FACTORS INFLUENCING DAILY FOOD-INTAKE PATTERNS IN BIRDS: A CASE STUDY WITH WINTERING COMMON CRANES

Avian foraging activity during daytime peaks after dawn and before dusk. The relative importance of each period of intensive foraging on the accumulated intake varies by species, individual, and day. We studied the relative importance of each period from direct observation of radio-tagged Common Cranes (Grus grus) during winter at a site of stopover and wintering in Spain. After a mathematical model was fitted, accumulated intake showed a typical anti-sigmoid shape, with greatest increases of intake after dawn and before dusk. The rise of accumulated intake decelerated until 50% of the day length, when the trajectory inflected and accelerated according to the fitted model. The trajectory of accumulated intake in territorial families that foraged in small home ranges with food predictably available but the cost of vigilance high decelerated until 20% of the day length, then accelerated for the rest of the day. In flocking nonterritorial cranes the inflection point was delayed until 60% of the day length, as expected since flocking cranes foraged in larger home ranges with food less predictable and the cost of vigilance lower. The inflection point was delayed in early winter and advanced in late winter. The changes in the inflection points of the functions for accumulated food intake may be straightforwardly applied to daily foraging routines of other species whose rates of intake may be recorded.