Wintering, rather than breeding, oceanic conditions may modulate 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 yr 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 of the North Atlantic Oscillation (NAO) index on survival during breeding and the Oceanic Niño index (ONI) during the non-breeding period within a capture-mark-recapture framework. Temporal trends along with 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 (first year) survival over the duration of the study period. Furthermore, models indicated increased young survival following an El Niño winter. We suggest this time-lagged effect reflects downstream propagation of environmental conditions, particularly food availability, or a potential carry-over effect of El Niño 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.
