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Résumé :
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In many migratory songbirds, males arrive earlier at stopover sites and at the breeding grounds (‘protandry’) and older birds precede younger ones, but less is known about age differences in protandry. In seasonal environments, differential timing by sex and age is thought to reflect selection imposed by seasonality (i.e., viability selection) and intrasexual competition for space and mates. Viability and sexual selection can also favor male-biased sexual size dimorphism (SSD), leading to a positive relationship between protandry and SSD among species. We evaluated whether the relationship between protandry during spring migration and SSD in wing length (SSDW) differed between age classes in 20 sexually dimorphic songbirds. Consistent with the hypothesis that older, higher-quality males can better afford to arrive early, we found a stronger relationship between protandry and SSDW among older birds than among first-time breeders. We also tested the relationship between protandry and sexual size dimorphism of the wingtip (SSDWT) and found that greater age differences in protandry were related to greater age differences in SSDWT. We conclude that larger body sizes, energetically efficient wing shapes, and experience together select for the earlier arrival timing of older males relative to first-time breeding males and females.
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