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 capt ure-recapture data of 2324 Serins (Serinus serinus) trapped in northea st Spain from 1985-1992 with Yunich platform traps and clap nets to te st this assumption. Results show that in autumn the platform trap capt ured fewer females than expected by chance. This is probably due to do minant 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 incre ase in the proportion of males from autumn to winter, which may be rel ated to differential mortality of the sexes.