Abstract
Molecular sexing revealed an unexpectedly strong female bias in the sex ratio of pre-breeding European Storm Petrels (Hydrobates pelagicus), attracted to playback of conspecific calls during their northwards migration past SW Europe. This bias was consistent across seven years, ranging from 80.8% to 89.7% female (mean annual sex ratio ± SD = 85.5% female ±4.1%). The sex ratio did not differ significantly from unity (i.e., 50% female) among (i) Storm Petrel chicks at a breeding colony in NW France, (ii) adults found dead on beaches in Southern Portugal, (iii) breeding birds attending nest burrows in the UK, captured by hand, and (iv) adults captured near a breeding colony in the UK using copies of the same sound recordings as used in Southern Europe, indicating that females are not inherently more strongly attracted to playback calls than males. A morphological discriminant function analysis failed to provide a good separation of the sexes, showing the importance of molecular sexing for this species. We found no sex difference in the seasonal or nocturnal timing of migration past Southern Europe, but there was a significant tendency for birds to be caught in sex-specific aggregations. The preponderance of females captured in Southern Europe suggests that the sexes may differ in migration route or in their colony-prospecting behaviour during migration, at sites far away from their natal colonies. Such differences in migration behaviour between males and females are poorly understood but have implications for the vulnerability of seabirds to pollution and environmental change at sea during the non-breeding season.
Highlights
Many species of bird exhibit marked differences between the sexes in aspects of their behaviour, including their foraging behaviour and migration strategies
Using molecular sexing from feathers and faeces, the aims of our study were: (i) To examine gender differences in extent and timing of northward migration into Europe of H. pelagicus; (ii) To investigate whether the sex ratios observed among samples of storm petrels caught on migration in Southern Europe are consistent with those obtained at other parts of the annual and life-history cycle; (iii) To examine whether there was evidence of gender grouping in the migratory passage of birds, as a probable indication of sexual segregation at sea [1]
Some differences in biometric measures were found between males and females H. pelagicus (Table S1), a morphological discriminant function analysis failed to provide satisfactory discrimination between the sexes
Summary
Many species of bird exhibit marked differences between the sexes in aspects of their behaviour, including their foraging behaviour and migration strategies These differences in foraging behaviour can potentially lead to differences in migration strategies with males and females migrating at different times, travelling by different migration routes, or travelling to and from different wintering grounds [1,2]. This can result in a complete segregation of sexes during migration, having a great impact on population dynamics of many bird species, including through differential mortality [3]. Within the Procellariiformes there are only three such studies; two of which were based on stable isotope signatures among various procellariid species [7,8] and the other used geolocator tracking devices to describe sex differences in migration routes of Balearic Shearwaters (Puffinus mauretanicus) [2]
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