Temporal variation in the traits of individuals and the extrinsic environment that influence nest success in an island songbird (2016)
Nest success is a key factor affecting the dynamics and life–history evolution of avian populations. While multiple factors affect nest success, their relative influence remains unclear, partly due to the short time periods over which many studies take place relative to the scales of temporal variation in the environment, and the traits of birds that make up populations. I examined the effects of two intrinsic (female age, inbreeding coefficient), two abiotic (rainfall, temperature), and three biotic (breeding densities, cowbird parasitism rates, and brood parasitism) factors potentially affecting nest success (≥ 1 fledged young) in an insular song sparrow (Melospiza melodia) population over 39 years. I also compared the influence of these factors in three 13–year intervals representing the early, mid and late periods of the study to estimate temporal influence. Over 39 years, song sparrow nesting success first increased (1 – 3 years), and then decreased with female age (3+ years), and declined in relation to the degree of inbreeding in females, increased rainfall during the nest period, and by increased breeding densities. Nests that were parasitized by brown–headed cowbirds, or that experienced increased risk of cowbird predation, tended to fail more often. Parallel analyses of nest success in the early, mid and late periods of the study showed that only female age and breeding densities explained success in all periods, whereas the effects of inbreeding, cowbirds, and rainfall were episodic through time. This discrepancy was due to temporal variation in abiotic or biotic conditions that affected which factors were most influential of success. Many studies of nest success in passerine birds are limited in duration and the number of variables that can be considered due to limits on the amount or quality of data, preventing the comparison of many biotic and abiotic factors reported to affect success in the literature. I show that over 39 years, the intrinsic effects of inbreeding, abiotic effects of climate and biotic effects of brood parasites on nest success were each influential but varied through time, indicating that any ranking of their relative influence on demography will also vary temporally.