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.