TRAPPING METHODS CAN BIAS AGE-RATIO IN SAMPLES OF PASSERINE POPULATIONS

In capture-recapture studies, productivity is usually estimated as the ratio of juveniles to adults trapped. However, because the validity o f productivity estimations relies on the assumption that the ratio of the capture probabilities for adults and young does not vary over samp les, the use of several trapping methods, with different associated ca pture probabilities for each age, could lead to erroneous inferences o n productivity. We investigated how the use of different trapping meth ods can affect the estimated age ratio of the sampled population. We u sed the captures of 5377 Serins Serinus serinus trapped from April to September in the years 1985 to 1994 in a suburban area of Barcelona (n ortheast Spain). There were three trapping methods: mist-nets, a Yunic k platform trap and a clap-net. A multifactorial approach using log-li near analyses showed that mist-nets captured a significantly higher pr oportion of juveniles than the other two methods. The tray captured mo re 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 untie sample of the population, and emphasizes t he need for caution when analysing population parameters from differen t bird trapping methods.