Time Allocation in the Kestrel (Falco tinnunculus), and the Principle of Energy Minimization

(1) Time allocation of the kestrel in the Netherlands was established by dawn to dusk observation of focal birds. Time budgets were analysed with respect to time of year, phase of the breeding cycle, sex and weather conditions. (2) The common vole, Microtus arvalis L., was the major food source (92% of food items caught) with minor additions, especially in summer, of common shrews, Sorex araneus L., songbirds and juvenile waders. (3) Flight-hunting and perching were the two main foraging modes. Flight-hunting yielded, on average, 2.2 small mammals h-1 in winter and 4.7 in summer. Perching yield dropped from 0.3 mammals h-1 in winter to below 0.1 in summer. Flight-hunting yield reflected seasonal variations in food availability, perching yield did not. (4) There was no stage in the annual cycle where available daylight limited the daily flight-hunting time. There were no weather conditions where daily flight-hunting exceeded an average of 3.6 h or 20% of the active day. Stringent periods, in the sense that all buffer time is used for foraging, were found neither in winter, when food was scarce, nor in summer when food demand was highest. (5) Experimentally increased hunger of the brood led to increased male parental effort in terms of time spent flight-hunting plus flying, with a recorded maximum of 60% of the active day. Flight-hunting yield did not increase. (6) In winter, kestrels minimized energy expenditure, not foraging time, by using the low-cost low-profit technique of perch-hunting. In summer they maximized daily energy gain within limits probably set by their rate of food assimilation