Differential Migration Timing and the Form and Function of Avian Wings
Abstract
A common differential migration strategy in birds is protandry, whereby males arrive earlier than females. The probable causes of protandry are well studied from the perspective of innate and physical environmental cues, but the influence of the social environment and wing morphology are less known. Theoretical models propose that sex ratio influences protandry; male-biased sex ratios are predicted to advance timing in males due to increased intrasexual competition. To empirically test this, I investigated the spring migration traits of male Yellow-rumped Warblers Setophaga coronata under differing sex ratios. An integrated automated telemetry approach was used, where locomotor movement of captive birds was quantified, followed by the use of Motus Wildlife Tracking System to quantify stopover departure timing post release. Males from the female-biased environment exhibited more locomotor movement and had an earlier onset of migratory restlessness, a proxy for the urge to migrate, suggesting that the composition of the social environment can influence migration behaviour. Next, I assessed the relationship between wing morphology and both differential arrival timing and flight performance in migratory passerines, as pointed wings are theorized to aid level-flight efficiency and impede take-off performance. A long-term migration monitoring dataset revealed that protandry and sexual size dimorphism of wing length co-vary by age, which is believed to reflect a coevolutionary response to sexual selection and viability selection. Larger males may be better able to afford the viability costs of early arrival and younger males may reduce competition with older males by arriving later. Wingtip morphology was characterized using the feather length measurements of 1929 individuals from 18 families and a flight tower was used to determine whether these morphologies influence take-off flight performance. Males, with larger and more convex wingtips, exhibited faster take-off speeds than females. Although older birds had more pointed wingtips than younger birds, this trait did not impact take-off speed. Overall, these findings suggest that males and older birds have competitive advantages and are likely better at escaping predation. Understanding the probable causes of differential migration provides insights on the basic knowledge of avian migration and helps to predict the consequences of future climate change.