Breeding Season Movements of Aleutian Terns <i>Onychoprion aleuticus</i>: Implications for Population Assessment and Monitoring

In 2017 and 2018, satellite transmitters were deployed (n2017 = 13, n2018 = 14) on Aleutian Terns Onychoprion aleuticus in two areas of Alaska, USA, to investigate breeding season movements and temporal patterns of colony activity. The effort was intended to aid in the development of a large-scale population-assessment framework. Across the two years, we observed dispersal to seven confirmed or potential (putative) colonies, none of which were previously documented, ranging in distance from 0.4 to 64.2 km from tag deployment sites. Although most terns did not associate with any known colonies during the breeding season, several individuals from the study area in western Alaska were sequentially associated with multiple colonies. Breeding season movements were usually within tens of kilometers of the capture location, but movements of several hundred kilometers were recorded occasionally. These results should be interpreted cautiously because we do not know the post-tagging nesting status of marked terns, nor were we able to determine whether there were tern behavioral changes related to capture and tagging. With this caution in mind and based on our findings, we recommend that future efforts to estimate total Aleutian Tern population sizes or trends consider the following: (1) estimates should not rely solely upon surveys of previously documented colonies; (2) estimation procedures should account for the potential movement of individual terns among colonies, the possible lack of association of a tern with any colony, the possible association of a tern with multiple colonies, and/or the possibility that adults do not attempt to breed each year; and (3) if necessary, studies should survey subsets of the Alaska breeding range in successive years to reduce the uncertainty around the size of the statewide tern population.