Supercharged penguins—supplementary feeding reshapes tawaki/Fiordland penguin winter migration

Migration in a species is primarily a response to resource availability, shelter, thermoregulation, predator avoidance, or seasonal environmental change. Tawaki/Fiordland penguins ( Eudyptes pachyrhynchus ) have earned the nickname “marathon penguins” as they migrate 2,500 km from their nest sites during the short period between breeding and annual moult, despite productive waters being found around their breeding sites in southern New Zealand. A similar directional tendency is evident during the subsequent winter migration, during which birds target subantarctic waters south of Tasmania, often reaching the polar front. However, during a previous study, two birds were tracked that not only moulted outside the species’ breeding range on the east coast of New Zealand’s South Island, but did so in the care of a rehabilitation centre. In care, birds are generally fed throughout the moult even though penguins cannot forage during the moult as all feathers are replaced simultaneously. Although these two birds ceased transmitting within days of release, they appeared to follow different trajectories—south and south-east—compared to wild birds that consistently travel west and south-west. The apparently aberrant behaviour of these east coast birds raised the question of whether moulting location and/or supplementary feeding during rehabilitation could affect subsequent winter migration. To examine this, between February and March 2020 we deployed satellite transmitters on 18 tawaki at the conclusion of their annual moult, comparing three groups: birds moulting on the west coast, birds moulting unassisted on the east coast, and birds released from rehabilitation facilities following supplementary feeding during the moult. West and east coast tawaki followed broadly similar routes—south-westward and southward respectively—reaching subantarctic waters south of Tasmania or south of mainland New Zealand. Supplementarily fed birds departed heavier than either wild group, ranged more widely and farther, resulting in longer trips and less focused movement patterns. Migratory differences among groups were driven principally by departure body condition, suggesting that supplementary feeding during the moult may substantially alter migratory behaviour and affect their subsequent annual life history. As such, this study provides an evidence-based foundation for refining rehabilitation protocols for penguin species undergoing moult in human care.