Agricultural intensification exacerbates female-biased primary brood sex-ratio in tree swallows

Impacts of agriculture practices are documented at every ecosystem level from landscape structure to biodiversity. Birds are especially affected by agricultural modifications as shown by the decline of farmland species in Europe and North America. Few studies have assessed the effects of such modifications on individual characteristics directly influencing population dynamics. Several bird studies showed that sex-ratio may be adaptive and that mother condition affects the production of sons and daughters. However, little is known about the connections between environmental and individual characteristics on sex allocation. Here we quantified the variation in primary sex-ratio in tree swallows (Tachycineta bicolor) nesting in contrasted environments associated with agricultural intensification in southern Qu,bec, Canada. We found that intensive agricultural practices affected female sex-ratio allocation in this area, resulting in more biased sex-ratio towards daughters throughout most of the hatching period. Yet, this bias towards daughters was reduced as the season progressed in the most intensively cultivated areas, suggesting that tree swallows have problems foreseeing the difficult growth and postfledging conditions that their nestlings will experience in such environments. Our results thereby support the hypothesis that intensive agricultural areas act as an ecological trap in our study system. We also found that effects of agricultural intensification on sex allocation differed among years and affected the relationships between sex-ratio allocation and hatching date. Our results suggest that agricultural intensification modifies female sex allocation in tree swallows, but the importance of the effects might vary among years and depend on timing of breeding.