No strong effects of leg-flagged geolocators on return rates or reproduction of a small long-distance migratory shorebird

Abstract

Small light-level geolocators have revolutionized research on avian migration and breeding ecology. However, proper evaluations of their impact on the life history of individuals compared to control individuals that experience the same conditions are still rare. Geolocator effects may be species specific and depend on the type of mounting, sex and size of individuals. While geolocators have been used extensively and without negative effects on large shorebirds, relatively little is known about their effects on small shorebirds, especially of those attached on leg-flags. We mounted 30 leg-flagged geolocators (15 on each sex) on Southern Dunlins (Calidris alpina schinzii) - a small, long distance migratory shorebird (40-52 grams) - and examined the effects of geolocators on return rates and reproduction through comparisons to a control group. The whole attachment weighed 1.5-2% of an individual's body mass. We found no evidence of lowered return rates. Out of 30 birds, 22 (73%) returned from both groups. Returning birds had similar breeding probability, timing of breeding, clutch size and nesting success. The proportion of unhatched eggs was higher in the geolocator group, but this difference was not significant. Inspection of unhatched eggs from the treatment group suggested no clear damage to eggs caused by geolocators. Our results suggest that at least one small wader species can withstand the extra weight imposed by appropriately sized geolocators. However, our study lasted only for one year, and long term evaluations that capture the full suite of environmental conditions and assess impact on brood care are needed.