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.
T
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
© 1997 British Trust for Ornithology
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