Fitxercontingut1980 by Mohamed Zamil

Bird Study (1997) 44, 348–354

Trapping methods can bias age ratio in samples of passerine populations J. DOMÈNECH* and JUAN CARLOS SENAR Museum of Zoology, Ap. Correus 593, 08080 Barcelona, Spain In capture–recapture studies, productivity is usually estimated as the ratio of juveniles to adults trapped. However, because the validity of productivity estimations relies on the assumption that the ratio of the capture probabilities for adults and young does not vary over samples, the use of several trapping methods, with different associated capture probabilities for each age, could lead to erroneous inferences on productivity. We investigated how the use of different trapping methods can affect the estimated age ratio of the sampled population. We used the captures of 5377 Serins Serinus serinus trapped from April to September in the years 1985 to 1994 in a suburban area of Barcelona (northeast Spain). There were three trapping methods: mist-nets, a Yunick platform trap and a clap-net. A multifactorial approach using log–linear analyses showed that mist-nets captured a significantly higher proportion of juveniles than the other two methods. The trap captured more adults, and to a lesser extent yearlings. The clap-net produced an intermediate age ratio. Results support the view that trapping method can influence the age ratio sample of the population, and emphasizes the need for caution when analysing population parameters from different bird trapping methods.

here is an increased interest in population dynamics and monitoring of bird populations,1-4 and the capture–recapture approach is becoming one of the standard methodologies. 5-8 Several long-term monitoring programmes, such as the British Constant Effort Sites (CES)9 scheme or the North American Monitoring Avian Productivity and Survivorship (MAPS)10 programme, rely on this approach; others are being developed in other countries. One of the parameters of interest in any monitoring programme is the estimation of productivity. In capture–recapture studies, productivity is usually estimated as the ratio of juveniles to adults,11,12 and this value has been found, for some species, to correlate quite well with the number of nestlings ringed.11 Mist-nets are generally used as the standard trapping method in any monitoring programme, but many other capture methods are potentially *Correspondence author Email: mzoolbcn@lix.intercom.es

available to the bird ringer. However, because the validity of productivity estimations relies on the assumption that the ratio of the capture probabilities for adults and young does not vary over samples, the use of several trapping methods, with different associated capture probabilities for each age, could lead to erroneous inferences on productivity. Hence, information on age biases associated with the different capture methods is of prime importance. For instance, mist-nets have been shown to trap more juvenile Great Tits Parus major than other methods.13 However, the effect of different biases has been shown in other contexts to vary in a complex way, so that the effect of a factor (e.g. age) can vary according to the levels of other factors.14–16 This paper analyses, in a multifactorial way, how the use of different trapping methods can affect the estimated age ratio of the sampled population. We compare three common capture methods: mist-nets, clap-nets and traps. Data show that different methods result in different

Trapping biases age ratios, which stresses the importance of consistency of the trapping regime when estimating various population indices. METHODS Serins were captured and recaptured between 1985 and 1994 in a suburban area of Barcelona (northeast Spain). The area includes three hectares of orchards and gardens. The data used for this paper are from birds trapped from April to September inclusive. This period is somewhat longer than the standard breeding period used in the CES9 scheme (May to August inclusive), because in Mediterranean areas juvenile Serins may already appear in April and females may still be breeding in August. A total of 5377 captures was used in the analysis, 2678 of them being previously unringed birds. All birds were individually ringed, and age and sex were determined according to Svensson.17 We used the following definitions: juveniles as birds in streaked plumage (Euring ages 3J); yearlings as birds in Euring codes 3 and 5; adults as birds in Euring codes 4 and 6. Yearlings should probably be split into the Euring 3 and 5 classes, as they may be behaving differently; however, sample size for Euring 3 birds (9% of total birds) was too small to allow for an independent class (they start to be captured around the end of September). Given that these Euring 3 birds are already some months old, we preferred to join them to the more ‘expert’ class of Euring 5 birds rather than to the highly inexperienced 3J birds. We only used juveniles which were additionally recaptured after the postjuvenile moult, so that they could be sexed retrospectively by plumage colour. Three trapping methods were used simultaneously: mist-nets, a Yunick platform trap18–20 and a clap-net.21 All three methods were available for the same length of time and in the same relative proportions (one trap, one clap-net, one or two mist-nets), except for the mist-net that was not always fixed. The clap-net and the trap were always associated with baited feeders and both were actively triggered by the trapper, but the mist-nets, a passive method, could be associated either with food or water. Only one trapping site was used for each trapping device. The maximum distance between trapping methods was 60 m.

