Competition-mediated prey availability drives Adélie penguin (Pygocelis adeliae) chick size, mass and condition at colonies of differing size in the southern Ross Sea

Understanding the relative effects of biotic and abiotic drivers of survival, and the interactions between them, is a key component in understanding the factors driving changes in animal populations.  Body size, mass and condition may be important determinants of an organism’s ability to survive periods of low resource availiability or high metabolic demand.  Such effects may be particularly important for naïve juveniles when they first become independent of their parents and must learn to forage for themselves.  Therefore, changes to adult foraging efficiency through intra-specific competition or environmental conditions are likely to impact chick size, mass and condition and may ultimately lead to lower post-fledging survival.  We examine how Adélie penguin chick size, mass and condition varied among breeding colonies of different sizes on Ross Island during a period of high environmental variability.  The presence of two giant icebergs from 2001 to 2005 increased sea ice concentrations, reducing adult foraging efficiency and providing a natural experiment to test the effects of environmental conditions and competition on chick size, mass and condition.  Our results show that the size, mass and condition of Adélie penguin chicks is greater during times when environmental conditions allow for more efficient parental foraging.  In addition, we show that in some cases, increased intraspecific competition may be a more important driver of chick size than abiotic factors, with chicks smaller and lighter at larger colonies. Understanding these patterns will allow better understanding of how such factors as climate change and altered food webs may affect changes in Adélie penguin populations in the Ross Sea and elsewhere.