Partitioning the sources of demographic variation reveals density‐dependent nest predation in an island bird population

Abstract

Ecological factors often shape demography through multiple mechanisms, making it difficult to identify the sources of demographic variation. In particular, conspecific density can influence both the strength of competition and the predation rate, but density-dependent competition has received more attention, particularly among terrestrial vertebrates and in island populations. A better understanding of how both competition and predation contribute to density-dependent variation in fecundity can be gained by partitioning the effects of density on offspring number from its effects on reproductive failure, while also evaluating how biotic and abiotic factors jointly shape demography. We examined the effects of population density and precipitation on fecundity, nest survival, and adult survival in an insular population of orange-crowned warblers (Oreothlypis celata) that breeds at high densities and exhibits a suite of traits suggesting strong intraspecific competition. Breeding density had a negative influence on fecundity, but it acted by increasing the probability of reproductive failure through nest predation, rather than through competition, which was predicted to reduce the number of offspring produced by successful individuals. Our results demonstrate that density-dependent nest predation can underlie the relationship between population density and fecundity even in a high-density, insular population where intraspecific competition should be strong.

Statistical modeling of patterns in annual reproductive rates

“…For models with the ZIP distribution, AIC c gave the same ranking and almost identical AIC c values as in the original publication of this data (Sofaer et al. ) (Appendix : Table S2). However, models with the ZICMP and ZIGP distribution had a different ranking and lower AIC c values than those with the ZIP distribution (Table ).…”

“…The ZIP model from the original publication of this dataset (Sofaer et al. ,b) had underdispersed residuals ( P = 0.003). The most parsimonious model of warbler fecundity had a ZICMP distribution, which was 4.9 ΔAIC c units better than the most parsimonious ZIGP and 34.1 ΔAIC c units better than the most parsimonious ZIP model (Table ).…”

“…We obtained data from the Dryad Digital Repository on the fecundity of a population of Orange‐crowned Warblers nesting on Santa Catalina Island, California, USA (Sofaer et al. ,b). During 7 yr of observation ( n = 181), fledglings per nest ranged from 0 to 6 with a median of 0 and a mean of 1.4.…”

Offspring growth and mobility in response to variation in parental care: a comparison between populations

“…During the breeding season, the sordida subspecies is largely endemic to California's Channel Islands, so the selective environments underlying population divergence encompass both latitudinal and insular effects, among others. For example, our study population on Santa Catalina Island breeds at high densities (Sofaer et al 2014) and exhibits a suite of traits associated with strong competition, including high testosterone and male aggression relative to the Alaska population (Horton et al 2010, Yoon et al 2012. We studied variation in parental care and its consequences for nestlings, evaluating the assumption that growth should reflect variation in per-offspring provisioning (Lack 1947), and the alternative that populations differ in nestlings' allocation to growth.…”

Temporal variation in the effects of individual and environmental factors on nest success

“…Influences level of male parental care, food availability, and likelihood of predation/parasitism Arcese and Smith 1988, Wingfield et al 1990, Arcese et al 1992, Sofaer et al 2014 Nest success rates decrease with increasing breeding population densities Local brood parasitism rate…”