Supplemental feeding as experimental tool to understand why birds do not lay earlier in the season

The timing of avian reproduction might be affected by the costs of egg
production in early spring. To study these costs, supplemental feeding
experiments have been conducted where birds are provided with food prior
to egg laying. We discuss the opportunities and pitfalls of such
experiments. We stress that supplemental feeding experiments alter the
conditions for the birds and hence can be used to measure the response in
laying date, but not to determine whether this response is adaptive. We
present two experiments, one on pied and one on great tits, both showing
no effect of supplemental feeding on laying date, but in great tits,
laying was affected in an additional treatment where night temperatures in
the nest boxes were manipulated. For both studies, we present ample data
on the ecological conditions of the year in which the experiments were
carried out. We furthermore present a mini-review of 31 studies on food
supplementation in passerines that examined effects on laying date,
focusing on between-year variation in the response to supplemental feeding
within the same studies. For 11 out of 19 species, at least two estimates
were available, and effects of food supplementation clearly varied between
species. The three studies done in the tropics showed a large effect of
supplemental feeding, while in the temperate zone population, this effect
was about one week for resident species, and small and non-significant for
migrant species. We argue that it is crucial that studies report on the
ecological conditions during the year(s) of the study to aid
interpretation of the results. We conclude that, in general, there is an
effect of supplemental feeding on the timing of reproduction in most
resident birds, but that the variation in the response needs to be
explored further.