The effect of supplemental food on time budget and body condition in the Black Redstart Phoenicurus ochruros

We studied the effect of supplementary food on time allocation, breeding success and parental body condition in Black Redstarts Phoenicurus ochruros in the Alps, NW. Italy. Time budgets of both ♂ ♂ and ♀ ♀ were significantly affected: individuals provided with supplementary food spent less time foraging and more time preening and vigilant. In accordance with predictions on the difference between sexes in parental investment, ♂ ♂ increased the time spent singing by 5 to 26% and reduced the number of feeds to nestlings from 5.4 to 2.0 h^-1. Females showed a higher rate of nestling feeding which offset reduced provisioning of nestlings by the ♂ ♂. The breeding success of control and supplemented pairs was similar. The mass of adults was monitored using electronic balances placed near the nest cavity. Control pairs decreased in mass while adults given supplementary food showed an increase. The difference between control and fed adults was greater in ♂ ♂ than ♀ ♀. The hypothesis of an adaptive reduction in mass by all individuals due to programmed anorexia is not supported. We discuss the alternative hypothesis of a cost of reproduction utilizing estimates of energy expenditure from time budgets.