How can a pelagic seabird provision its chick when relying on a distant food resource? Cyclic attendance at the colony, foraging decision and body condition in sooty shearwaters

Abstract

1. Sooty shearwaters are abundant in waters too distant from their colonies to account for the observed average frequency with which the chicks are fed. This, and the unexplained cyclic pattern of attendance at their colonies, have led to contradictory interpretations related to evolution of the extreme life histories of pelagic seabirds and their relationship with the marine environment. 2. Study of the provisioning behaviour of individual parent sooty shearwaters indicates that they can rely on productive distant waters, probably 1550. km away, to build up their body reserves and to provision their chick. These long foraging trips on average last 11. days and account for 84% of the foraging time. 3. Birds use this stored energy to cover the costs of performing several successive foraging trips of short duration in nearby less productive waters. These brief trips double the energy flow to the chick and take only 16% of the foraging time but are at the expense of adult body condition, i.e. there is a net energy loss. Foraging successively with short and long trips allows parents to increase by 20% the energy flow to the chick over what it would get if foraging was exclusively in distant waters. 4. The results suggest that maximal fitness in this pelagic seabird is not achieved by maximization of foraging efficiency but is the result of a trade‐off system whereby at some stages birds forage with net energy losses. The decision to feed close to or far from the colonies, i.e. to allocate to the chick or to store body reserves, is not related to any endogenous rhythm, nor to the nutritional status of the chick, nor to the length of the previous foraging trip. Rather, it is under the sole control of adult body condition, with the possible existence of a threshold body mass around 750 g. Thus, adult body mass plays a central role in foraging decisions, linking foraging and allocation. 5. The synchronized return of foraging birds every 2. weeks is not due to environmental factors. By modelling attendance patterns with an increasing variance in the foraging routine, it is shown that the periodicity of feeding times is retained throughout the chick rearing period, with a cyclicity of 14. days. This result indicates that cyclic attendance at the level of the whole colony is an emergent property of the two‐fold foraging strategy of individual adults. 6. One main consequence of this system appears to be a reduction in competition close to the breeding grounds and this may help explain the existence of huge populations of sooty and short‐tailed shearwaters that rely on distant food resources.