Wintering, rather than breeding, oceanic conditions contribute to declining survival in a long-distance migratory seabird
Steep declines in Arctic skua populations in the southern extent of their breeding range have been reported during the last half of the 20th century. We used 24 years of ringing and re-encounter data from the Faroe Islands, North Atlantic, to investigate if patterns in survival probabilities can be explained by large scale climatic events. Having first determined the migratory phenology and wintering regions, we estimated the effects on survival of the North Atlantic Oscillation (NAO) index during breeding and Oceanic Nino index (ONI) during the non-breeding period within a capture-mark-recapture framework. Temporal trends, and direct and time-lagged effects of the environment on annual survival were modelled. We found support for a substantial decrease in adult annual survival, from ca. 0.93 in 1985 to ca. 0.77 in 2008, and weak support for a decrease in young (1st year) survival over the duration of the study period. Furthermore, models indicated increased young survival following an El Nino winter. We suggest this time-lagged effect reflects downstream propagation of environmental conditions, particularly food availability, or a potential carry-over effect of El Nino conditions positively impacting the performance of the parents in the subsequent breeding season, leading to improved young survival prospects. While adult mortality cannot be attributed to the oceanic climate oscillations tested here; the negative trend in survival may account for the substantial population declines observed over the last decades.