Trapping methods can bias age ratio in samples of passerine populations

Abstract

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.