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The day was divided into two parts: time 1 up until three hours after sunrise, time 2 after this. The age ratio of the sampled Serin population varied between these two periods: a higher percentage of juveniles was trapped in the early morning, whereas yearlings and adults were more often trapped in the second time period (χ22 = 29.97, P < 0.001). We included residence status (new ringed birds versus recaptured ones) in the analysis, in addition to age, sex and time period, to standardize for a possible bias effect of this factor (see ref. 22). We use residence status in a broad sense, to distinguish birds which probably do not yet have experience of traps (new ringed birds) from those already familiar with them (recaptured ones). Given the high trapping effort invested in our area (one catching session each week, during the whole year; total number of recaptures obtained in the whole period was 4409), it is likely that already ringed birds are in fact residents, whereas unringed birds are newly arrived or transients. The usual time lag between successive capture sessions (one week) and the fact that Serins are rarely retrapped within the same day, excludes the possibility that transients staying for short periods in the area would be classified as residents. The five-factor contingency table relating the number of birds trapped with each method and factors sex, age, residence status and time of day was analysed using backwards stepwise log–linear analysis, with an automatic model selection procedure which derives the simplest model that fitted the data (the ‘best model’, which includes the fewest interactions necessary to fit the observed table).23 The statistic used to determine whether to keep or drop factors from the model was the likelihood ratio chi-square.23 RESULTS The best model explaining the variation in the five-factor log–linear analysis (χ24 = 34.16, P = 0.73) included five interactions (Table 1) (note that five of the two-way interactions are included in the three-way interactions). The first one related trapping method to age and time. The exact effects of this interaction were, however, complex. Mist-nets captured a significantly higher proportion of juveniles

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Table 1. Marginal association χ2 values of the log–linear five-factorial independence test between the variables capture method (clap-net vs. platform trap vs. mist-net), sex, age (1st year juveniles vs. 2nd year juveniles vs. adults; Euring ages 3 vs. 5 vs. 4/6), residence status (newly ringed bird vs. recaptures) and time. Hypothesis

χ2

Method × Residence Method × Sex Method × Age Method × Time Residence × Sex Residence × Age Residence × Time Sex × Age Sex × Time Age × Time Method × Residence × Sex Method × Residence × Age Method × Residence × Time Method × Sex × Age Method × Sex × Time Method × Age × Time Method × Residence × Sex × Age Method × Residence × Sex × Time Method × Residence × Age × Time Method × Sex × Age × Time Method × Residence × Sex × Age × Time

2 2 4 2 1 2 1 2 1 2 2 4 2 4 2 4 4 2 4 4 4

246.06 1.35 108.73 25.39 9.12 252.26 12.02 8.61 0.25 12.78 0.91 35.28 0.34 1.82 3.10 15.02 4.94 3.84 1.51 1.17 2.88

P < 0.001 0.51 < 0.001 < 0.001 0.003 < 0.001 0.001 0.01 0.62 0.005 0.64 < 0.001 0.84 0.77 0.21 0.01 0.29 0.15 0.82 0.88 0.58

Table 2. Contingency tables relating capture method to age. Age Capture method

Juveniles

Yearlings

Adults

Clap-net

1234 (1130)

550 (578)

145 (221)

Trap

1434 (1681)

986 (859)

448 (329)

483 (340)

74 (174)

23 (66)

Mist-net

Statistics in Table 1. Expected frequencies within parentheses.

(83% of birds captured at mist-nets were juveniles; Table 2) than the other two methods. The trap captured more adults, and to a lesser extent more yearlings, than the mist-net and the clap-net (Tables 1 & 2, Fig. 1). The effect of time period was nevertheless variable: time did not affect mist-net catches, which always captured more juveniles (χ22 = 2.98, P = 0.23): nor did it affect adults, which were more often captured in the trap independently of the time period (χ22 = 4.89, P = 0.087) (Fig. 1). However, for yearlings there was a shift with time from the clap-net to the trap, and for juveniles there

was an increase in the percentage of birds trapped at the mist-net in the second time period, which is the reason for the three-way interaction between trapping method, age and time period (Fig. 1; Table 1). The second interaction showed that residents were more often captured with the trap and this tendency was greater if they were adults (Table 1, interaction trapping method × residence × age; Fig. 2). The third interaction related time and residence, so that the proportion of unringed birds was greater during time period 1 (Tables 1 & 3).

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Trapping biases

Trap

Mist-net

351

Clap-net

100

Proportion of Serins (%)

0 Time 1

Time 2

Time 1

Juveniles

Time 2

Time 1

Yearlings

Time 2

Adults

Figure 1. Proportion of Serins trapped by the three trapping methods analysed by age and time period.

Trap

Mist-net

Clap-net

100

Proportion of Serins (%)

0 R

Juveniles

Yearlings

Adults

Figure 2. Proportion of Serins trapped by the three trapping methods analysed by age and residence status. R, residents; T, transients.

