Secondary sex ratio covaries with demographic trends and ecological conditions in the barn swallow
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Parents are expected to invest more in the sex that benefits most from the local environment. When the quality of breeding sites varies spatially and natal dispersal of males and females differs, parents in high-quality habitats should skew their progeny sex ratio in favor of the less dispersing sex. We tested this prediction in the barn swallow (Hirundo rustica L.), by relating the proportion of male offspring around fledging (secondary sex ratio) of first and second broods to the ecological quality (presence of livestock farming and relative surface of hayfields in the foraging range) and local demographic trends of the farms where the colonies were located. Consistent with our predictions, the proportion of male offspring, which are more philopatric than females, increased with the extent of hayfields, which are high quality, preferred foraging habitats. Moreover, the proportion of male offspring in second broods was smaller in colonies with positive demographic trends, possibly indicating density-dependent effects on sex ratio. Independent of the mechanism generating uneven sex ratio (zygote sex ratio adjustment or sex-related pre-fledging mortality), barn swallows breeding under favorable conditions overproduced the sex that is more likely to benefit from such conditions.
KeywordsSex allocation Natal dispersal Demographic trend Habitat quality Julliard’s model
We are grateful to all farm owners that allowed us to perform this study in their cowsheds and houses. Paolo Bonvini, Matteo Bosetti, Valentina Mangoni, Davide Sala, Beatrice Sicurella, Enrico Steiner and Massimiliano Mori greatly helped during field works. We also thank the administration of the Parco Regionale Adda Sud for their logistic support. De Anna Estella Beasley kindly provided comments on a previous version of this manuscript, improving its readability.
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