Foraging distribution of a tropical seabird supports Ashmole’s hypothesis of population regulation
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Many animals reproduce in large aggregations, which can vary in size from dozens to millions of individuals across species, time and space. The size of breeding colonies is a complex trade-off between multiple costs and benefits to an individual’s fitness, but the mechanisms by which colony size affects fitness are still poorly understood. One important cost of breeding in a large colony is the spatial constraint in resource use due to the need to regularly return to a central location. Large aggregations, like seabird breeding colonies, may therefore deplete food resources near the colony, forcing individuals to travel farther to find food, which may ultimately limit their reproductive output and population size. This hypothesis, proposed in 1963 by Ashmole for tropical oceanic islands, has so far not been tested at tropical seabird colonies, where food availability is less predictable than in colder waters. We compare the foraging distribution of a common tropical seabird, the masked booby Sula dactylatra, breeding on two islands in the South Atlantic that differ in the size of the breeding seabird community by 2 orders of magnitude, but are surrounded by similar oligotrophic waters. Foraging trips from the island with the smaller colony were on average 221 km (61 %) and 18.0 h (75 %) shorter because birds from the smaller colony rarely spent the night at sea and foraged on average 64 km (46 %) closer to the colony. Energy expenditure was significantly lower, and nest survival higher (47 vs. 37 %, n = 371) on the island with the smaller colony. These results are fully consistent with the predictions from Ashmole’s hypothesis and indicate that competition for food around tropical oceanic seabird colonies may indeed be a limiting factor for populations. Identifying important feeding areas for seabirds based on their foraging range may need to account for colony size of both the target and potential competitor species.
KeywordsAshmole’s halo Central-place forager Coloniality Interference competition Hidden Markov model Marine protected area Top predator
We appreciate the enthusiastic assistance and advice during data collection from Ian Fisher, Richard Hesketh, Dane Wade, Catherine Supple, Natasha Williams, Kenickie Andrews, Pete Mayhew, Annette Broderick and Brendan Godley, Judith Brown, Phil Lambdon, Shayla Ellick, Dave Higgins, the St. Helena National Trust, and staff in the Environmental Management Division of the Environmental and Natural Resources Directorate on St. Helena. The work on Ascension Island was partly funded by a Darwin Grant (# 19026) to Ascension Island Government and the University of Exeter (Annette Broderick and Brendan Godley), managed on-island by NW and SW. The work on St. Helena was partly funded by Enterprise St. Helena and the Seabird Group. Nigel Butcher and Andrew Asque assisted with preparation of loggers and equipment. Samantha Patrick, Phil Taylor and Mark Miller generously shared unpublished R code to analyse tracking data. Three anonymous reviewers, Henri Weimerskirch and Charles Brown provided helpful comments that improved the manuscript.
This research study was carried out under permission and with collaboration of the Ascension Island Government Conservation Department and the St. Helena Environmental Management Directorate. The capture and handling of birds and attachment of unconventional marks were carried out under licence from the British Trust for Ornithology.
Conflict of interest
The authors declare that they have no conflict of interest.
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