Density-dependent factors (e.g. predation, disease and parasites, and competition) are key processes regulating species population sizes. Specifically, competition for limiting resources drives community structure and assemblage. The partitioning of limited resources based on behavioral adaptations or phenotype can lead to niche specialization, enabling the coexistence of closely related species. Seabirds often aggregate in high densities to form mixed-species nesting colonies on remote islands. New Zealand supports the greatest number of endemic bird species globally, but multiple invasive mammal species have been introduced to a majority of New Zealand’s islands, causing the extinction of 41% of New Zealand’s birds and the abandonment of many seabird colonies. Invasive mammal eradication, initiated in the early 1900s, has led to >100 islands that are in some form of recovery, and reassembly of seabird colonies offers an ideal study system to examine the early stages of competition and its influence on community assemblages during island recovery. We monitored seabird nests of five different species at an island that had been cleared of invasive mammalian predators and has been undergoing community reassembly. Here, we use stable isotopes to identify the degree of dietary specialization and relate dietary specialization to chick growth and survival.
Results/Conclusions
For our known-age chicks (n = 29), we found that age was most strongly related to the index of wing length for all of our study species. Predictions of age generated from the top AICc-ranked generalized linear model were similar to that of observed ages (ρ = 0.95). We then used this model to assign age estimates to chicks of unknown ages (n = 223), generate growth curves, and quantify growth rates of age and mass, tarsus, and culmen lengths. Chick diets identified from regurgitated prey items consisted largely of cephalopods and some crustaceans, with larger species appearing to specialize on larger prey items. Using stable isotopes, we relate δ13C values (with low values signifying lipid-rich prey items) and δ15N (signifying trophic level) to chick growth rates. Between species, diets and growth rates varied but became less variable when compared within a single species. While evolutionary history separated our study species into distinct foraging niches, our results suggest there is low intraspecific competition between breeding seabirds. Our results highlight the utility of stable isotopes in tracking competition within and between breeding populations during community reassembly.