted of 19 males and 25 females, or 1.0:1.4 sex ratio.
SURVIVAL AND FECUNDITY OF SWALLOW-TAILED KITES—Meyer
13
Table 1. Adult swallow-tailed kites (n = 43) radio-tagged with VHF and satellite transmitters and monitored from 1 May 2000 to 30 April 2004 to determine survival and fecundity. One VHF bird (5.475) was recaptured and fitted with a satellite transmitter (Sat 16082), making 44 entries in the list.
Agea
Sex
Capture date
State
Location (county)
Transmitter typeb
Transmitter ID #c
AHY AHY AHY AHY AHY AHY AHY AHY AHY AHY AHY AHY AHY AHY AHY AHY AHY AHY AHY AHY AHY AHY AHY AHY AHY AHY AHY AHY AHY AHY AHY AHY AHY AHY AHY AHY AHY AHY AHY AHY AHY AHY AHY AHY
M F F M F M M F F F M F M M F F M F M F M F F M F F F F F M M F F M M F F F M F M M F F
05/31/00 05/30/01 05/30/00 05/30/00 05/22/01 05/18/00 05/26/00 05/26/00 05/22/00 05/22/00 05/19/00 05/15/00 05/15/00 05/09/00 05/06/00 05/02/00 05/01/00 05/01/00 05/15/01 05/20/01 05/23/01 05/25/01 05/28/01 06/05/01 06/05/01 06/07/01 06/08/01 05/13/02 05/14/02 05/20/02 05/22/02 05/23/02 05/27/02 05/29/02 05/30/02 06/03/02 06/07/02 06/08/02 06/19/02 06/19/02 06/13/03 06/18/03 06/20/03 06/24/03
FL FL FL FL FL FL FL FL FL FL FL FL FL FL FL FL FL FL FL FL FL FL FL FL FL FL FL FL FL FL FL FL FL FL FL FL GA GA GA GA GA GA GA GA
Polk Polk Polk Polk Levy Seminole Levy Levy Levy Levy Levy Levy Levy Levy Levy Levy Levy Levy Levy Levy Levy Orange Levy Levy Levy Levy Polk Levy Levy Levy Levy Levy Levy Levy Levy Levy Long Brantley Camden Camden Glynn Camden Brantley Long
VHF VHF Sat VHF Sat VHF VHF VHF VHF VHF VHF VHF VHF VHF VHF VHF VHF VHF VHF Sat VHF Sat VHF Sat VHF VHF VHF Sat Sat Sat Sat Sat Sat Sat Sat VHF Sat Sat Sat Sat Sat Sat VHF VHF
5.502 5.521 16081a 5.489 16083a 5.266 5.275 5.475 5.455 5.443 5.204 5.249 5.364 5.191 5.335 5.117 5.171 5.145 6.045 16082 6.086 16085a 6.108 16086 6.148 6.168 6.255 36308 36309 36310 36311 36312 36313 36315 36314a 5.747 36316 16081b 16083b 16085b 36314b 41244 4.847 4.884
aAHY
= after hatch year. 19 satellite transmitters included 6 birds tagged in Georgia during Avian Research and Conservation Institute’s contract research for the Department of Natural Resources. cSatellite ID numbers repeat on different individuals in subsequent years (distinguished by a or b suffix) because Service Argos assigns a limited number of identifiers, which must be reused. bThe
14
FLORIDA FISH AND WILDLIFE CONSERVATION COMMISSION FINAL REPORT
Measuring Breeding Effort In 4 breeding seasons, from 2001 to 2004, we searched for adult swallowtailed kites that had been fitted with VHF or satellite transmitters 1 or more years before. Of the 24 birds we relocated, we were able to confirm the breeding status (i.e., whether or not they produced eggs) of 13 individual females (11 in Florida and 2 in Georgia) over the 4-year period. The 13 females nested 22 of the 24 times when they were present during a suitable time period for nesting, resulting in a nesting effort of 92%. Although our focus was to determine the breeding effort of females, we were able to confirm the breeding status of 7 of the 11 males relocated during the breeding season. Six of the 7 males nested, a breeding effort of 86%. Monitoring Radio-tagged Adults Kites with VHF transmitters were monitored during a total groundtracking effort of about 1,080 hours from 2001 to 2004. During the same period, we searched for the same birds during 16 flights totaling 61 hours. The first of the 19 solar-powered satellite transmitters was deployed in May 2000. Three more were placed on kites in 2001, 12 in 2002 (8 in Florida and 4 in Georgia), and 2 in 2003 (Table 1). Since May 