Later breeding in northern gannets in the eastern Atlantic

Species with breeding distributions spanning a broad latitudinal range typically experience a correspondingly wide range of environmental conditions, and may also be subject to temporal changes in conditions operating either across their range or more locally. Phenological records are potentially useful for elucidating how species adjust to this spatial and temporal variation, particularly in the context of responses to climate change, and have been widely used in studies of terrestrial birds. In contrast, despite the fact that many marine environments are also changing markedly as a result of climate warming, studies of the phenology of seabirds are comparatively rare. We used data from a wide range of sources to examine trends in breeding time in relation to latitude (49 to 62°N) and year (1980 to 2007) in the northern gannet Morus bassanus at 17 colonies in the eastern Atlantic. We found significant spatial and temporal effects but no significant interaction between latitude and year, indicating a consistent shift in breeding time at gannetries in the central and southern parts of the breeding range between 1980 and 2007. On average, median hatch date became 1.98 d later for every 1° latitude shift north, such that breeding in the Faeroe Islands was 26 d later than in northern France, and breeding was delayed by 0.21 d yr-1, i.e. 6 d later in 2007 compared to 1980. There was no evidence that the trend towards later breeding was related to temporal changes in either local (late-winter sea surface temperature) or large-scale (winter North Atlantic Oscillation) ocean climate. We believe that this is the first evidence of a trend towards later breeding in a marine predator across a major part of its breeding range. More generally, our findings accord with other recent studies suggesting contrasting phenological responses between marine and terrestrial north temperate birds.