Satellite Telemetry Reveals Strong Fidelity to Migration Routes and Wintering Grounds for the Gull-Billed Tern (Gelochelidon nilotica)

Abstract

The Western Gull-billed Tern (Gelochelidon nilotica) breeds in only 16 locations in southern California, USA and the Pacific coast and Gulf of California in Mexico. Relatively little information is available on migration and wintering locations for the population. This is the first project to use satellite telemetry to document Gull-billed Tern migration routes, important stop-over locations, and non-breeding areas for the northwestern-most breeding population that nests in San Diego, California. A total of eleven deployments occurred over a period of four breeding seasons. Of the 11 deployments, five terns provided information on complete migration cycles consisting of both a fall and a successive spring migration. Four terns provided information on two fall and one spring migration, and one tern provided information on three fall and three spring migrations. Migration routes in the fall and spring are similar, with individuals crossing inland over the Peninsular Mountain Range of Baja California to stop over in the Rio Colorado Delta of Mexico, before beginning southward travel along the continental Mexico coastline and reversing the route for spring migration. The wintering distribution of individuals from the San Diego breeding colony appears to be small, consisting of a 250-km span of coastal wetlands ranging from Bahia Tobari, southern Sonora south to Santa Maria-La Reforma, northern Sinaloa, Mexico. Long term monitoring of individual movements suggests strong fidelity to both breeding and wintering grounds, with individuals returning to San Diego each season to breed and then returning to previously used winter locations.