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 performance of great and blue tit (Parus major L., P. caeruleus L.) parents, and its consequences for the growth and fledging weight of nestlings. Because fledging weight is a determinant of subsequent survival and therefore fitness, foraging decisions of the parents play a key role in the reproductive system of tits. The analysis quantifies (i) the rate at which energy is delivered to the nestlings in relation to prey size and abundance, and (ii) the growth rates of nestlings and the resulting fledging weight in relation to the 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 searching time per item was 2.5-3 min instead of 5-6 min before and after the peak. Searching time was significantly reduced when the birds returned to the foraging site where the preceding prey was found. This accords with the clumped distribution of caterpillars within the canopy. 3. The foraging performance (in mg caterpillars per min) was maximal when caterpillars were both abundant and large, i.e. shortly before they left the trees for pupation. The high feeding frequency and the large prey then caused a peak energy flow rate to the nestlings of 4-5 times the rate before or after the caterpillar peak. This suggests that the foraging success and rate of food delivery by tit parents was primarily determined by the abundance and size of prey. 4. The growth rate of nestlings, as well as their fledging weight was correlated with the rate of food delivery. Low feeding performance of the parents resulted therefore in poor relative growth rates of only 0.3-0.6 of the rate achieved under optimal conditions and, as a consequence, in a low fledging weight. This indicates that tit parents have restricted options to adjust prey delivery rates according to the requirements of the brood. 5. The results give insight into the chain of causal mechanisms through which an environmental factor (availability of food) has a strong and immediate effect on fitness (growth, fledging weight and, thus, survival of the nestlings). The importance of caterpillar size for foraging success and prey delivery rates of parent tits makes clear, why the phase of best foraging conditions is shorter than the period during which caterpillars are available. The relationships we quantified give a proximate explanation for the great effects that temperature and caterpillar growth have on the between-year variation in selection intensity for laying date observed in other studies.