Wintering, rather than breeding, oceanic conditions contribute to declining survival in a long-distance migratory seabird

Steep declines in Arctic skua populations have been reported during the last half of the 20th century in the southern extent of their breeding range. We used 24 years of available ringing and re-encounter data from the Faroe Islands, North Atlantic, to determine if patterns in survival probabilities can be explained by large scale climatic events. Having first determined the migratory phenology and wintering regions, we tested the North Atlantic Oscillation (NAO) index during breeding and Oceanic Niño index (ONI) during the non-breeding period within a capture-mark-recapture framework to model direct and time-lagged effects of the environment on annual survival. We found differential effects in the two age-classes examined: young and adults. Overall, three models were equally supported. We found strong support for a substantial decrease in adult annual survival over the study period, from ca. 0.93 probability of survival in 1985 to ca. 0.77 in 2008, and support for a decrease in young survival over the duration of the study period. Furthermore, we found support for increased chick survival following an El Niño winter. We suggest this reflects a potential carry-over effect of El Niño conditions positively impacting the performance of the parents in the subsequent breeding season, leading to improved chick survival prospects. The negative trend of adult survival cannot be attributed to the oceanic climate oscillations tested here; however, this result may account for the substantial population declines observed during the last decades.