Why do Pied Flycatchers Lose Weight
Abstract from Functional Ecology
2nd May 1996
The easy answer that it takes a lot of effort to find food for their young - the cost of reproduction - is not the only theory available at the moment. It could be that the birds are reducing their weight to save costs associated with flying about their territory finding food for their young.
Hillstrom tried to test these theories for Swedish Pied Flycatchers for some of which he provided supplementary food - mealworms on a tray near their nestbox. For the males, but not the females, the adults showed a higher loss of body mass through the period when the young were being fed in the nest. This supports the cost of reproduction theory. For the females there was a clear relationship between the number of young fledged and fledging success and the weight loss of the females - but not the males.
So there was some support for the alternative theory. Possibly the two theories are not mutually exclusive and the birds could be really clever. They could be adjusting their body mass to optimise the amount of effort they put into rearing their young and to maximise their overall lifetime reproductive success.
Heady stuff and the sort of thing that very careful study is showing that birds may be capable of sorting out - but not, of course, consciously.