This study used miniature archival light loggers (geo-locators) to map the migration route of the Arctic tern. The method itself is not novel, and has been used successfully for more that a decade, but due to the weight of loggers, studies were limited to large-sized seabirds, such as albatrosses and shearwaters. Within the last couple of years, however, technological development has allowed these loggers to be reduced in size and weight, opening up a whole new array of small to medium-sized birds to such study.

By recording and storing ambient light intensity, the geolocators reveal information on sunrise and sunset. When these data are combined with time recordings, two daily geographical positions can be calculated and migration routes can be mapped. Compared to conventional satellite transmitters, geolocators are much lighter and can be carried by smaller birds, and the cost is significantly lower. The disadvantage of geolocators is that they cannot transmit data, and the targged bird has to be caught again at the end of the study period – a logistic challenge that normally requires that the bird nest at the same location two years in a row – and the accuracy of a position obtained by geolocator (approx. 185 km on average), is less precise than satellite transmitter positions

In this study, we fitted miniature archival light loggers (Mk14 geolocators, mass 1.4 g; British Antarctic Survey, Cambridge), attached to plastic leg rings, to 50 breeding Arctic terns in Northeast Greenland and 20 loggers on Arctic terns in Northwest Iceland in July 2007. Logger, ring, tape, and cable tie weighed 2.0g, approximately 1.9% of mean adult body mass. The following year we returned to the study sites and were able to retrieved ten loggers in Greenland and one logger in Iceland. More birds with loggers were seen in the colony, but these could not be recaptured. A low return rate was expected as Arctic terns may shift colonies between years, or simply skip a breeding season altogether.

In order to equip Arctic terns with geo-locators the bird had to be trapped on the nest. Breeding Arctic terns were trapped during incubation in 2007, and caught again in 2008, after a full circle of migration.

This is a very simple, gentle, low-tech method to trap ground nesting birds such as terns (like here an Arctic tern), gulls and shorebirds. From the back of the trap to the stick, a thin black line runs over the eggs.

Footage provided by Thomas Grue ( and Ulrik Bang (