HATCHING ASYNCHRONY, SIBLING HIERARCHIES AND BROOD REDUCTION IN THE CHINSTRAP PENGUIN PYGOSCELIS-ANTARCTICA

We studied patterns of chick growth and mortality in relation to egg s ize and hatching asynchrony during two breeding seasons (1991 and 1992 ) in a colony of chinstrap penguins sited in the Vapour Col rookery, D eception Island, South Shetlands. Intraclutch variability in egg size was slight and not related to chick asymmetry at hatching. Hatching wa s asynchronous in 78% (1991) and 69% (1992) of the clutches, asynchron y ranging from 1 to 4 days (on average 0.9 in 1991 and 1.0 days in 199 2). Chicks resulting from one-egg clutches grew better than chicks in families of two in 1991. In 1992, single chicks grew to the same size and mass at 46 days of age as chicks of broods of two, suggesting food limitation in 1991 but not in 1992. In 1991, asymmetry between siblin gs in mass and flipper length was significantly greater in asynchronou s than in synchronous families during the initial guard stage, but the se differences disappeared during the later creche phase. In 1992, asy mmetry in body mass increased with hatching asynchrony and decreased w ith age. Only the effect of age was significant for flipper length and culmen. Asymmetries at 15 days were similar in both years, but signif icantly lower in 1992 than in 1991 at 46 days of age. There were relat ively frequent reversals of size hierarchies during both phases of chi ck growth in the two years, reversals being more common in 1991 than i n 1992 for small chicks. In 1991, survivors of brood reduction grew si gnificantly worse than chicks in non-reduced broods. In both years, ch icks of synchronous broods attained similarly large sizes before fledg ing as both A and B chicks of asynchronous broods. In 1991, chick mort ality rate increased during the guard stage due to parental desertions , decreased during the transition to creches (occurs at a mean age of 29 days) and returned to high constant levels during the creche stage, when it is mostly due to starvation (in total 66% of hatched chicks s urvived to fledging). In contrast, in 1992, mortality was relatively h igh immediately after hatching and almost absent for chicks older than 3 weeks (87% of chicks survived to fledging). Mortality affected simi larly one- and two-chick families. In 1991, asynchronous families suff ered a significantly greater probability of brood reduction than synch ronous families, but this probability was not significantly related to degree of asymmetry between siblings. No association between asynchro ny and mortality was found in 1992. These results show that there is f ood limitation in this population during the creche phase in some year s, that asynchronous hatching does not facilitate early brood reductio n and that it does not ensure stable size hierarchies between siblings . Brood reduction due to starvation is not associated to prior asymmet ry and does not facilitate the survival or improve the growth of the s urviving chick. Asynchronous hatching may be a consequence of thermal constraints on embryo development inducing incubation of eggs as soon as they are