2000, these 19 transmitters have yielded over 8,000 satellite locations of LC 0 or better. These data for 19 individual kites represent nearly continuous, year-round status reports for periods ranging from 68 to 713 days from deployment to the date and time of the last signal detected (mean = 337 days Âą 207 SD) (Table 2). The satellite transmitter on the longest surviving kite (#16082) has operated for 1,182 days, with an additional 361 days of VHF (ID# 5.475) monitoring time prior to this period. Viewing the number of transmitters that became undetectable in relation to time since deployment suggests a clustering of last transmissions around 200 days (Fig. 1). The geographic and, therefore, seasonal distribution of last locations also appeared to be clumped (Fig. 2), occurring least frequently in the U.S. during the breeding season, with the rest evenly spread over the migration (northbound and southbound) and winter seasons. When the kitesâ&#x20AC;&#x2122; time allotment per season is considered (i.e., proportion of the year spent breeding, migrating southbound, etc.), signal losses were disproportionately more likely to occur during northbound migration and the winter and were least likely during the breeding season (Fig. 3).
SURVIVAL AND FECUNDITY OF SWALLOW-TAILED KITES—Meyer
15
Table 2. Movement data, winter locations, date and location of last fix, and elapsed time from transmitter deployment to last signal (n = 15) for 19 individual swallow-tailed kites tracked by satellite from 1 May 2000 to 30 April 2004. The data span a period that included 28 southbound migrations (2000–2003) and 19 northbound migrations (2001–2004) of radio-tagged birds.
Bird ID
Sex
Year
Depart Florida
16081a
F
16086
M
16082d
F
16083a 16085a 36311d
F F M
36313
F
16083bd
M
36310
M
36316d
F
38308
F
36312 36314a 36315 36309 16081b 16085b 36314b 41244
F M M F F F M M
2000 2001 2001 2002 2001 2002 2003 2001 2001 2002 2003 2002 2003 2002 2003 2002 2003 2002 2003 2002 2003 2002 2002 2002 2002 2002 2002 2003 2003
7/21 7/19 8/12 8/11 7/29 7/17 7/23 n/a 8/8 8/13 8/9 8/5 7/31 8/10 8/19 8/9 7/31 8/8 8/12 8/10 8/8 8/7 8/20 9/2 8/7 8/24 8/21 8/18 8/23
aDistance
Distance southa (km)
Arrive for winter
8,763
11/18
8,312 8,341 8,728 9,188 9,215
10/12 10/12 10/22 9/28 10/13
8,715 8,235 8,440 8,222 7,669 9,463 8,045 9,275 8,160 7,790 7,741 8,866 7,898 9,415 6,869
11/18 10/15 10/20 11/15 10/6 10/26 10/6 10/23 9/24 11/18 10/26 10/27 10/26 11/14 10/13
9,198
10/13
7,584 8,379
10/7 11/21
Date of last Last location locationb
Last X coord
Last Y coord
Elapsed timec (d)
8/20/01
South
-76.595
1.636
448
3/20/03
North
-92.291
29.431
663
7/28/01 Breeding 11/19/01 Winter
-82.73 -53.918
29.183 -22.623
68 178
5/10/04 Breeding
-83.51
34.156
713
11/8/03
Winter
-54.191
-21.383
557
12/22/03 12/29/02 3/24/03 11/11/02 4/26/03 11/20/02 10/23/02 4/6/04 2/10/04
Winter Winter North South North South South North Winter
-61.648 -53.334 -85.043 -76.919 -81.257 -76.855 -64.027 -78.509 -60.236
-16.302 -21.439 24.752 0.779 26.684 5.341 -10.173 26.692 -14.125
588 210 298 166 347 165 126 297 237
south = Length of migratory route, in kilometers, from breeding grounds to winter range. when last satellite signal was detected. cElapsed time = elapsed radio time, in days, from transmitter deployment to last signal detection. dAs of 30 April 2004 this bird was still alive, and its transmitter was still active. bSeason
16
FLORIDA FISH AND WILDLIFE CONSERVATION COMMISSION FINAL REPORT
Fig. 1. Number of days after deployment when the last location was received for 15 swallow-tailed kites carrying solar-powered satellite transmitters from 2000 to 2004.
