Prey selection and foraging performance of breeding Great Tits Parus majorin relation to food availability

We studied the nestling diet and the foraging performance of Great Tits in relation to prey abundance in the field. Numerous experimental studies pres ent data on foraging decisions in captive Great Tits. Little is, however, k nown about prey selection in the field in relation to the food available an d 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 p art of Great Tit reproduction. During the early breeding season up to 75% o f the prey biomass delivered to the nestlings were spiders, which is in con trast 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 th e Great Tits change their preferences and 80-90% of the delivered prey mass es were caterpillars, as reported by other authors. This 'switching' betwee n prey occurred within a few days. It was not related to the changes in abu ndance but to size of caterpillars. The rate at which caterpillars were del ivered to the nestlings (in mg/nestling/h) was strongly correlated with the caterpillar biomass available (in mg/m of branches) and nestling growth ra te 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 betw een food supply, growth and breeding success.