Ecological and phenological covariates of offspring sex ratio in barn swallows

Non-random sex allocation in relation to parental, ecological and phenological factors has been investigated in several correlational studies of birds, mostly based on few breeding seasons and relatively small sample sizes, which have led to different results. We investigated sex ratio of nestling barn swallows (Hirundo rustica) in relation to adult sex ratio, laying date, clutch size, colony size and meteorological conditions in a sample of 553 broods (>2200 nestlings) during 10 years. At the population level, nestling sex ratio varied among years and deviated from parity in two years. Sex ratio among adults did not predict offspring sex ratio in the current or the following year. At the within-family level, the proportion of sons increased with laying date in large clutches, did not vary among clutches of intermediate size, and tended to decline with laying date in small clutches. Large colonies harbored more sons. The proportion of males increased with temperature during laying whereas the effects of temperature during the pre- or post-laying periods and that of rainfall were non-significant. These patterns of variation of offspring sex ratio did not differ between years. Thus, we identified several potential causal sources of variation in barn swallow offspring sex ratio, including temporal, phenological and ecological factors. The observation of an association of offspring sex with temperature during laying is novel for birds and may be mediated by effects on maternal steroid hormones profile. The ecological and evolutionary implications of present findings are discussed in the light of adaptive sex allocation theory.