Fig. 2. Distribution of the last locations for 15 swallow-tailed kites with solar-powered satellite transmitters. Symbols indicate the season of last location, for which the seasonal limits were based on the accumulated satellite locations.
SURVIVAL AND FECUNDITY OF SWALLOW-TAILED KITES—Meyer
17
Fig. 3. Percentage of signal losses in each season compared to the proportion of the year spent in that activity for 15 swallow-tailed kites carrying solar-powered satellite transmitters from 2000 to 2004. Seasons were delineated with reference to the accumulated satellite-tracking data.
Estimating Survival We referred to the temporal and geographic distributions of signal losses when assigning annual status codes to each bird (Table 3). To select the season when a signal was last detected, we considered each bird’s position and prior movements, not simply the calendar date. The yearly sets of status codes, grouped into 4 different scenarios ranging from most (Run 1) to least (Run 4) conservative, resulted in the annual survival curves depicted in Figs. 4–7. Mean estimates (all 4 years) for annual survival were 0.678 (Run 4) to 0.936 (Run 1) (Table 4), with confidence intervals spanning ± 0.150–0.20. The median of Runs 2 and 3 was 0.807. Confidence intervals (Table 4) were omitted from Figs. 4–7 to make the graphs more readable. Based on the most conservative of the 3 derivations of the log-rank test (Cox and Oakes 1984), survival did not differ among years, between sexes, or between birds with VHF versus satellite transmitters.
18
FLORIDA FISH AND WILDLIFE CONSERVATION COMMISSION FINAL REPORT
Table 3. Summary data and annual status for adult swallow-tailed kites (n = 43 individuals) tracked by VHF telemetry and satellite from 2000 to 2004. One bird was tracked first by VHF and then by satellite telemetry (treated as 1 bird in the analysis).
Transmitter ID #a typeb 4.847 4.884 5.117 5.145 5.171 5.191 5.204 5.249 5.266 5.275 5.335 5.364 5.443 5.455 5.475 5.489 5.502 5.521 5.747 6.045 6.086 6.108 6.148 6.168 6.255 16081a 16081b 16082 16083a 16083b 16085a 16085b 16086 36308 36309 36310 36311 36312 36313 36314a
VHF VHF VHF VHF VHF VHF VHF VHF VHF VHF VHF VHF VHF VHF VHF VHF VHF VHF VHF VHF VHF VHF VHF VHF VHF Sat Sat Sat Sat Sat Sat Sat Sat Sat Sat Sat Sat Sat Sat Sat
Sex
Captured
F F F F M M M F M M F M F F F M M F F M M F F F F F F F F M F F M F F M M F F M
06/20/03 06/24/03 05/02/00 05/01/00 05/01/00 05/09/00 05/19/00 05/15/00 05/18/00 05/26/00 05/06/00 05/15/00 05/22/00 05/22/00 05/26/00 05/30/00 05/31/00 05/30/00 06/03/02 05/15/01 05/23/01 05/28/01 06/05/01 06/07/01 06/08/01 05/30/00 06/08/02 05/20/01 05/22/01 06/19/02 05/25/01 06/19/02 06/05/01 05/13/02 05/14/02 05/20/02 05/22/02 05/23/02 05/27/02 05/30/02
Date last observed
Season last observed
06/30/04 06/30/03 05/17/02 12/06/00 05/18/02 05/28/02 12/19/01 03/23/01 11/20/00 05/14/02 08/02/00 11/20/00 12/10/01 07/02/02
Breeding Breeding Breeding Winter Breeding Breeding Winter Breeding Winter Breeding Southbound Winter Winter Breeding
05/19/02 05/19/02 04/04/01 06/30/04 05/23/03 05/29/03 05/28/03 12/06/01 05/28/03 08/21/02 08/20/01 11/20/02 06/30/04 07/28/01 06/30/04 12/16/01 10/23/02 03/20/03 12/22/03 04/26/03 11/08/03 06/30/04 12/29/02 05/10/04 03/24/03
Breeding Breeding Northbound Breeding Breeding Breeding Breeding Winter Breeding Southbound Southbound Southbound Breeding Breeding Breeding Winter Southbound Northbound Winter Northbound Winter Breeding Winter Breeding Northbound
Statusc 4/30/01 4/30/02 4/30/03 4/30/04 b f b f b b b a f b b b b b b b b a
b
b
e
b b f
e e
b f f f b b b b
e
b b b f b b c b d
e b e e b b b b f
b
b e e e e
c b
b
b
b
c b
d a b d b b c b a
d c b b
SURVIVAL AND FECUNDITY OF SWALLOW-TAILED KITESâ&#x20AC;&#x201D;Meyer
19
Table 3. Continued.
