Differences in perceived predation risk associated with variation in relative size of extra-pair and within-pair offspring

Extra-pair paternity (EPP) is a widespread phenomenon in birds. Researchers have long hypothesized that EPP must confer a fitness advantage to extra-pair offspring (EPO), but empirical support for this hypothesis is definitively mixed. This could be because genetic benefits of EPP only exist in a subset of environmental contexts to which a population is exposed. From 2013-2015, we manipulated perceived predator density in a population of tree swallows (Tachycineta bicolor) breeding in New York to see whether fitness outcomes of extra-pair and within-pair offspring (WPO) varied with predation risk. In nests that had been exposed to predators, EPO were larger, longer-winged, and heavier than WPO. In non-predator nests, WPO tended to be larger, longer-winged, and heavier than EPO, though the effect was non-significant. We found no differences in age, morphology, or stress physiology between extra-pair and within-pair sires from the same nest, suggesting that additive genetic benefits canno...