Patterns of Per-Brood and Per-Offspring Provisioning Efforts in the Willow Tit Parus montanus

Abstract

Nestling feeding behaviour of Willow Tits Parus montanus was studied in natural and artificially size-altered broods at Oulu, northern Finland. Per-brood provisioning effort was positively and per-offspring effort negatively correlated with brood size. Brood size manipulations (about + 30% relative to control broods) did not significantly affect the patterns of per-offspring provisioning effort, i.e. parents of enlarged broods could increase their per-brood effort significantly as compared with control broods. Thus, Nur's refined trade-off model of optimal nestling feeding was supported. On the other hand, no measurable extra costs (in physical condition, body mass or size) to parents of enlarged broods were detected; however, the young of enlarged broods had lower body masses than those of control and reduced broods confirming earlier results in the same population. Thus, in broods above the modal size parents primarily tried to ensure their own rather than their offspring's survival. At the beginning of the nestling period, when females brooded the young, male contribution to provisioning was higher. At the end of the nestling period male load sizes were larger but female visiting rates slightly higher, resulting in sexually equal total feeding effort. This was expected for a monogamous passerine with biparental care. Feeding effort increased with offspring age owing to the increased load sizes, increased female feeding rates and prolonged working days. However, the temporal increase in female visiting rates may be explained by the decreased brooding effort. A temporal increase in feeding effort is consistent with the corresponding increase in nest defence intensity, found in our earlier studies.