Habitat and sampling related bias in sex-ratio of trapped Blackcaps Sylvia atricapilla

Abstract

Herremans, M. 1987. Habitat and sampling related bias in sex-ratio of trapped Blackcaps Sylvia arricapilla. Ring. & Migr. 10: 31-34. Random sampling at a coastal site showed sex-ratios rather close to, but in some years significantly different from equality. Trapping of Blackcaps by artificially induced landfall using tape lures revealed an overall sex-ratio bias in favour of males (61 5%. N=5,067). However, results were greatly variable, depending on habitat conditions and selectivity of sampling. Trapping in habitat suitable for the species resulted in relatively more females (54.7% males) than sampling in "unsuitable habitat" (66.2% males) and, similarly, "random sampling" resulted in more females (55.5% males) than samples "selected" by tape-luring birds down from higher vegetation (64.6% males). However, random sampling in suitable habitat (53.4% males, N=8.382) also proved distinct from random sampling in unsuitable habitat (65.0% males, N=1,864), indicating that females are less likely to come down into, or remain in, strange habitats. In both cases a clear habitat assessment at night seems involved in the birds decision. Marc Herremans. Primes Lydialaan 65. B-3030 Heverlee. Belgium.