Determinants of reproductive success in a neotropical migratory songbird: Timing, site-selection and mating strategies.

摘要

Identifying and understanding sources of individual variation in fecundity is of central importance to life history theory and population ecology. In this study, I first examined how factors on the breeding grounds in eastern Maryland affected the reproductive success of the Ovenbird ( Seiurus aurocapilla), a small Neotropical migrant warbler. Early breeding was favored by the increasing probability of predation for Ovenbird nests over time, but this effect was moderated by a countervailing pattern in the risk of brood parasitism by Brown-headed Cowbirds (Molothrus ater). Levels of extra-pair paternity (46% of broods, 28% of nestlings) were in the middle of values found for other warblers, but surprisingly high considering the Ovenbird's dull, sexually monochromatic plumage. Rates of non-monogamy in the social mating system were also relatively high, and included divorce within- and between-seasons, polygyny and, rarely, a type of polyandry where two males fed nestlings of a single brood. Differences among sites in the spring arrival dates of males, mating success, and measures of reproductive success were not consistent, suggesting that variation in forest patch size and edge proximity was not sufficient to produce effects like those found by most fragmentation studies. In contrast, nest-site selection had important fitness consequences. Females selected nest sites with dense local herbaceous vegetation, which afforded protection against nest predators and cowbirds. Although deep leaf litter contained more arthropods, Ovenbirds' main food source, females did not select nest sites with deep litter, and the relationship between nestling condition and litter depth was not uniformly positive. In the second part of this study, I investigated connections between the breeding season and preceding events. Surprisingly, the timing and body condition of males arriving to the breeding grounds had little effect on measures of reproductive performance. Stable-carbon isotope values in blood samples from males arriving at the breeding grounds did not suggest a link between winter habitat quality and spring arrival time or body condition. These finding suggest that the scope for individual-level carry-over effects from winter to summer is limited in Ovenbirds, and that the effects of breeding grounds events on individual fecundity are dominant.