We studied the nestling diet and the foraging performance of Great Tits in relation to prey abundance in the field. Numerous experimental studies present data on foraging decisions in captive Great Tits. Little is, however, known about prey selection in the field in relation to the food available and the consequences this has for the food delivery rate to nestlings. Since the foraging performance of the parents is one of the main determinants of fledging weight and juvenile survival, foraging behaviour is an important part of Great Tit reproduction. During the early breeding season up to 75% of the prey biomass delivered to the nestlings were spiders, which is in contrast with other studies. Only when caterpillars reached a size of 10–12 mg (approximately the average size of the spiders caught at that time) did the Great Tits change their preferences and 80–90% of the delivered prey masses were caterpillars, as reported by other authors. This ‘switching’ between prey occurred within a few days. It was not related to the changes in abundance but to size of caterpillars. The rate at which caterpillars were delivered to the nestlings (in mg/nestling/h) was strongly correlated with the caterpillar biomass available (in mg/m of branches) and nestling growth rate was significantly influenced by the mass of available caterpillars. The results provide evidence why perfect timing of breeding is so important for the Great Tit, and contribute to the understanding of the causal link between food supply, growth and breeding success.

In seasonal environments with limited time and energy resources, double-brooded birds face trade-offs in the timing of their two reproductive attempts and in the effort allocated to the first and the second broods. In the Barn Swallow Hirundo rustica a long care period for the first brood enhances the survival of first-brood chicks, but also delays the start of the second brood, which in turn reduces the survival prospects of second-brood chicks. Probably as a response to this trade-off, double-brooded Barn Swallows reduce the period of post-fledging care for first-brood fledglings. By radiotracking whole families, we investigated the determinants of this behaviour and its consequences for the survival of the first-brood fledglings. The end of the females' investment in post-fledging care of the first brood was related to the beginning of egg synthesis for the second clutch. With the start of egg synthesis, females significantly reduced provisioning rates to the firstbrood fledglings to less than one-fifth of the previous rates, while the proportion of time they spent foraging remained high. Assuming that the females' foraging success was constant, we conclude that their energy income was allocated to egg production rather than fledgling provision. Males did not compensate for the females' reduced feeding rates. Thus the start of egg production for the second clutch had a marked effect on the quantity of food received by first-brood fledglings. In parallel with the changes in parental behaviour and provisioning rates, we observed a marked drop in the daily survival rate of first-brood chicks. These results support the hypothesis that females face a strong tradeoff in the allocation of energy to subsequent broods. Energy allocation to a second clutch involves a cost in terms of reduced provisioning, and as a result the survival of first-brood chicks is compromised. This is probably outweighed by the improved success of an early second brood.

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