At Marion Island, there was considerable correlation in numbers of adults breeding at study colonies for both Macaroni Eudyptes chrysolophus and Eastern Rockhopper E. chrysocome filholi Penguins, over 26 and 22 years respectively, suggesting over-wintering conditions may influence the proportions of birds breeding. For both species the time of arrival of females for breeding, and for Rockhopper Penguins the mass of females on arrival, was significantly related to breeding success. Therefore, over-wintering conditions may also affect breeding success. Trends in breeding success at study colonies were more strongly correlated for Macaroni Penguins than Rockhopper Penguins. Macaroni Penguins have a greater foraging range than Rockhopper Penguins when breeding, and may be more influenced at this stage by wider-scale environmental phenomena. For Macaroni Penguins, breeding success was significantly correlated with mass of chicks at fledging. For both species, mass on arrival of males was significantly correlated with that of females. Although both species had low weights on arrival after the El Niño Southern Oscillation event of 1997/98, there was no significant correlation in mass on arrival between the two species. It is likely that at Marion Island their over-wintering grounds are different.