Volume 48, No. 1

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Inter-colony and interspecific differences in the isotopic niche of two sympatric gull species in Newfoundland




MAYNARD, L.D. & DAVOREN, G.K. 2020. Inter-colony and interspecific differences in the isotopic niche of two sympatric gull species in Newfoundland. Marine Ornithology 48: 103 - 109

Received 14 June 2019, accepted 25 December 2019

Date Published: 2020/04/15
Date Online: 2019/03/30
Key words: stable isotopes, carbon, nitrogen, gulls, Larus, diet, isotopic niche


Large gulls are omnivorous predators that are typically associated with coastal environments, but gull colonies vary in their proximity to the coast. Because the diet of central-place foragers is often dictated by resource availability within range of the central place, we investigated inter-colony and interspecific differences in the diet of Great Black-backed Gulls Larus marinus and Herring Gulls L. argentatus at multiple inshore colonies (< 20 km) and one offshore colony (> 60 km; Funk Island) on the northeast Newfoundland coast, Canada. Values of δ15N and δ13C in whole blood of gull chicks and adults (incubating) were used to compare isotopic niche breadth (standard ellipse area) and trophic level (δ15N) between species (adults and chicks) and colonies (chicks only). Herring Gull chicks had higher δ15N at the offshore colony relative to inshore colonies, indicating that these chicks were provisioned with higher trophic level resources (e.g., seabird eggs/chicks) compared to lower trophic level resources (e.g., benthic invertebrates) at inshore colonies. Great Black-backed Gull chicks had higher δ15N than Herring Gull chicks at all colonies, indicating they were consistently provisioned with higher trophic level resources. Isotopic niche breadth was broader for Great Black-backed Gull chicks raised inshore relative to offshore, indicating a wider variety of resources provisioned at inshore colonies relative to the offshore colony. Incubating adult Great Black-backed Gulls incorporated higher trophic level prey and had a narrower isotopic niche breadth than incubating Herring Gulls at the same inshore colony, indicating that they have a more specialized diet of higher trophic level than Herring Gulls. We suggest, based on our results, that the differential availability of food resources proximal to colonies influences the diet of these two sympatric gull species, thereby informing region-specific gull management 


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