PATTERNS OF PER-BROOD AND PER-OFFSPRING PROVISIONING EFFORTS IN THE WILLOW TIT PARUS-MONTANUS

Nestling feeding behaviour of Willow Tits Parus montanus was studied i n natural and artificially size-altered broods al Oulu, northern Finla nd. Per-brood provisioning effort was positively and per-offspring eff ort negatively correlated with brood size. Brood size manipulations (a bout +/- 30% relative to control broods) did not significantly affect the patterns of per-offspring provisioning effort, i.e. parents of enl arged broods could increase their per-brood effort significantly as co mpared with control broods. Thus, Nur's refined trade-off model of opt imal nestling Feeding was supported. On the other hand, no measurable extra costs (in physical condition, body mass or size) to parents of e nlarged broods were detected; however, the young of enlarged broods ha d lower body masses than those of control and reduced broods confirmin g earlier results in the same population. Thus, in broods above the mo dal size parents primarily tried to ensure their own rather than their offspring's survival. At the beginning of the nestling period, when F emales brooded the young, male contribution to provisioning was higher . At the end of the nestling period male load sizes were larger but fe male visiting rates slightly higher, resulting in sexually equal total feeding effort. This was expected for a monogamous passerine with bip arental care. Feeding effort increased with offspring age owing to the increased load sizes, increased Female feeding rates and prolonged wo rking days. However, the temporal increase in female visiting rates ma y be explained by the decreased brooding effort. A temporal increase i n feeding effort is consistent with the corresponding increase in nest defence intensity, found in our earlier studies.