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

• Published in: Ecology and evolution 2014-07, Vol.4 (13), p.2738-2748
• Main Authors: Sofaer, Helen R., Sillett, T. Scott, Langin, Kathryn M., Morrison, Scott A., Ghalambor, Cameron K.
• Format: Article
• Language: English
• Subjects: Abiotic factors; Animal behavior; Animal breeding; Bird populations; Competition; Demographics; Demography; density dependence; Fecundity; island syndrome; nest predation; Offspring; Original Research; Partitioning; Population density; Predation; Reproductive failure; Studies; Survival; Terrestrial environments; Variation; Vertebrates; zero‐inflated model
• ISSN: 2045-7758; 2045-7758
• DOI: 10.1002/ece3.1127

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. Population density can shape demographic rates by affecting both competition and predation, but these effects are rarely partitioned. We found that rainfall and density‐dependent nest predation had stronger effects on fecundity than intraspecific competition in an island songbird population where we expected competition to be strong. Our results challenge assumptions about the drivers of demographic variation, while our analytical approach highlights methods that separate the processes affecting reproductive failure from those affecting the number of young produced by successful individuals.