Transmitter ID #a typeb 36314b 36315 36316 41244
Sat Sat Sat Sat
Sex
Captured
Date last observed
M M F M
06/13/03 05/29/02 06/07/02 06/18/03
04/06/04 11/11/02 06/30/04 02/10/04
Season last observed Northbound Southbound Breeding Winter
Statusc 4/30/01 4/30/02 4/30/03 4/30/04 d c b
b d
aID
# = frequency (VHF) or Service Argos tracking number (satellite). = satellite transmitter, VHF = VHF transmitter. cStatus indicates levels of certainty regarding each birdâ&#x20AC;&#x2122;s fate; no entry means the bird had not yet been radio tagged or had already died. Four pairs of satellite-tracked birds have the same ID numbers, distinguished by a and b suffixes, because Service Argos issues a limited number of IDs that must be reused. a = confirmed dead before the end of the yearly interval; b = confirmed alive at the end of the yearly interval; c = satellite birds that stopped reporting, with no prior evidence of impending radio failure, from 2 areas where losses were clustered in South America; d = satellite birds that stopped reporting, with no prior evidence of impending radio failure, in areas with no other known losses; e = VHF birds that were alive when their radios expired at the end of their expected life, 2+ years after deployment; f = VHF birds that were alive when last detected in the U.S. prior to fall migration, en route on migration, or on their winter range, but which were not found in the U.S. the following spring. bSat
Fig. 4. Estimates of annual survival for adult swallow-tailed kites from 1 May 2000 to 30 April 2001, Runs 1 through 4. Run 1 was the most conservative scenario, with only those birds confirmed dead or alive assigned to those categories. Runs 2, 3, and 4 were progressively less conservative, with increasing numbers of censored birds gradually reassigned to the confirmed dead class.
20
FLORIDA FISH AND WILDLIFE CONSERVATION COMMISSION FINAL REPORT
Fig. 5. Estimates of annual survival for adult swallow-tailed kites from 1 May 2001 to 30 April 2002, Runs 1 through 4. Run 1 was the most conservative scenario, with only those birds confirmed dead or alive assigned to those categories. Runs 2, 3, and 4 were progressively less conservative, with increasing numbers of censored birds gradually reassigned to the confirmed dead class.
Fig. 6. Estimates of annual survival for adult swallow-tailed kites from 1 May 2002 to 30 April 2003, Runs 1 through 4. Run 1 was the most conservative scenario, with only those birds confirmed dead or alive assigned to those categories. Runs 2, 3, and 4 were progressively less conservative, with increasing numbers of censored birds gradually reassigned to the confirmed dead class.
SURVIVAL AND FECUNDITY OF SWALLOW-TAILED KITESâ&#x20AC;&#x201D;Meyer
21
Fig. 7. Estimates of annual survival for adult swallow-tailed kites from 1 May 2003 to 30 April 2004, Runs 1 through 4. Run 1 was the most conservative scenario, with only those birds confirmed dead or alive assigned to those categories. Runs 2, 3, and 4 were progressively less conservative, with increasing numbers of censored birds gradually reassigned to the confirmed dead class.
