title: 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.
authors: Graduate Degree Program in Ecology and Biology Department, Colorado State University 1878 Campus Delivery, Fort Collins, Colorado, 80523.
authors: Helen R
authors: HR
authors: Sofaer
authors: Migratory Bird Center, Smithsonian Conservation Biology Institute, National Zoological Park MRC 5503, Washington, District of Columbia, 20013-7012.
authors: T Scott
authors: TS
authors: Sillett
authors: Graduate Degree Program in Ecology and Biology Department, Colorado State University 1878 Campus Delivery, Fort Collins, Colorado, 80523.
authors: Kathryn M
authors: KM
authors: Langin
authors: The Nature Conservancy 201 Mission St, 4th Floor, San Francisco, California, 94105.
authors: Scott A
authors: SA
authors: Morrison
authors: Graduate Degree Program in Ecology and Biology Department, Colorado State University 1878 Campus Delivery, Fort Collins, Colorado, 80523.
authors: Cameron K
authors: CK
authors: Ghalambor
keywords: Demography; density dependence; fecundity; island syndrome; nest predation; zero-inflated model