Using geolocator data to reveal incubation periods and breeding biology in Red Knots Calidris canutus rufa

Shorebirds that nest at low densities in the Arctic are difficult to study. Information on breeding phenology, incubation period, and incubation outcomes can aid an understanding of population dynamics, particularly for species whose populations are declining. We describe a technique for using data from light-archival geolocators placed on Red Knots Calidris canutus rufa to determine information about breeding, and report on theproportion of knots that reached Arctic breeding grounds, attempted incubation, and appeared to incubate to full term, as well as total time spent in the Arctic. We captured 19 knots that were fitted with geolocators in New Jersey, Massachusetts, Florida, Texas, and S Argentina, and reached Arctic breeding areas. The median arrival date in the Arctic was 10 June, the median departure date was 22 July, and the mean time in the Arctic was 44 ±2.3 days (range 28–65 days). Because of 24 hr sunlight in the Arctic summer, prolonged periods of a mainly dark signal indicate that the geolocator was not exposed to light, and we inferred that the bird was incubating. On the basis of this assumption, we found that 85% initiated incubation (N = 20), 65 % (N = 17) incubated for 18–24 days, and one incubated for 30 days. Geolocator output showed that knots rarely entered salt water while in the Arctic. All the knots for which we obtained data reached their Arctic breeding grounds and a high proportion incubated nests.