Nutrient sources for forest birds captured within an undisturbed petrel colony, and management implications

Abstract

Where seabird breeding colonies occupy forested habitats, unusual nutrient links between marine and terrestrial ecosystems can occur. In such circumstances, the dietary characteristics of forest birds inhabiting pristine seabird colonies have rarely been investigated. In this study, carried out in a mixed-species colony of petrels (Procellariiformes), we measured the stable isotopic (δ¹³C, δ¹⁵N) signatures of individual feathers of New Zealand Bellbirds (Meliphagidae : Anthornis melanura melanura; n = 6) and Red-crowned Parakeets (Psittacidae : Cyanoramphus novaezelandiae novaezelandiae; n = 15). The δ¹⁵N of Parakeet feathers encompassed a wide range, from +7.1‰ to –0.9‰. From the high maximum δ¹⁵N, relative to global temperate forest foliage, we conclude that some Parakeets were feeding within the petrel colony where they were captured. However, the low minimum δ¹⁵N, relative to foliage from the petrel colony, implies that Parakeet isotopic composition could not be fully explained by potential dietary items from the petrel colony. Bellbird δ¹⁵N (range +3.4‰ to +9.6‰) was more enriched than that of Parakeets, consistent with their higher trophic level, but also consistent with a dietary intake consisting of items from both within and beyond the petrel colony. Bellbird isotopic ratios were strongly enriched in δ¹³C, which could be explained only by consumption of invertebrates with marine isotopic enrichment. The marine invertebrates could have been associated with breeding petrels, or have come from the littoral zone near the site of capture. The importance for both species of food sources both inside and outside of petrel colonies implies that petrel colonies offer foraging opportunities for forest birds but that the birds also use non-colony areas. Consequently, petrel colonies are integral parts of the wider terrestrial landscape which they inhabit.