Chick growth and nest departure in Baltic Black Guillemots Cepphus grylle

Abstract

During apparently good food years, Black Guillemot Cepphus grylle chicks from the Gulf of Finland grew at aconstant rate, showing the typical peak (asymptotic) weight and the subsequent weight recession prior to fledging . Single chicks fledged heavier than twins, but no differences emerged in departure age. Among twins, growth rate and fledging success were similar for A- and B-chicks. Chicks from artificially enlarged broods of three fledged lighter, had significantly shorter tarsus and a longer nest-rearing period than control broods of twins and singles, indicating food stress . The rate of parental feeding was positively correlated with brood size, but the increase was not proportional to brood size, so that chicks in larger broods received a smaller number of feedings per chick than those in smaller broods. The diet was similar among broods, containing 90% of Eelpout Zoarces viviparus. Despite the increase in provisioning effort, parents of enlarged broods did not discriminate between their own and alien offspring . Nest departure age was strongly affected by the combined effect of the developmental maturity of the chick's body dimensions and the depth of the nest cavity . In deep cavities, chicks needed to wait longer to attain the right body mass/ wing length ratio for climbing out than in low ones. Because this study did not follow birds after fledging, immediate effects of lower body weight and shorter tarsus on survival cannot be assessed .