0.867 1.000 0.875 1.000
0.936 0.075 0.037
2000–2001 2001–2002 2002–2003 2003–2004
Mean (all years) SD SE
Year (1 May–30 April) Survival
1.038 0.001 0.001
1.037
0.713
0.704 0.013 0.009
1.039
Upper CI
0.695
Run 1 Lower CI
0.854 0.080 0.040
0.867 0.894 0.737 0.917 0.687 0.103 0.052
0.695 0.755 0.539 0.760
Run 2 Lower Survival CI
1.020 0.059 0.030
1.039 1.033 0.935 1.073
Upper CI
0.759 0.116 0.058
0.867 0.852 0.650 0.667
Survival
0.549 0.150 0.075
0.695 0.660 0.441 0.399
Run 3 Lower CI
0.960 0.080 0.040
1.039 1.007 0.859 0.933
Upper CI
0.678 0.074 0.037
0.765 0.714 0.619 0.615
Survival
0.462 0.098 0.049
0.563 0.521 0.411 0.351
Run 4 Lower CI
0.895 0.058 0.029
0.966 0.908 0.827 0.880
Upper CI
Table 4. Annual survival for 4 years (1 May 2000–30 April 2004) under each of 4 scenarios (Run1–Run 4) for assigning mortality to lost radio signals. Run 1 was the most conservative scenario, with only those birds confirmed dead or alive assigned to those categories. Runs 2, 3, and 4 were progressively less conservative, with increasing numbers of censored birds gradually reassigned to the confirmed dead class. An empty cell indicates that there were too few data for a meaningful estimate.
22 FLORIDA FISH AND WILDLIFE CONSERVATION COMMISSION FINAL REPORT
SURVIVAL AND FECUNDITY OF SWALLOW-TAILED KITES—Meyer
23
DISCUSSION Age-specific survival is one of the vital demographic rates essential for modeling animal populations. Radio telemetry provides valuable opportunities for estimating the survival of birds, thus contributing to demographic modeling and projections of population trends. As with other survival-study methodologies, care is needed in applying telemetry results to statistical analyses to avoid biases and unsound inferences, but also to make the fullest possible use of the data, which are difficult and costly to obtain. In a comparative study of 49 bird species examining the influence of demographic variables on the population’s intrinsic rate of growth, Saether and Bakke (2000) found that the relative contributions of fecundity and adult survival to population growth were strongly correlated with life history. Fecundity’s contribution was greatest in species with large clutch sizes and high adult mortality rates, whereas adult survival’s contribution was highest among species with delayed maturation and small clutches, such as swallowtailed kites. We used VHF and satellite telemetry to investigate adult survival of swallow-tailed kites, a long-lived and widely ranging species of critical conservation concern. In the process of collecting the necessary data, we also identified the likely seasonal and geographic correlates of mortality, and we measured breeding effort of adult females to facilitate (along with existing data on nest success, productivity, and age of first reproduction) estimating fecundity. Fecundity is defined as the number of female offspring that fledge per adult female in the population. This must take into account age at first reproduction (i.e., not all females in the population are of reproductive age), nest success, productivity (number of young fledging per successful nest), and breeding effort (the proportion of reproductive-age females that actually attempt to breed in any year). Fecundity is not a single term in population models but rather the suite of variables listed above, each of which is estimated and entered individually. Previous studies have produced useful estimates of age at first reproduction, nest success, and productivity, but we lacked an estimate for breeding effort. Breeding effort was high (92%) in our study. This estimate should be considered tentative, given the relatively small sample of females and the limited geographic focal areas of our study. Our result, however, is a useful first approximation of breeding effort and probably can be taken as a likely upper limit of what occurs across a wider range. It also is valuable in light of
24
FLORIDA FISH AND WILDLIFE CONSERVATION COMMISSION FINAL REPORT
our previous inability to make more than a poorly informed guess about female breeding effort. Swallow-tailed kites are highly social and nest in loose colonies or neighborhoods (Robertson 1988, Meyer 1995). We have always observed large numbers of apparently non-breeding kites in nest areas but have not known whether they were sub-adults (i.e., birds that had not reached reproductive age) or breeding-age birds that were foregoing nesting. It appears, at least in our study area, that most of these non-breeding birds were sub-adults. Although the speciesâ&#x20AC;&#x2122; mating system has not been studied rigorously, it appears to be mainly monogamous. If this is the case, and if the sexes have equal survivorship, then males also must attempt to nest in high proportion each year. In presenting the results of this 4-year study of adult survival, we offer tentative interpretations based on a respectable sample that, although not large, supports a useful range of survival estimates applicable to population modeling of swallow-tailed kites as