Maternal energy expenditure does not change with flight costs or food availability in the pied flycatcher (Ficedula hypoleuca): costs and benefits for nestlings

We manipulated parental work; load without changing brood size in a populat ion of pied flycatchers Ficedula hypoleuca by removing two primaries (7 and 9) from each wing of females, thus reducing wing area and increasing fligh t costs. At other nests, we offered supplementary food in the form of live mealworms (10-20 g daily from hatching) to reduce brood demand and thus par ental foraging costs. Other nests were left as controls. The daily energy e xpenditure of females feeding 12-day-old nestlings was measured with doubly labelled water (D2O)-O-18. Females in both treatments expended the same am ount of energy, fed at the same rate and had similar body masses to birds i n the control group. No effect of treatment on male mass and feeding effort was detected. More nestlings, however, died in nests of handicapped female s. Nestlings of handicapped females had significantly lower body mass and h aematocrit values than nestlings in food-supplemented nests, with nestlings in control nests occupying an intermediate position. The effects of both t reatments on nestling mass, haematocrit values and mortality rates were onl y noticeable in nests infested with mites. Maternal energy expenditure is a pparently constrained and offspring pay the costs imposed by reduced provis ioning rate or increased demand caused by ectoparasites, while receiving be nefits when food supply improves. The presumption that avian reproductive c osts derive from changes in a flexible energy output may not be met in many cases.