Foraging movements of Leach's storm-petrels Oceanodroma leucorhoa during incubation

Knowledge of foraging movements during the breeding season is key to understanding energetic stresses faced by seabirds. Using archival light loggers (geolocators), a Bayesian state-space model, and stable isotope analysis, we compared foraging movements of Leach's storm-petrels Oceanodroma leucorhoa during their incubation periods in 2012 and 2013. Data were collected from two colonies, Bon Portage Island and Country Island, which are 380 km apart along the coast of Nova Scotia, Canada. Based on allometry for procellariiform mass, predicted foraging ranges for Leach's storm-petrels are 200 km; however, observed maximum distances from the colony were 3 to 5 times that. Storm-petrels from Country Island travelled 1015 ± 238 km southeast to the Laurentian fan and south of the Grand Banks whereas storm-petrels from Bon Portage Island travelled 613 ± 167 km southeast, beyond the continental slope, east of Georges Bank. The average distance travelled in a return trip was 2287 ± 603 km and 1303 ± 351 km for Country Island and Bon Portage Island, respectively. There were no differences between years in cumulative distances travelled within islands, but foraging trips did not last as long in 2013 (4.7 ± 1.5 d) as they did in 2012 (6.2 ± 2.1 d). Stable isotope analyses indicated that, during the incubation period, prey items from Country Island were from higher trophic levels and possibly had higher energy content than those from Bon Portage Island, perhaps explaining the more distant and longer foraging trips for Country Island birds.