ÂŠ 1997 British Trust for Ornithology, Bird Study,

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Table 3. Contingency tables relating residence to the variables age, and time. Age

Time

Residence

Juveniles

Yearlings

Adults

Time 1

Time 2

Residents

1155 (1582)

1025 (803)

519 (309)

1246 (1370)

1453 (1329)

Transients

1996 (1569)

585 (802)

97 (307)

1483 (1359)

1195 (1319)

Statistics in Table 1. Expected frequencies within parentheses.

Table 4. Contingency tables relating sex to the variables age, and residence. Age Sex

Juveniles

Yearlings

Males

447 (431)

Females

328 (344)

Residence Adults

Residents

Transients

855 (894)

366 (342)

1085 (1045)

583 (623)

752 (713)

249 (273)

793 (833)

536 (496)

Statistics in Table 1. Expected frequencies within parentheses.

The fourth interaction showed that a higher proportion of resident birds were males (Tables 1 & 4) and the last interaction showed that a higher proportion of adult and juvenile birds were males (Tables 1 & 4). DISCUSSION The results support the view that trapping method can bias the age ratio sample of the population. This can lead to erroneous inferences on productivity whenever different capture devices are used. The most important effect found was that mist-nets captured a higher proportion of juveniles than the trap or the clap-net. This could be due to the fact that a bird willing to obtain bait can avoid a mistnet, especially if it already has some experience with these nets,13,24,25 but it is almost impossible for a bird to take bait unless it goes into the trap or the clap-net area (i.e. risking capture). Additionally, unlike mist-nets, a trap or a clapnet is not trapping the bird each time it enters because on non-trapping days the trap and clap-net areas are used as feeders). Therefore, the age bias obtained can be explained either because of an increase in experience with age per se22,26,27 or because of the probability of mistnet-shyness increasing with number of contacts and therefore with age.

ÂŠ 1997 British Trust for Ornithology, Bird Study,

We also found that adults, and to a lesser extent yearlings, were more often captured at the traps. This could again be related to the greater experience of adults26,27 which might perceive a lower predation risk of feeding at the trap because of its elevated position (see ref. 19). The effect of residence (i.e. experience with the trapping devices) on the interaction between trapping method and age has been stressed elsewhere: newly ringed birds (e.g. transients) are trapped more often with the clap-net than with the trap because they are probably newly arrived immigrants that may be wary of entering an unfamiliar wire funnel. However the effect of the trap may be lower for juvenile birds because their more confident behaviour makes them prone to search for food by copying other foragers (e.g. resident adults).22 The effect of time of day on the interaction between trapping method and age is difficult to explain. Birds may have a greater need for food early in the morning (time 1), so that a greater proportion of birds is caught by baited trapping methods (trap and clap-net) during this period, causing in turn a decrease by the proportion of birds at the mist-nets. This could be more pronounced for inexperienced juveniles. But, whatever the cause, the interaction exists and should be taken into account.

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Trapping biases We also detected that newly ringed Serins showed a greater tendency to be caught soon after dawn (time 1). This would be expected if we assume that, for a bird moving to a new area, this should be the best time to start exploratory movements. This time preference has also been found in transient Siskins Carduelis spinus.28 The interactions found between sex and residence and between sex and age (i.e. a greater proportion of males in the resident and adult classes) may be a by-product of differential mortality rates between the sexes, which has already been described for several other cardueline finches.29 Because males live longer, they have a higher probability of becoming residents and becoming adults. The higher dispersal rate of females (pers. obs.) could also explain these interactions. The greater proportion of males in the juvenile class is probably an artefact of the juvenile sexing procedure (see Methods): if a greater proportion of juvenile females than males disperse before they can be sexed, the juvenile subpopulation sexed as female will be reduced with respect to that of males. All of this stresses the need for caution when analysing population parameters from different bird trapping methods. Also, because different species may behave differently, it is likely to be invalid to compare estimates of population parameters for different species. However, provided that the samples are always obtained with the same method, inter-year comparisons, which are so important to monitor bird populations,3,9 and which are in fact what CES or MAPS attempt to do, should still be valid. ACKNOWLEDGEMENTS We are grateful to: C. du Feu and W.J. Peach for critical comments on the paper; D. Boné, L. Boné, J. Cascales, J.L. Copete, L.M. Copete, A. Degollada, J. Figuerola and D. Valera for their assistance in the field. We are also grateful to L. Arroyo for her help in the laboratory. This study is a contribution to DGICYT research projects PB92-0044-C02 and PB95-0102-C02. REFERENCES 1. Ralph, C.J. & Scott, J.M. (1981) Estimating Numbers

