Abstract
The unusually high quality of census data for large waterbirds in Europe facilitates the study of how population change varies across a broad geographical range and relates to global change. The wintering population of the greylag goose Anser anser in the Atlantic flyway spanning between Sweden and Spain has increased from 120 000 to 610 000 individuals over the past three decades, and expanded its wintering range northwards. Although population sizes recorded in January have increased in all seven countries in the wintering range, we found a pronounced northwards latitudinal effect in which the rate of increase is higher at greater latitudes, causing a constant shift in the centre of gravity for the spatial distribution of wintering geese. Local winter temperatures have a strong influence on goose numbers but in a manner that is also dependent on latitude, with the partial effect of temperature (while controlling for the increasing population trend between years) being negative at the south end and positive at the north end of the flyway. Contrary to assumptions in the literature, the expansion of crops exploited by greylag geese has made little contribution to the increases in population size. Only in one case (expansion of winter cereals in Denmark) did we find evidence of an effect of changing land use. The expanding and shifting greylag population is likely to have increasing impacts on habitats in northern Europe during the course of this century.
Highlights
Global warming is unequivocal: the mean surface temperature of the Earth has increased about 0.85°C since 1880, when long-term recording started at multiple sites [1], and there is high confidence that the average annual temperatures in the Northern Hemisphere over the period 1983–2012 have been the warmest for the last 800 years [1]
There is ample evidence of the ecological impacts that this rise in temperature has had on range shifts to keep up with climate change [2,3,4]
Milder winter conditions due to climate warming may allow birds to remain near to the breeding grounds during winter
Summary
Global warming is unequivocal: the mean surface temperature of the Earth has increased about 0.85°C since 1880, when long-term recording started at multiple sites [1], and there is high confidence that the average annual temperatures in the Northern Hemisphere over the period 1983–2012 have been the warmest for the last 800 years [1]. In the Northern hemisphere, migratory birds usually fly long distances between breeding and wintering grounds, spending the winter at lower latitudes, taking advantage of seasonal changes in food availability and day length [5]. Milder winter conditions due to climate warming may allow birds to remain near to the breeding grounds during winter. A pattern of colonization from lower to higher latitudes so as to occupy the newly available habitats may be expected. The main potential advantages of wintering near the breeding grounds are to avoid the mortality associated with migration, to arrive earliest and in better condition at the breeding grounds, and to occupy the highest quality habitat, enhancing reproductive success [6,7,8]. The main disadvantage is a high thermoregulatory cost as a consequence of more unfavorable winter conditions and sudden changes in availability of resources (e.g. due to snow fall) [9,10]
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