Correlations between population density, natural selection and phenotypic change are widespread and may comprise an eco-evolutionary feedback loop, yet we still know very little about the causes of connections between them. Isolating the mechanistic links in eco-evolutionary feedback loops, both in terms of identifying sources of variation in traits and in terms of determining how and why natural selection varies with population density, can provide key insight into avian population dynamics. Here, we summarize more than a decade of findings in western bluebirds (Sialia mexicana) to illustrate the multiple and potentially interacting mechanisms that can cause simultaneous changes in traits and population density. In previous work, we discovered correlated changes in aggression, population density and natural selection during the process of colonization. Here we provide evidence that density-dependent selection, maternal effects and demographic consequences of selection on aggression may all play a role in driving feedback between phenotypic change and population density. Thus, this system provides an example of the multiple mechanistic links that can produce such feedback loops and emphasizes the importance of investigating alternative hypotheses for correlated patterns of ecological and phenotypic change even when there is strong evidence that natural selection is acting on a trait. Ultimately, identifying these mechanisms is crucial, as eco-evolutionary feedback have the potential to explain avian population cycles, range dynamics, population persistence, and even patterns of species coexistence.
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We would like to thank Niels Dingemanse and Pim Edelaar for inviting us to participate in their symposium and an anonymous reviewer for comments on an earlier version of this manuscript. Members of Mountain Bluebird Trails and residents of the Hayes Creek neighborhood kindly allowed us to monitor nests on their properties. This work was supported by grants from the US National Science Foundation (DEB-0918095 and DEB-1350107 to RAD) and the US National Science Foundation Graduate Research Fellowship Program (DGE 1143953 to SMA).
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