Foraging ecology, fluctuating food availability and energetics of wintering brent geese E. Tinkler, W. I. Montgomery & R. W. Elwood School of Biological Sciences, Ecology and Evolutionary Biology, Medical Biology Centre, Queen’s University, Belfast, UK Keywords foraging; energetics; brent geese. Correspondence R. W. Elwood, School of Biological Sciences, Ecology and Evolutionary Biology, Medical Biology Centre, Queen’s University, Belfast, UK. Email: r.elwood@qub.ac.uk Editor: Andrew Kitchener Received 12 April 2008; revised 25 February 2009; accepted 9 March 2009 doi:10.1111/j.1469-7998.2009.00578.x Abstract Changing energy requirements and dramatic shifts in food availability are major factors driving behaviour and distribution of herbivores. We investigate this in wintering East Canadian High Arctic light-bellied brent geese Branta bernicla hrota in Northern Ireland. They followed a sequential pattern of habitat use, feeding on intertidal Zostera spp. in autumn and early winter before moving to predominantly saltmarsh and farmland in late winter and early spring. Night-time feeding occurred throughout and made a considerable contribution to the birds’ daily energy budget, at times accounting for 450% of energy intake. Nocturnal feeding, however, is limited to the intertidal, possibly because of predation risk on terrestrial habitat, and increases with moonlight. The amount of Zostera spp., declined dramatically after the arrival of birds, predominantly, but not entirely, due to consumption by the birds. Birds gained fat reserves in the first 2 months but then this was dramatically lost as their major food source collapsed and their daily energy intake declined. Single birds consistently fared worse than paired birds and pairs with juveniles fared better than those without suggesting a benefit of having a family to compete for food. Many birds leave the Lough at this time of reduced Zostera spp. for other sea inlets in Ireland but some remain. Body condition of the latter gradually improved in early spring and reflected a heavy reliance on terrestrial habitats, particularly farmland, to meet the birds’ daily energy require- ments. However, even in the period immediately before migration to the breeding ground, the birds did not regain the amount of abdominal fatness observed in November. The dramatic changes in available food and requirements of the birds drive the major changes seen in foraging behaviour as the birds evade starvation in the wintering period. Introduction Food availability plays a central role in determining the survival, reproductive success and movements of animals (Drent & Prins, 1987; Ganter, Prokosch & Ebbinge, 1997; Percival & Evans, 1997; Clausen et al., 1998; Rowcliffe et al., 2001; Nolet et al., 2002). Generally, the distribution and abundance of animals is closely related to the distribution and abundance of their food supply, particularly during the non-breeding season (Percival & Evans, 1997). For migra- tory species, such as arctic breeding birds, site selection during the non-breeding season must allow for the replen- ishment of energy reserves and accumulation of adequate energy stores to reproduce on the return to the breeding grounds (Meijer & Drent, 1999; Prop, Black & Shimmings, 2003). The nutritional condition of birds leaving the winter- ing site is a key determinant of fitness (Ankney & Macinnes, 1978; Drent & Daan, 1980; Ebbinge & Spaans, 1995). A major problem, however, is that depletion of resources may result in birds being unable to meet their energy require- ments over extended periods (Owen, Wells & Black, 1992; Pettifor et al., 2000). In general, the distribution of animals can be viewed in terms of the ‘ideal free’ distribution (Fretwell, 1972) that assumes all animals are equal, free to go where they will do best and are ‘ideal’ in having complete information about available resources. Thus food quantity and depletion are critical in determining the movements in winter between food patches within and between habitats (Vickery et al., 1995; Percival, Sutherland & Evans, 1998; Rowcliffe et al., 2001). Foraging animals, however, are unlikely to distribute themselves solely on the basis of food quality and avail- ability. Plants high in a specific nutrient such as nitrogen and/or digestibility can affect food selection (Boudewijn, 1984; Prop & Vulink, 1992; McKay, Bishop & Ennis, 1994; Vickery et al., 1995; Hassall & Lane, 2005), as can the efficiency of overall energy gain from the food items (Madsen, 1988; Mathers, Montgomery & Portig, 1998b). The selection of a habitat may also be influenced by factors not directly related to food supply such as predation risk, Journal of Zoology Journal of Zoology 278 (2009) 313–323 c  2009 The Authors. Journal compilation c  2009 The Zoological Society of London 313 Journal of Zoology. Print ISSN 0952-8369