Carry-Over Effects and Plumage Polymorphism in Swainson's Hawks
AuthorBriggs, Christopher William
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The maintenance of genetic diversity in the face of forces such as genetic drift and natural selection has intrigued scientists for decades. Such processes should seemingly oppose diversity in a stable environment. However, environments are rarely stable in natural systems and processes can be complex. We investigate how a plumage polymorphism is maintained in a population of Swainson's Hawks (<italic>Buteo swainsoni</italic>) in northern California, USA. Swainson's Hawks are highly polymorphic in the belly, flanks and underwing coverts ranging from white to dark brown and seemingly everything in between. Generally, these morphs can be grouped into 3 general classes; light, intermediate and dark. We first examined potential fitness differences among morph classes. Specifically we examined two primary hypotheses of heterosis (i.e., heterozygote advantage) and apostatic (i.e., frequency dependant) selection. Both mechanisms have been cited before in predator populations as potential mechanisms to maintain a polymorphism. However, we found no evidence of differences in any fitness parameter between the morph classes including; nest success, nest productivity, recruitment of offspring or lifetime reproductive success. There was marginal evidence of differences in survival between morph classes with dark individuals having a slightly higher adult survival compared to intermediate and dark morph. We also examined sexual selection in this population. Individuals did not appear to mate assortatively (i.e. there was no preference for a mate based on one's own mate class). For females, there was similarly no evidence for imprinting. In contrast, males chose mates that consistently matched the maternal morph. Further, males selected mates more consistently than we expect by chance. Finally, males that were not able to select mates with the same morph class as their mother had a lower lifetime reproductive success. This result indicates that these males may not have invested as heavily in reproduction, or were lower quality males that could not attract a mate of the correct morph. This sexual selection of the males may aid in the maintenance of the polymorphism over time.We also examined carry-over effects, processes and events in one season that affect an individual or population in another. Carry-over effects are garnering greater attention in studies of migratory species. Part of this newfound interest stems from tools and techniques allowing researchers to follow individuals or glean greater insights about foraging locations in different times of the year. We used feathers as indices of body condition from both the wintering grounds in Argentina and the breeding area. Specifically, we used the average daily growth of the feather, the level of corticosterone in the feather, and number of fault bars in the retrices and remiges of breeding hawks. We used deuterium levels in feathers to determine where each feather was grown to ensure that indices of condition came from a known location. Average daily growth was correlated with mass adjusted for body size, but corticosterone in feathers and fault bars were not. For males, average daily growth of feathers grown in Argentina and number of fault bars on worn feathers were correlated with nest success demonstrating carry-over effects and that individual in good body condition on their wintering grounds had higher reproductive performance. In contrast, measures of feather condition in females were not correlated with nest success. Corticosterone in feathers was higher in breeding males, but not in females. As males are the primary providers of prey for both the female and young nestlings their condition may drive the ability of a nesting attempt to succeed because an individual in poor condition may not be able to effectively provision the female or offspring.