Effects of geolocator deployment by using flexible leg-loop harnesses in a small wader

Miniature light-level geolocators are rapidly gaining popularity as tools in studies of bird migration. In waders such devices have so far typically been mounted on the tibia. In smaller wader species with relatively short and thin legs, however, attaching geolocators to the back by using a harness seems to be the best option currently available. However, recent literature warned against using this method in waders since it remains unknown how birds would cope with the harness when accumulating migration-related energy stores. We examined potential effects of an elastic leg-loop harness used for attaching geolocator backpacks in free-ranging Temminck’s Stints Calidris temminckii breeding in Norway. There were no indications that these devices hampered the birds’ locomotion while in the breeding area, no tagged birds deserted their nest and breeding success appeared normal. We found no significant difference in annual return rates between birds carrying geolocators (11/29; one additional bird had lost the geolocator) and colour-ringed birds (26/49) from a separate study in the same area 30 years earlier. This conclusion did not change when analysing sexes separately. Moreover, birds that returned did not differ from those that did not in body mass and body condition, and body mass of returning birds did not differ in the two breeding seasons. In returned birds that were successfully re-trapped (n = 9) we found no evidence for geolocators and harnesses causing injuries or abrasion to the skin, but in most individuals back feathers were abraded in a restricted area where the geolocator had been positioned. Although these results are positive with regard to the potential for using leg-loop harnesses to attach geolocators to small waders, effects may vary in relation to the strategy of pre-migratory fat accumulation in experimental birds. More studies are needed to evaluate this hypothesis.