The foraging performance of great and blue tits (Parus major and P-caerulens) in relation to caterpillar development, and its consequences for nestling growth and fledging weight

1. We analysed the effect of prey density and size on the foraging performa nce of great and blue tit (Parus major L., P. caeruleus L.) parents, and it s consequences for the growth and fledging weight of nestlings. Because fle dging weight is a determinant of subsequent survival and therefore fitness, foraging decisions of the parents play a key role in the reproductive syst em of tits. The analysis quantifies (i) the rate at which energy is deliver ed to the nestlings in relation to prey size and abundance, and (ii) the gr owth rates of nestlings and the resulting fledging weight in relation to th e rate of food delivery by the parents. 2. The searching time per prey item increased exponentially with decreasing prey biomass. During the peak abundance of caterpillars, the average searc hing time per item was 2.5-3 min instead of 5-6 min before and after the pe ak. Searching time was significantly reduced when the birds returned to the foraging site where the preceding prey was found. This accords with the cl umped distribution of caterpillars within the canopy. 3. The foraging performance (in mg caterpillars per min) was maximal when c aterpillars were both abundant and large, i.e. shortly before they left the trees for pupation. The high feeding frequency and the large prey then cau sed a peak energy flow rate to the nestlings of 4-5 times the rate before o r after the caterpillar peak. This suggests that the foraging success and r ate of food delivery by tit parents was primarily determined by the abundan ce and size of prey. 4. The growth rate of nestlings, as well as their fledging weight was corre lated with the rate of food delivery. Low feeding performance of the parent s resulted therefore in poor relative growth rates of only 0.3-0.6 of the r ate achieved under optimal conditions and, as a consequence, in a low fledg ing weight. This indicates that tit parents have restricted options to adju st prey delivery rates according to the requirements of the brood. 5. The results give insight into the chain of causal mechanisms through whi ch an environmental factor (availability of food) has a strong and immediat e effect on fitness (growth, fledging weight and, thus, survival of the nes tlings). The importance of caterpillar size for foraging success and prey d elivery rates of parent tits makes clear, why the phase of best foraging co nditions is shorter than the period during which caterpillars are available . The relationships we quantified give a proximate explanation for the grea t effects that temperature and caterpillar growth have on the between-year variation in selection intensity for laying date observed in other studies.