Carlson A. & Moreno J. (1986) Foraging behaviour and parental care in the Fieldfare. ARDEA 74 (1): 79-90

The foraging behaviour of Fieldfares Turdus pilaris L. feeding nestlings was studied in a small breeding colony in the Botanical Garden of Uppsala. Earthworms were the most common prey taken and were preferentially delivered to the nestlings, while almost all other prey captured were consumed by the adults after capture. Parents also delivered the largest worms to the young. The time taken to pull the worms out of the ground and to handle them before loading and consumption increases with worm length, which probably determined the negative correlation between load size (no. of worms loaded) and the length of the worms included in the load. Large worms were sometimes partially consumed, the largest part being usually taken to the young. Smaller than average worms were sometimes consumed after the bird had already loaded larger worms. There was no clear loading effect as envisaged by central place foraging theory, but the size of the worms already loaded clearly affected the decision to return to the nest or to try to augment the load size. The foraging behaviour of Fieldfares seems to be so flexible, that no simple central place foraging model appears applicable. Sequential models where each decision depends on decisions taken earlier in+ a foraging bout seem more appropriate but their complexity would make them difficult to test. Feeding rates, no. of prey delivered, length of worms carried, no. young fed on each visit and brooding behaviour of females were studied in 2 pairs with 4 and 6 young of 2, 8 and 12 days of age. Males maintained constant feeding rates, delivered less but larger worms and maintained constant food delivery rates (mass of worms delivered per hour) as nestlings aged. Females increased their feeding rates, delivered the same number of worms on each visit and increased the size of the worms delivered and their food delivery rates as nestlings aged. Less old nestlings were fed on each visit than was the case with young nestlings, and the same number of young were fed with the same load mass in both nests. The simplest explanation for these trends is that there is a constraint on prey size when young are small, so parents have to deliver small worms, but they can then load several of them as small prey are easy to handle, and feed several young on each visit. Later when the constraint on prey size disappears, they can take larger worms, but they will have to reduce the load size (due to the handling constraint) and feed fewer young on each visit. This is exactly what males do. Females react to the relaxation of the nestling size constraint by taking larger worms, although they do not carry fewer worms. The absence of a load size - prey size trade-off is best explained by their intensive brooding when nestlings are small (almost 80% of the time). The need to return quickly to the nest after satisfying their own needs explains the small load sizes at the beginning. We have thus a pattern of parental care where males maintain a constant contribution to nestling feeding during the whole nestling period, while it is females that, by relaxing their nest attendance and increasing their food contribution, respond to the increasing needs of the young. Such a marked role separation appears not to be a common pattern in altricial species. There is an indication of a brood size dependent increase in energy expenditure for the females and of a more rapid growth in the smaller brood.