Body size is associated with yearling breeding and extra-pair mating in the Island Scrub-Jay

Large body size is an important determinant of individual fitness in many animal species, especially in island systems where habitat saturation may result in strong intraspecific competition for mates and breeding territories. Here we show that large body size is associated with benefits to yearling breeding and extra-pair mating in the Island Scrub-Jay (Aphelocoma insularis), endemic to Santa Cruz Island, California. This species is ∼20% larger than its mainland congener, consistent with the island syndrome, indicating that body size may be a trait under selection. From 2009 to 2013, we quantified the reproductive success of a marked population of Island Scrub-Jays, tracked which yearlings acquired a breeding territory and bred, and measured the occurrence of extra-pair paternity. Two potential contributors to fitness were positively related to body size. Larger yearling males were more likely to breed, possibly due to greater behavioral dominance during aggressive encounters. Larger males were also less likely to lose paternity to extra-pair males and, anecdotally, extra-pair males were larger than the social male cuckolded. This study provides evidence that larger males may have a fitness advantage over smaller males by breeding earlier and avoiding paternity loss, but estimates of lifetime reproductive success are ultimately needed for Island Scrub-Jays and other long-lived species. LAY SUMMARY The body size of an animal can influence its ability to pass on its genes. In some species larger individuals may be better able to secure a place to breed, and larger males may father a greater proportion of offspring in their nest. We examined whether body size is related to territory acquisition, breeding, and offspring parentage in the Island Scrub-Jay, a species that is restricted to Santa Cruz Island, California, and is larger than its sibling species on the nearby mainland. Yearling jays that were larger in body size were more likely to acquire a territory and breed, and larger males of any age were less likely to lose paternity to extra-pair matings. These results suggest that larger Island Scrub-Jays have greater reproductive success.