Resource partitioning between incubating and chick-rearing brown boobies and red-tailed tropicbirds on Christmas Island

Joan Navarro,Lena Braun,Janos C Hennicke,Carola Sanpera,Rocio Moreno

doi:10.1186/s40555-014-0027-1

Abstract

Background: In oligotrophic tropical marine environments, the main mechanism explaining the coexistence of sympatric seabirds is segregation by habitat or segregation by prey within the same habitat. Both types of segregation can play a role during the breeding season due to different constraints associated with different phases of the breeding cycle. By using stable isotope analyses, we investigated intra- and interspecific foraging segregation in two tropical seabird species, the red-tailed tropicbird Phaeton rubricauda and the brown booby Sula leucogaster, breeding sympatrically on Christmas Island, Indian Ocean. We compared isotopic values of δ 13 C and δ 15 N in blood from incubating and chick-rearing adults of both species. Results: The results showed small but significantly interspecific and intraspecific differences in δ 13 C and δ 15 N values. Differences in δ 13 C values suggest spatial segregation in the main foraging grounds between the two species during the breeding season as well as between incubating and chick-rearing brown boobies. In contrast, red-tailed tropicbirds probably exploited similar foraging habitats during both breeding stages. δ 15 N values did not indicate diet-related differences, neither within nor between species, suggesting a highly opportunistic feeding behavior to cope with the limited prey available in the oligotrophic marine environment. Conclusions: Competition for prey in breeding red-tailed tropicbirds and brown boobies seems to be reduced by spatial segregation enabling both species to successfully reproduce in sympatry in an oligotrophic tropical marine environment.

Highlights

In oligotrophic tropical marine environments, the main mechanism explaining the coexistence of sympatric seabirds is segregation by habitat or segregation by prey within the same habitat
The principle underlying this approach is that stable isotope ratios of nitrogen (δ15N) and carbon (δ13C) in predators reflect those of their prey species in a predictable manner (Karnovsky et al 2012). δ15N values show a stepwise enrichment between 3‰ to 5‰ with each trophic level and are reliable indicators of the consumer's trophic position (Inger and Bearhop 2008). δ13C values indicate consumer foraging areas discriminating between inshore/benthic and offshore/pelagic feeding (Forero and Hobson 2003; Inger and Bearhop 2008)
We aimed to investigate the main mechanisms that may explain the coexistence of two abundant sympatric tropical seabird species, the red-tailed tropicbird Phaethon rubricauda and the brown booby Sula leucogaster on Christmas Island (Indian Ocean; Figure 1)

Summary

Introduction

In oligotrophic tropical marine environments, the main mechanism explaining the coexistence of sympatric seabirds is segregation by habitat or segregation by prey within the same habitat. Differences in δ13C values suggest spatial segregation in the main foraging grounds between the two species during the breeding season as well as between incubating and chick-rearing brown boobies. The analysis of stable isotopes has been shown to be an effective technique to investigate the trophic structure of marine food webs and resource allocation of sympatric seabirds (Cherel et al 2008, Kojadinovic et al 2008; Moreno et al 2013). The principle underlying this approach is that stable isotope ratios of nitrogen (δ15N) and carbon (δ13C) in predators reflect those of their prey species in a predictable manner (Karnovsky et al 2012). The principle underlying this approach is that stable isotope ratios of nitrogen (δ15N) and carbon (δ13C) in predators reflect those of their prey species in a predictable manner (Karnovsky et al 2012). δ15N values show a stepwise enrichment between 3‰ to 5‰ with each trophic level and are reliable indicators of the consumer's trophic position (Inger and Bearhop 2008). δ13C values indicate consumer foraging areas discriminating between inshore/benthic and offshore/pelagic feeding (Forero and Hobson 2003; Inger and Bearhop 2008)

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Conclusion