Sex bias in captured Little Ravens Corvus mellori varies with entry aperture size in a modified Australian Crow Trap

摘要

Sex bias is reported among an increasing number of bird populations. However, where sex ratio determination involves captured birds, the possibility of a sex bias associated with trapping must be excluded before a population-level sex bias can be reasonably inferred (Weston et al. 2004). For cage traps, sexual size dimorphism may influence the rate at which species enter or escape from traps. While trapping Little Ravens Corvus mellori, at Cheetham Wetlands, Melbourne (37°53'36"S, 144°47'33"E) with a modified Australian Crow Trap (ACT), we noticed substantial size variation among Little Ravens, and initially experienced high rates of escape from the trap prompting us to decrease the size of the entrance points (i.e. the size of the apertures); a tactic which dramatically decreased escape rates. These events enabled us to test the prediction that aperture size might influence the sex of birds captured in a modified Australian Crow Trap, specifically with larger aperture sizes resulting in a higher escape rate of the smaller sex.