Complex interactions between predation risk, stress physiology, and extra-pair paternity in a free-living songbird
Extra-pair mating is a common reproductive strategy among socially monogamous birds, but its relevant costs and benefits, especially to females, remain elusive. Many studies have tested the hypothesis that extra-pair offspring (EPO) are superior in quality to their maternal half-siblings (within-pair offspring; WPO), but few have examined whether this relationship varies as a function of environment. In this study, we explore how increased exposure to perceived predation risk influences mating decisions and patterns of reproductive investment in Tree Swallows (Tachycineta bicolor). Using a series of stationary mounts, we manipulated predator density and disturbance in a local population of swallows during the summers of 2013 and 2014. Microsatellites were used to genotype individuals, and growth (nestlings) and plasma corticosterone levels (adults and nestlings) were assayed. In so doing, we sought to evaluate three hypotheses for how increased exposure to predators might influence mating decisions or outcomes: (1) through a strategic shift in extra-pair behavior that favors nestlings better equipped to survive under high threat of predation; (2) through changes in stress physiology that modulate the propensity to seek extra-pair mates or invest differentially in EPO; or (3) through changes in reproductive investment that affect relative success of EPO vs WPO.
Extra-pair rate (measured as the proportion of EPO within a brood of nestlings) was unaffected by repeated exposure to predator mounts. Similarly, we observed only weak correlations between predator exposure and adult and nestling plasma corticosterone. However, there were strong interactions between predator exposure and extra-pair status on nestling growth, and these varied across the two years of our study. In 2013, EPO from predator-exposed boxes were significantly larger and heavier than EPO from control boxes, or WPO from either treatment. In 2014, WPO from control boxes were significantly larger and heavier than any other class of nestlings. Future work should strive to elucidate the mechanism underlying these patterns, especially in light of the annual variation we observed. Additionally, it will be important to understand whether and how environmentally-mediated variation in nestling growth between EPO and WPO might scale up to influence demographic processes such as survival and recruitment. Extra-pair mating is often treated as a static property of a population, but our results suggest that it is a dynamic and nuanced series of decisions driven by a complex web of ecological interactions.