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of Terrestial Birds. Studies in Avian Biology, Vol. 6. 2. Koskimies, P. & Väisänen, R.A. (1991) Monitoring Bird Populations. Zoological Museum, Finnish Museum Natural History, Helsinki. 3. Greenwood, J.J.D., Baillie, S.R., Gregory, R.D., Peach, W.J. & Fuller, R.J. (1994) Some new approaches to conservation monitoring of British breeding birds. Ibis, 137 (suppl. 1), 16–28. 4. Perrins, C.M., Lebreton, J.D. & Hirons, G.J.M. (1991) Bird Population Studies: Relevance to Conservation and Management. Oxford University Press, Oxford. 5. White, G.C., Anderson, D.R., Burnham, K.P. & Otis, D.L. (1982) Capture–Recapture and Removal Methods for Sampling Closed Populations. Los Alamos National Laboratory, LA 8787-NERP, Los Alamos. 6. Pollock, K.H., Nichols, J.D., Brownie, C. & Hines, J.E. (1990) Statistical inference for capture–recapture experiments. Wildl. Monogr., 107, 1–97. 7. Lebreton, J.D. & North, P.M. (1993) Marked Individuals in the Study of Bird Population. Birkhäuser Verlag, Basel. 8. Lebreton, J.D., Burnham, K.P., Clobert, J. & Anderson, D.R. (1992) Modeling survival and testing biological hypothesis using marked animals: a unified approach with case studies. Ecol. Monogr., 62, 67–118. 9. Baillie, S.R. (1990) Integrated population monitoring of breeding birds in Britain and Ireland. Ibis, 132, 151–166. 10. Tautin, J. (1993) The influence of capture–recapture methodology on the evolution of the North American bird banding program. In Marked Individuals in the Study of Bird Population (eds J.D. Lebreton & P.M. North), pp. 185–196. Birkhäuser Verlag, Basel. 11. Du Feu, C. & McMeeking, J. (1991) Does constant effort netting estimate juvenile abundance? Ringing Migr., 12, 118–123. 12. Peach, W.J. (1993) Combining mark–recapture data sets for small passerines. In Marked Individuals in the Study of Bird Population (eds J.D. Lebreton & P.M. North), pp. 107–122. Birkhäuser Verlag, Basel. 13. Bauchau, V. & van Noordwijk, A.J. (1995) Comparison of survival estimates obtained from three different methods of recapture in the same population of the great tit. J. Appl. Stat., 22, 1031–1037. 14. Borras, A. & Senar, J.C. (1986) Sex, age and condition bias of decoy trapped Citril finches (Serinus citrinella). Misc. Zool., 10, 403–406. 15. Senar, J.C., Copete, J.L. & Domenech, J. (1994) The use of decoys to trap birds and associated biases: an example in the Siskin Carduelis spinus. Butll. GCA, 11, 23–30. 16. Figuerola, J. (1995) Does the use of a tape lure bias samples of Curlew Sandpipers captured with

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mist-nets? J. Field Ornithol., 66, 497–500. 17. Svensson, L. (1992) Identification Guide to European Passerines. L.Svensson, Stockholm. 18. Yunick, R.P. (1971) The squirrel-proof trap platform. EBBA News, 34, 120–122. 19. Yunick, R.P. (1971) A platform trap. EBBA News, 34, 122–125. 20. Yunick, R.P. (1971) A trap transfer cage. EBBA News, 34, 125–126. 21. Bateman, J.A. (1979) Trapping. A Practical Guide. David & Charles, London. 22. Senar, J.C. (1988) Trapping finches with the Yunick Platform Trap: the residency bias. J. Field Ornithol., 59, 381–384. 23. Norusis, M.J. (1986) SPSS/PC+ Advanced Statistics. SPSS Inc., Chicago. 24. Schmidt, K.H., Jackel, S. & Croon, B. (1986) Mistnetting at bird-feeders – a convenient method to

study Great Tit populations? J. Ornithol., 127, 61–67. 25. Wooller, R.D. (1986) Declining rates of capture of birds in mist-nets. Corella, 10, 63–64. 26. Enoksson, B. (1988) Age- and sex-related differences in dominance and foraging behaviour of nuthatches Sitta europaea. Anim. Behav., 36, 231–238. 27. Hogstad, O. (1989) Subordination in mixed-age bird flocks – a removal study. Ibis, 131, 128–134. 28. Senar, J.C., Burton, P.J.K. & Metcalfe, N.B. (1992) Variation in the nomadic tendency of a wintering finch Carduelis spinus and its relationship with body condition. Ornis Scand., 23, 63–72. 29. Breitwisch, R. (1989) Mortality patterns, sex ratios, and parental investment in monogamous birds. Curr. Ornithol., 6, 1–50.

(MS received 9 July 1996; revised MS accepted 6 February1997)

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