Growth and its relationship to fledging success of African black oystercatcher Haematopus moquini chicks

Abstract

We investigated the growth of African black oystercatcher Haematopus moquini chicks on Robben Island, South Africa, over three austral summers, 2001–2004. Using a robust regression analysis to determine the growth parameters of chicks of known and unknown age we found that oystercatchers from our study population had a Gompertz growth rate coefficient that was 2% less than predicted for body mass based on the equation for waders. Leg growth lagged initially, then increased and slowed again as the chicks became older, whereas wing growth was slow initially but increased with age. Chicks with small growth rate coefficients for body mass exhibited retarded growth of all body measures except wing length. This enabled these chicks to fledge in a shorter period of time than their slow growth would otherwise allow. The growth rate of body mass was observed to vary greatly between chicks. Fast-growing African black oystercatchers had a shorter pre-fledging period; were larger at fledging and were more likely to fledge successfully. African black oystercatchers display sibling rivalry, and once a dominance relationship is established, the larger chick remains so during the pre-fledging period. Larger siblings fledged earlier and at a heavier mass than the smaller siblings and this may improve their chances of survival. Neither hatching date nor brood size influenced the growth rate coefficients.