Patterns of Fluctuating Selection on Morphological and Reproductive Traits in Female Tree Swallow (Tachycineta bicolor)
Temporally replicated studies are essential to describe and understand selection in natural populations. Selection patterns can differ among life stages representing different fitness components. Despite the increasing number of long-term studies, yearly estimates of fluctuation in strength and direction are mostly available from studies conducted on a limited number of years. Based on a population of Tree swallows (Tachycineta bicolor) monitored over 10,200 km2 in Southern Québec, Canada, since 2004, we investigated how patterns of selection may change across breeding stages by dividing the overall selection at the nesting stage (number of fledglings produced) into hatchling (number of hatchlings produced) and fledgling (number of hatchlings having successfully fledged) selection stages. We assessed fluctuation in selection gradients on two morphological (body mass and wing length) and two reproductive (laying date and clutch size) traits in females. We found significant positive selection gradients for body mass and clutch size and negative selection gradients for laying date, though the latter only during the fledgling selection stage. We also found that selection gradients on reproductive traits significantly fluctuated in direction and/or strength among years but only during the hatchling breeding stage. Our results thus emphasize the need to consider how selection events may be fluctuating in time and among breeding stages and the importance of these patterns for the maintenance of phenotypic variation in wild populations.
KeywordsSelection gradients Fluctuating selection Laying date Clutch size Body mass Wing length Birds
We would like to thank all the students and field assistants who helped gathering the data over the years, as well as the 40 farms owners for providing access to their land. We thank Cédric Frenette Dussault and three anonymous reviewers for comments on a previous draft version. This work was funded by grants from the Fonds de Recherche du Québec—Nature et Technologies (FRQNT) (D. G., F. P., M. B.), by Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery grants (D. G., F. P., M. B.) and by the Canada Research Chairs program (F.P., M. B.).
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