Evans P.R. (1976) Energy balance and optimal foraging strategies in shorebirds: some implications for their distributions and movements in the non-breeding season. ARDEA 64 (3-4): 117-139

geen Nederlandse samenvatting beschikbaar op internet In an attempt to understand the geographical distribution and movements of shorebirds in the non-breeding season, circumstances are reviewed which make it difficult for a bird to obtain sufficient food to balance its daily energy requirements. The survey considers first the factors which lead to increased energy requirements, reduced hunting effectiveness of the bird, reduced prey availability and reduced time for feeding. The effects of wind, tide, daylenght and temperature on energy balance are then summarized, with examples. Alternative foraging strategies provide a possible means of reducing energy expenditure when food availability decreases. When prey availability is high, hunting methods which require a high rate of energy usage may provide higher rates of net gain of energy (to be used for maintenance) than energetically less costly methods, but the reverse may be true when prey are scarce. The optimal foraging method, considered as that which minimizes the time required for feeding, may then change with prey availability. However, if starvation, not predation, is the chief source of mortality, the optimal foraging method may be that by which the food needed to provide the daily maintenance energy is minimized. If the energetic costs of foraging exceed the energy gained as food in a given time, a bird does best not to forage. This has been observed in very cold and windy weather. The northern geographical limits to shorebird distributions in the non-breeding season are probably set by average temperature and daylenght. Many species may be unable to utilize intertidal feeding areas in tropical regions if these are fringed by mangroves. Within the 'wintering' areas, some individuals of most species move from place to place, while others remain in a single site, to which they may return in subsequent autumns. Movements of most species cannot be related directly to adverse physical conditions in the environment, though Knot come to Britain in larger numbers in cold winters. Few species perform 'hard-weather movements'; most stay in, and attempt to survive, adverse conditions. Laboratory measurements of heat loss may enable prediction of survival times, but field studies of feeding behaviour and prey availability in different geographical areas will be needed to understand further the patterns of distribution and movements of shorebirds in the non-breeding season.