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 body mass during the reproductive period. Mass loss of females after hatching of the young has commonly been regarded as a result of increased reproductive stress. Alternatively, the loss of body mass during this period is an adaptive trait that increases feeding ability and reduces the risk of predation through reduced wing loading. The reproductive stress hypothesis was tested by providing extra food inside the nestbox to female Pied Flycatchers Ficedula hypoleuca during incubation and hatching. A pairwise design was used, with one supplemented and one control female in each pair (n = 32), to account for variation in hatching date. Body mass was strongly reduced after hatching, in particular in females with high initial body mass. However, mass reduction was similar in supplemented and control females. Hence, the result was consistent with the adaptation for flight hypothesis but not with the reproductive stress hypothesis

Slagsvold T. & Johansen M.A., ARDEA 86 (2): 203-211