Causal and functional correlates of brood amalgamation in the chinstrap penguin Pygoscelis antarctica : parental decision and adult aggressiveness

Despite the high number of species in which creching behaviour has been analysed, the factors determining brood desertion and chick aggregations remain relatively poorly understood. We analysed cre- ching behaviour of chinstrap penguins (Pygoscelis ant- arctica) to test whether: (1) timing of chick aggregations was mainly determined by the growth stage of chicks or by adult physiological constraints; (2) the creche acts as a protective mechanism against predation or as a defence against conspecific adult aggressiveness. Our results show that chick desertion was not related to chick growth rate and was driven primarily by a parental decision, determined by adult physical constraints imposed by moulting needs and the short breeding sea- son in Antarctica. With respect to the functional meaning of brood amalgamations, our results suggest that they are originated by the aggressive behaviour of adults, although the forces driving them could depend greatly on ecological conditions and vary among species and populations. Finally, because brood amalgamations can be determined by adult aggressiveness contrasting with the origin and typical definition of the term ‘‘creche’’ (which implies the nursery concept), we propose the use of the term ‘‘brood amalgamation’’ for the penguin ‘‘creches’’.Peer reviewe