How different provisioning strategies result in equal rates of food delivery:: an experimental study of blue tits Parus caeruleus

Food provisioning in birds requires a considerable amount of time and usually has to be traded-off against other parental and non-parental activities. I investigated experimentally the rate at which blue tit Parus caeruleus parents deliver food to their brood after a change in food availability. The main argument behind this study is that parents enjoying an additional food source may use less time for self-feeding and therefore use more time for food provisioning. This could increase the rate at which food is brought to the nest. However, a prey choice model that takes the energetic needs of the parent into account allows for the possibility that the food-supplemented parents would deliver the same amount of food by increasing prey size (through an increase in prey selectivity) and reducing visit rate. The field data indicate that the parents changed provisioning strategy when food-supplemented: they fed the chicks natural food less frequently, but brought larger larvae. On the whole, delivery rate of natural food was the same or lower than in controls. The results suggest that food-supplemented parents used the time saved to increase their degree of food selectivity. When the gains from an increased delivery rate are not worth the increased costs (mainly resulting from an increased visiting rate), the parent with low energetic need may increase selectivity to provide the same amount of food to the brood as the unmanipulated parent, but at a lower cost.