BROOD SIZE, FOOD PROVISIONING AND CHICK GROWTH IN THE PIGEON GUILLEMOT CEPPHUS-COLUMBA

We studied the effects of brood size and chick age on food provisioning rates to Pigeon Guillemot Cepphus columba nests, and differences in growth parameters between chicks in broods of one and broods of two. Provisioning rates increased with chick age in the first part of the nestling period, and then declined. This decline was steeper in 1985, when overall provisioning rates were higher. Per-chick provisioning rates were lower to broods of two, resulting in one of the chicks having a lower growth rate, and possibly a lower fiedging mass, than its sibling. Growth parameters of the faster growing chick in broods of two did not differ significantly from those of solo chicks. The decline in provisioning rate late in the nestling period is probably not due to reduced food availability. It may be due to reduced energetic demands of the chicks or to a deliberate strategy on the parents' behalf to induce fledging. At present we lack the information to distinguish between these two hypotheses.