Vol. 43, No. 4
Ian C. T. Nisbetand Carolyn S. Mostello
Fig. 1. A Common Tern with a geolocator mounted on its leg. The geolocator was attached to a custom-made plastic leg flag using marine epoxy adhesive, and secured with two loops of dental floss sealed with superglue. The light sensor is the small white rectangle. The small white flag was placed on the right leg so that the bird could be located if it returned but had lost the geolocator assembly during the winter. (Photograph by C. S. Mostello).
In recent years the study of bird migration has been revolutionized by the development and use of miniaturized tracking devices. It is now possible to track individual birds wherever they go, across mountains and deserts, through the night, and far out at sea. As yet, satellite transmitters and GPS receivers can be used only on large birds; the only devices available to remotely track small birds that cover great distances are light level geolocators, which use changes in light intensity to determine locations. Geolocators were originally developed by the British Antarctic Survey for tracking albatrosses, but their engineers progressively made them smaller so that they can now be used on small birds of many species. In recent years they have been used to track, among others, Veeries (Heckscher et al. 2011; Hobson and Kardynal 2015), Northern Wheatears (Bairlein et al. 2012), Black Swifts (Beason et al. 2012), Ovenbirds (Hallworth et al. 2015), and—most recently—Blackpoll Warblers on their autumn migration across the ocean from New England to the West Indies (DeLuca et al. 2015).