Summary
Population counts are useful for monitoring populations as they can indicate both the ecological history and the present condition of a population. We have conducted an annual population census of Adélie penguins at Béchervaise Island since the breeding season of 1990/91 as part of our long term monitoring program. Each population count is conducted at the beginning of the breeding season when these birds come ashore. While most mature penguins return to the island every year, up to 20% of each cohort are absent for at least one season. When away from the island during the breeding season, these birds form part of an ‘itinerant population’ whose whereabouts is unknown. Young birds contribute significantly to the itinerant population as fledglings generally do not return until their second or third year and can even take up to nine years to return to the island for the first time. We estimate that the number of birds in this itinerant population can be as large as 1200 birds, with the current size of the Béchervaise Island penguin colony being approximately 4300 birds. The existence of this itinerant population has consequences for both the use of population counts for population monitoring and the management of these populations. Interannual variation in population counts is due not only to birth and death processes but also migration between the itinerant population and the Béchervaise Island population. Surveys of nearby islands show that the itinerant population does not simply inhabit these colonies. These results suggest that our monitoring needs to be conducted at a much broader scale to obtain a greater understanding of the spatially explicit nature of this and other populations.