long as one takes the relatively broad confidence intervals into consideration. Our results also point to the likely locations and possible causes of a large portion of the annual mortality of the U.S. population of swallow-tailed kites, thereby illuminating conservation and management opportunities, or the critical lack thereof. The occurrence of a disproportionately large number of apparent deaths relative to the amount of time spent on northbound migration and on the winter range in South America indicates that specific areas used during these activities are particularly risky for the kites. The clustering of apparent mortality in these areas cannot be explained as a result of the kites simply spending most of their time there because relatively little time is spent compared to the number of losses (see Fig. 3). Ongoing research by Avian Research and Conservation Institute over the next few years, with support from FWC and others, will continually augment our survival datasets, allowing periodic reanalysis and increasing veracity of our interpretations. All survival analyses require some assumptions, regardless of how the data are collected (Bennetts and Kitchens 1997). Conditions in the study area should represent those of the larger range of the species. Individual study subjects, furthermore, should be representative of the greater population. Our need to find a large number of nests to support trapping and radio tagging, a very difficult process, required manageable logistics. This restricted the spatial range of our study and resulted in the selection of most of our marked kites from relatively small areas mainly in northwestern peninsular Florida and the coastal plain of southeastern Georgia. These 2 areas represent roughly the best and the worst of what we have studied for swallow-tailed kite breeding habitat based on nesting success, productivity, and activity ranges
SURVIVAL AND FECUNDITY OF SWALLOW-TAILED KITESâ&#x20AC;&#x201D;Meyer
25
and prey delivery rates of adults rearing young. In addition, the sex ratio of our radio-tagged birds was skewed toward females (19 males versus 25 females, or 1.0:1.4) as a result of our trapping success. Some inaccuracies in our survival estimates may have resulted from these factors. Subsequent analyses of longer-term datasets based on a more broadly sampled study population should improve the accuracy of our estimates. In addition to these general assumptions, some specific ones apply to analyses based on telemetry data (Bennetts and Kitchens 1997). The first is that transmitters should not influence survival. Ambrose and Riddle (1988) concluded that the maximum distance flown by radio-tagged peregrine falcons (Falco peregrinus) was about the same as that for banded falcons without transmitters. In a study of albatrosses and petrels of various genera, Phillips et al. (2003) found no significant increase in foraging trip duration, mass of food delivered, or nesting success for radio-tagged birds. In our own experience, with this and previous studies of swallow-tailed kites, we have had no indication that either the VHF or satellite transmitters or the harness attachment adversely affected survival. There was not significant difference in survival in the present study between birds carrying VHF versus satellite transmitters (P > 0.39). The second assumption regarding telemetry data in survival studies is that censored and uncensored individuals have the same survival. There is no reason to suspect that this assumption was violated in our study. The third assumption is that the sample size does not negatively affect the staggered entry design. When samples are relatively small, as for the present study, the number of birds at risk at any given time should be taken into account. If this number is small and confidence intervals are broad, the results should be viewed with caution. Transmitter reliability and detectability also are important considerations. We have found both the VHF and satellite transmitters used in this study to be highly reliable, to operate consistently, and to be consistently detectable. Aerial surveys for VHF signals repeatedly located the same individuals, even though they were moving around the state, and rarely added new individuals to the pool of detected kites, indicating that VHF detectability did not compromise the quality of our results. Satellite transmitters are consistently detectable except for very brief, occasional periods of 1 or 2 lost transmission cycles. This was probably due to extended periods of cloudy weather that precluded adequate recharging. As for reliability, we presently are tracking 2 birds with satellite transmitters that have been operating for 3.2 years (a
26
FLORIDA FISH AND WILDLIFE CONSERVATION COMMISSION FINAL REPORT
