Nutrient or energy limitation on egg formation: A feeding experiment in great tits

1. We tested the hypothesis that protein availability rather than energy availability constrains egg formation in great tits (Parus major L.) by providing them with two food supplements of different protein content in the prelaying and laying period of 1991 and 1992. 2. Timing of breeding, clutch size and egg size did not differ between the two food supplements. Thus, the hypothesis that a low protein content of supplement restricts laying in great tits cannot be supported. 3. Food supplementation, independent of food type and year, led to an advancement of laying by 5.6 days. Average clutch size of supplemented birds, corrected for laying date and year, was 0.55 eggs above the population average. 4. Supplementary fed great tits did not breed more successfully than controls. This suggests that by adding food early in laying, birds were induced to lay at a less favourable time and/or produced clutches that were too large to care for successfully under normal food condition. Thus food early in the season might not only energetically restrict laying birds, but also provide clues about future conditions during chick rearing. 5. The comparison of results of published supplemental feeding experiments shows that the birds responded more strongly to additional feeding in poor environments. Almost two-thirds of the variation in changes of clutch size due to experimental treatment can be explained by year quality. Thus, only in poor environments do reproductive traits seem to be limited by energy availability early in the season, but other factors become more important in good environments.