ORIGINAL PAPER Foraging distribution of a tropical seabird supports Ashmole’ s hypothesis of population regulation Steffen Oppel 1 & Annalea Beard 2 & Derren Fox 3 & Elizabeth Mackley 1 & Eliza Leat 3 & Leeann Henry 2 & Elizabeth Clingham 2 & Nathan Fowler 3 & Jolene Sim 3 & Julia Sommerfeld 4 & Nicola Weber 3 & Sam Weber 3 & Mark Bolton 1 Received: 20 November 2014 /Revised: 10 March 2015 /Accepted: 11 March 2015 # © Crown Copyright 2015 Abstract 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 hy- pothesis, 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 wa- ters. We compare the foraging distribution of a common trop- ical 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 hy- pothesis 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. Keywords Ashmole’ s halo . Central-place forager . Coloniality . Interference competition . Hidden Markov model . Marine protected area . Top predator Introduction The evolution of aggregated breeding systems, generally re- ferred to as colonies, is believed to have benefited from infor- mation exchange, cooperation, efficient defence against pred- ators and several other benefits (Wittenberger and Hunt 1985; Danchin and Wagner 1997; Brown and Brown 2001). Despite advantages to breeding in large aggregations, there are distinct costs such as competition for resources or disease transmis- sion, and the complex trade-off between costs and benefits leads to large variation in colony size across species, space and time (Wittenberger and Hunt 1985; Brown and Brown 2001). However, the mechanisms that determine the size of colonies are generally poorly understood (Brown and Brown 1996; Brown et al. 2013). Communicated by C. R. Brown Electronic supplementary material The online version of this article (doi:10.1007/s00265-015-1903-3) contains supplementary material, which is available to authorized users. * Steffen Oppel steffen.oppel@rspb.org.uk 1 RSPB Centre for Conservation Science, Royal Society for the Protection of Birds, The Lodge, Sandy, Bedfordshire SG19 2DL, UK 2 Environmental Management Division - Marine Section, St. Helena Government, Jamestown STHL 1ZZ, Saint Helena 3 Ascension Island Government Conservation Department, Georgetown ASCN 1ZZ, Ascension 4 Research and Technology Centre, University of Kiel, Büsum, Germany Behav Ecol Sociobiol DOI 10.1007/s00265-015-1903-3 Author's personal copy