swallow-tailed kite) and 3.8 years (a short-tailed hawk). Besides our own good experience, Britten et al. (1999) found that only 2 of 42 (4.7%) small satellite transmitters failed prematurely on peregrine falcons. In addition to the similar survival of birds with VHF and satellite transmitters, despite the transmitters’ different mass and size, the temporal and spatial clustering of lost signals is the best evidence that losing radio contact with marked kites usually was not due to the influence of the transmitters on the birds’ performance, to poor detectability, or to failed or lost transmitters. We cannot select a single value as an estimate of annual survival of adult swallow-tailed kites, but we offer a suggestion. The highest value reported, 0.936 for Run 1, probably is too high (Table 4). Given the nature of this species’ far-ranging movements, it is unlikely that all the birds that could not be confirmed dead disappeared due to radio failure or loss. The lowest estimate, 0.678 for Run 4, is equally unlikely because some radio failure or loss probably occurred. We do not believe this happened frequently, however, based on the considerations listed above. The actual survival rate of adult swallow-tailed kites in our study probably fell somewhere between the intermediate estimates based on Runs 2 (0.854) and 3 (0.759). We suggest that, at least for now, the range of 76–85% be used as an estimate of adult survival for modeling the demographics of the U.S. population of swallow-tailed kites. Depending on the population’s (and model’s) sensitivity to adult survival, both the high and low estimates may result in the same prediction for a population trend, whether declining, stable, or increasing. If this is the case, we will have learned much and can then decide on the potential value of increasing adult survival. Will management intervention targeting adult survival be cost-effective given other possible options for improving the status of the population? The accumulated longterm datasets for Florida’s swallow-tailed kites, such as that used here, will enable us to address such questions.
SURVIVAL AND FECUNDITY OF SWALLOW-TAILED KITESâ&#x20AC;&#x201D;Meyer
27
RECOMMENDATIONS Most opportunities to intervene in the management and conservation of swallow-tailed kites are on the breeding range here in the U.S. Unfortunately, mortality is disproportionately more frequent elsewhere, particularly during the southbound migration and winter in South America and as the kites approach the U.S. coast on northbound migration. Specific sources of mortality in South America should be investigated. It is possible that these threats can be addressed, particularly if these deaths are due to human persecution that is focused geographically. The U.S. Fish and Wildlife Service has offered the Avian Research and Conservation Institute funding to pursue conservation of privately owned agricultural lands in Brazil where a very large portion of the U.S. kite population overwinters. We also have received support from the Disney Wildlife Conservation Fund for satellite tracking of swallow-tailed kites from the Brazilian breeding population to determine their winter range. This may also illuminate sources of threats in South America to the migratory U.S. breeding population. It also would be wise to investigate ways to augment the reproductive output of the U.S. population to mitigate as much as possible losses incurred while Floridaâ&#x20AC;&#x2122;s swallow-tailed kites are beyond the reach of management and conservation.
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LITERATURE CITED Ambrose, R. E., and K. E. Riddle. 1988. Population dispersal, turnover, and migration of Alaska peregrines. Pages 677–684 in T. J. Cade, J. H. Enderson, C. G. Thelander, and C. M. White, editors. Peregrine falcon populations: their management and recovery. The Peregrine Fund, Boise, Idaho, USA. Beissinger, S. R. 1995. Modeling extinction in periodic environments: Everglades water levels and snail kite population viability. Ecological Applications 5:618–631. Bennetts, R. E., and W. M. Kitchens. 1997. The demography and movements of snail kites in Florida. Technical Report Number 56. U.S.G.S. Biological Resources Division, Florida Cooperative Fish and Wildlife Research Unit. Britten, M. W., P. L. Kennedy, and S. Amborse. 1999. Performance and accuracy evaluation of small satellite transmitters. Journal of Wildlife Management 63:1349–1358. Cely, J. 1979. Status of the swallow-tailed kite and factors affecting its distribution. Pages 144–150 in D. M. Forsythe and W. B. Ezell Jr., editors. Proceedings of the first South Carolina endangered species symposium. South Carolina Wildlife and Marine Resources Department, Columbia, South Carolina, USA. Cox, D. R., and D. Oakes. 1984. Analysis of survival data. Monographs on statistics and applied probability 21. Chapman and Hall, New York, New York, USA. Grier, J. W. 1980. Modeling approaches to bald eagle population dynamics. Wildlife Society Bulletin 8:3116–3322. Kaplan, E. L., and E. Meier. 1958. Nonparametric estimation from incomplete observations. Journal of the American Statistical Association 53:457–481. Lee, E. T. 1980. Statistical methods for survival data analysis. Lifetime Learning Publishers, Belmont, California, USA.
