Mass loss in female Pied Flycatchers Ficedula hypoleuca during late incubation: Supplementation fails to support the reproductive stress hypothesis

In most species of altricial birds, females experience great changes in bod y mass during the reproductive period. Mass loss of females after hatching of the young has commonly been regarded as a result of increased reproducti ve stress. Alternatively, the loss of body mass during this period is an ad aptive trait that increases feeding ability and reduces the risk of predati on through reduced wing loading. The reproductive stress hypothesis was tes ted by providing extra food inside the nestbox to female Pied Flycatchers F icedula hypoleuca during incubation and hatching. A pairwise design was use d, with one supplemented and one control female in each pair (n = 32), to a ccount for variation in hatching date. Body mass was strongly reduced after hatching, in particular in females with high initial body mass. How ever, mass reduction was similar in supplemented and control females. Hence, the result was consistent with the adaptation for flight hypothesis but not wit h the reproductive stress hypothesis.