Migration front of post-moult emperor penguins

Abstract

The moult is arguably the most critical period in the life of emperor penguins (Aptenodytes forsteri). Birds from western Ross Sea colonies travel yearly to and from the pack ice of the eastern Ross Sea to moult. Despite the suspected large numbers of penguins involved, this migration had never been directly observed. Here, we provide the first description of a migratory front of penguins travelling east to west between their moulting habitat and to the breeding colonies. Early autumn ship-bound visual surveys showed density of birds increased significantly as we approached the eastern Ross Sea and was not related to ice type, per cent ice cover or primary productivity. This supports the hypothesis of a dense “source” of post-moult birds in the eastern Ross Sea migrating in near-synchrony and gradually dispersing towards breeding colonies in the southwest and northwest Ross Sea. Emperor penguins travelled alone or in small groups of up to 8 individuals, concentrating around narrow leads or isolated water holes, and were occasionally seen far from open water, suggesting they move primarily by swimming, complemented by tobogganing. Their new coats indicated they had completed the moult. Aggregations of birds and guano stains suggested they were feeding while migrating.