Sex allocation in the sexually monomorphic fairy martin

Abstract

Offspring sex ratios were examined at the population and family level in the sexually monomorphic, socially monogamous fairy martin Petrochelidon ariel at five colony sites over a 4‐year period (1993–1996). The sex of 465 nestlings from 169 broods was determined using sex‐specific PCR at the CHD locus. In accordance with predicted sex allocation patterns, population sex ratios at hatching and fledging did not differ from parity in any year and the variance in brood sex ratios did not deviate from the binomial distribution. Further, brood sex ratio did not vary with hatching date during the season, brood number, brood size or colony size. The sex ratio of broods with extra‐pair young did not differ from those without, while the sex ratio of broods fathered by males that gained extra‐pair fertilizations did not differ from broods fathered by other males. Extra‐pair chicks were as likely to be male as female. Neither the total number of feeding visits to the brood nor the relative feeding contribution by the sexes varied significantly with brood sex ratio. Brood sex ratios were also unrelated to paternal size, condition and breeding experience or maternal condition and breeding experience. However, contrary to our prediction, brood sex ratio was negatively correlated with maternal size. Generally, these results were consistent with our expectations that brood sex ratios would not vary with environmental factors or parental characteristics, and would not influence the level of parental provisioning. However, the finding that females with longer tarsi produced an excess of daughters is difficult to reconcile with our current understanding of fairy martin life history and breeding ecology.