Trap type can bias estimates of sex ratio

Sex ratio is a parameter of interest in many population studies. A key assumption of these studies is that the trapping method used does not bias the estimate of sex ratio in the sampled population. We use capture-recapture data of 2324 Serins (Serinus serinus) trapped in northeast Spain from 1985-1992 with Yunich platform traps and clap nets to test this assumption. Results show that in autumn the platform trap captured fewer females than expected by chance. This is probably due to dominant males monopolizing the small baited surface of the trap (trap: 0.36 vs. clap net: 3.24 m(2)). The data additionally show a real increase in the proportion of males from autumn to winter, which may be related to differential mortality of the sexes.