Doctor of Philosophy
Morbey, Yolanda E.
Guglielmo, Christopher G.
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.
Summary for Lay Audience
Birds often exhibit different migration timing patterns, such as males and older birds arriving at breeding sites before females and younger birds. Evidence suggests that these differences can be caused by internal cues and factors from the physical environment. However, less is known about whether wing morphology and the social environment can influence migration timing and associated traits. Regarding the social environment, theories predict that spring migration timing may advance because of increased local levels of male-male competition. To test this theory, I studied the spring migration traits of male Yellow-rumped Warblers housed with differing sex ratios and used radio telemetry to track their locomotor movements during spring migration. Males living with more females exhibited more movement and had an earlier urge to migrate in the spring, suggesting that the composition of the social environment can indeed influence migration. As for wing morphology, wing size and shape could influence migration timing and overall flight performance because long, pointed wings are expected to benefit level-flight, such as migratory flights, whereas rounded wings are expected to increase take-off performance, which could benefit birds in male-male competition. To test whether wing morphology relates to spring migration timing and take-off flight I analyzed a long-term migration dataset and conducted field work over three migration seasons to quantify sex and age differences in wing morphology. I found that the difference between male and female arrival timing of older and younger birds was mirrored by differences in wing length; older males commonly had the longest wings and earliest arrival timing, followed by younger males, older females, and lastly, younger females. This supports the theory that larger, older males can better withstand arriving early in unfavourable weather conditions and that smaller, younger males can reduce competition with older males by arriving later. Males with larger, more convex wingtips had faster take-off speeds, while older birds, with more pointed wingtips, had similar take-off speeds as younger birds. My findings suggest that males and older birds are likely better competitors. Overall, these findings are important for understanding migration and for predicting the consequences of future environmental change.
Deakin, Jessica E., "Differential Migration Timing and the Form and Function of Avian Wings" (2023). Electronic Thesis and Dissertation Repository. 9779.
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