Growth and sibling rivalry and their relationship to fledging success of African Black Oystercatcher, Haemotopus moquini, chicks

We investigated the growth of African Black Oystercatchers, Haematopus moquini, on Robben Island, South Africa, over three austral summers, 2001–2004. Oystercatcher chicks from our study population were found to have a Gompertz growth rate coefficient that was 2% less than predicted for body mass based on the equation for waders. The growth rate of body mass was observed to vary greatly between chicks. Fast-growing African Black Oystercatchers had a shorter prefledging period, were larger at fledging and were more likely to fledge successfully. 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 enables these chicks to fledge in a shorter period of time than their slow growth would otherwise allow. Neither hatching date nor brood size influenced the growth rate coefficients. African Black Oystercatchers display sibling rivalry, and once a dominance relationship is established, the larger chick remains so during the prefledging period. The larger sibling fledges earlier and at a heavier mass; this may improve their chances of survival. African Black Oystercatcher chick growth 69 Introduction Oystercatcher chicks are semi-precocial waders (del Hoyo et al. 1996), which means that they are mobile soon after hatching but rely entirely on parental feeding until after fledging (Dircksen 1932, Lind 1965, Groves 1984, Hockey 1984a, 1996). Thus, prey availability, parental food provisioning and territory quality can influence the growth and survival of chicks (Groves 1984, Ens et al. 1992, Kersten & Brenninkmeijer 1995, Hazlitt et al. 2002). Kersten and Brenninkmeijer (1995) reported that fledging success of Eurasian Oystercatcher, Haematopus ostralegus, chicks increased with growth rate and that chicks fledging at an earlier age exhibited a faster growth rate. Groves (1984) found that brood size impacted the growth rate of American Black Oystercatcher, Haematopus bachmani, chicks; chicks from one-chick broods exhibited greater masses than chicks from two-chick broods of the same age. In addition, dominance relations and sibling rivalry, between siblings of two-chick broods resulted in mass differences between siblings; larger chicks weighed up to 48% more upon fledging (Groves 1984). Oystercatchers start incubation once the clutch is complete, thus resulting in relatively synchronous hatching (Hockey 1983a, Ens et al. 1996). Chicks usually hatch within a few hours of each other, and chicks hatched first are brooded while the remaining eggs hatch (Hockey 1996). Chicks leave the nest shortly after the last chick hatches (Hockey 1996). Although siblings begin their life at roughly the same size and receive their first feeding at the same time, Safriel (1981) noted that within two weeks after hatching sibling rivalry results in significant mass differences between Eurasian Oystercatcher chicks. Hazlitt et al. (2002) proposed that parent American Black Oystercatchers adjust their effort according to the quality of their territory. Parental provisioning and food availability could therefore influence sibling rivalry and result in the development of dominance of one chick within the brood and thus a size difference between siblings. The results reported here are part of a larger study about the breeding biology of African Black Oystercatchers: e.g. Tjørve and Underhill (submitted manuscript-a, submitted manuscript-b, submitted manuscript-c). The aim of this study was to describe the growth of African Black Oystercatcher chicks and to determine and discuss the factors that affect their growth and successful fledging. We compare different methods of analysing growth and discuss their applicability to the growth data of African Black Oystercatchers. The rate of development of the embryo during the incubation period may influence development after hatching. Variation in embryonic rate may be paralleled by chick growth rate – factors not related to feeding. In this study, however, we only discuss post-hatching influences – the impact of hatch date, brood size, sibling rivalry and possible factors affecting the establishment and maintenance of weight differences in two-chick broods. We have also investigated how growth rate affects fledging success, the duration of the prefledging period and the size of fledglings. African Black Oystercatchers breed over the austral summer (Leseberg et al. 2000), October to April. First hatchlings are observed in late November. Chicks grow during the long, warm period of southern summer. We compared the growth of