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Loery, G. K., H. Pollock, J. D. Nichols, and J. E. Hines. 1987. Age-specificity of black-capped chickadee survival rates: analysis of capture-recapture data. Ecology 68:1038–1044. Mertz, D. B. 1971. Life history phenomena in increasing and decreasing populations. Pages 361–399 in G. P. Patil, E. C. Pielou, and W. E. Waters, editors. Statistical ecology. Volume 2. Pennsylvania State University Press, University Park, Pennsylvania, USA. Meyer, K. D. 1995. Swallow-tailed kite (Elanoides forficatus). In A. Poole and F. Gill, editors. The birds of North America. No. 138. Academy of Natural Sciences, Philadelphia, Pennsylvania, and American Ornithological Union, Washington, D.C., USA. _____. 2004a. Conservation and management of the swallow-tailed kite. Final report. Florida Fish and Wildlife Conservation Commission, Tallahassee, Florida, USA. _____. 2004b. Demography, dispersal, and migration of the swallow-tailed kite. Final report. Florida Fish and Wildlife Conservation Commission, Tallahassee, Florida, USA. _____, and M. Collopy. 1995. Status, distribution, and habitat requirements of the American swallow-tailed kite (Elanoides forficatus) in Florida. Final report. Florida Fish and Wildlife Conservation Commission, Tallahassee, Florida, USA. _____, and M. W. Collopy. 1996. American swallow-tailed kite. Pages 188–196 in J. Rodgers, H. Kale II, and H. Smith, editors. Rare and endangered biota of Florida. Volume 5: Birds. University Press of Florida, Gainesville, Florida, USA. Millsap, B. A., J. Gore, D. Runde, and S. Cerulean. 1989. Setting priorities for the conservation of fish and wildlife species in Florida. Florida Game and Fresh Water Fish Commission, Tallahassee, Florida, USA. Nichols, J. D., J. Bart, R. J. Lampert, W. J. L. Sladen, and J. E. Hines. 1992. Annual survival rates of adult and immature eastern population tundra swans. Journal Wildlife Management 56:485–494.
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_____, G. L. Hensler, and P. W. Sykes Jr. 1980. Demography of the Everglade kite: implications for population management. Ecological Modeling 9:215–232. Phillips, R. A., J. C. Xavier, and J. P. Croxall. 2003. Effects of satellite transmitters on albatrosses and petrels. Auk 120:1082–1090. Pollock, K. H., S. R. Winterstein, C. M. Bunck, and P. D. Curtis. 1989. Survival analysis in telemetry studies: the staggered entry design. Journal of Wildlife Management 53:7–15. Robertson, W., Jr. 1988. American swallow-tailed kite. Pages 109–131 in R. S. Palmer, editor. Handbook of North American birds. Volume 4. Yale University Press, New Haven, Connecticut, USA. Saether, B.-E., and O. Bakke. 2000. Avian life history variation and contribution of demographic traits to the population growth rate. Ecology 81:642–653. SAS Institute. 1988. SAS/STAT User’s guide. Release 6.03. SAS Institute, Cary, North Carolina, USA. Service Argos. 1996. User’s manual. Landover, Maryland, USA. White, G. C., and R. A. Garrott. 1990. Analysis of wildlife radio-tracking data. Academic Press, New York, New York, USA. Zimmerman, G. M., and K. D. Meyer. 2004. Migration ecology of swallowtailed kites in Cuba, Mexico, and Belize. Final report. Florida Fish and Wildlife Conservation Commission, Tallahassee, Florida, USA.
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