Movements and Asynchronous Nesting of the Peruvian Tern Sternula lorata in Chile and Peru

The Peruvian Tern Sternula lorata is an endangered seabird that nests asynchronously in different parts of its nesting range from northern Chile to Ecuador. Asynchronous nesting raises the possibility of counting individuals more than once during annual population censuses. We deployed 10 light-level archival tags (geolocators) to terns at the nesting ground in Mejillones, Chile, at the southern end of this species’ range during 06–11 September 2015, and we retrieved three the following year at the same location. Results showed that the tagged birds remained at the nesting grounds in Mejillones from July to December then migrated north, some in stages, to non-breeding areas along the coast of Peru and possibly Ecuador. Here they remained until early to mid-July 2016, although some movement among specific sites occurred between December 2015 and July 2016. They returned to their nesting areas more rapidly in early to mid-July. We detected no indications of additional nesting by these individuals away from Mejillones, Chile. Though sample size is small, these observations indicate at least that terns that breed in northern Chile likely do not nest again in Peru/Ecuador, though they may stay for extended periods. Therefore, we recommend that population estimates sum the counts of all nests and/or nesting pairs at all sites throughout the range each calendar year and count large roosting flocks separately. In addition, we suggest further studies to examine the possibility of geographically and temporally separated breeding populations.