Climate change is expected to result in climatic zones moving poleward and an associated shift in the geographic ranges of bird species. A suite of studies in the northern hemisphere are now demonstrating such distributional shifts.
Climate change is expected to result in climatic zones moving poleward and an associated shift in the geographic ranges of bird species. Evidence for such shifts is now becoming apparent. It remains difficult to causally link such distributional shifts specifically to climate change as a wide range of additional factors also influence species distributions. However, thanks to recent research in Europe and North America, a remarkably consistent pattern is emerging that provides compelling evidence that climate change is responsible for significant northward shifts in the distribution of avian communities across the northern hemisphere.
In the UK, Thomas and Lennon (1999) investigated changes in bird species distribution by comparing two breeding bird atlases (1968–72 and 1988–91). They showed that southern birds had shifted their mean northern range boundary 18.9 km northwards. In contrast, there had been no southward range expansion in northern species, suggesting that climate change and not other factors, such as land-use changes or population growth, was responsible. A study in Scandinavia used a similar method to examine two Finnish bird atlases (Brommer 2004). It found that breeding birds in Finland had moved their northern range limit by about the same amount (18.8 km) in roughly half the time (twelve years). Again there was no significant southward shift in the distribution of northern species. The poleward range expansions revealed in these European studies coincided with the most rapid period of global warming in 10,000 years.
Using data from the American Breeding Bird Survey (BBS), Hitch and Leberg (2007) investigated whether a similar pattern had occurred in North America. They found that the northern limit of a subset of birds with a southern distribution had shifted 61.1 km northwards over a 26-year period. As in Europe, there had been no change in the southern boundary of northern species. Another study analysing data on the winter distribution of 254 North American bird species between 1975 and 2004 found a poleward shift in the mean northern range boundary of 44.4 km (La Sorte and Thompson 2007). It also revealed that similar trends were occurring within the interior of species’ ranges; with northward shifts in both the centre of occurrence (13.5 km) and the centre of abundance (30.9 km).
Taken collectively, these studies provide compelling evidence that climate change has already resulted in significant poleward range shifts in numerous bird species. It seems highly unlikely that such consistent trends, reported across two continents, could be satisfactorily explained by factors other than climate change. Given that to date temperature rises have been relatively modest, these findings suggest that future global warming will result in substantially altered avian communities. Although there are likely to be ‘winners and losers’ amongst bird species, those found in polar regions or at high altitudes are likely to experience significant range contractions as a result of continued rising temperatures. Species with poor dispersal abilities or highly specialised ecological requirements will be particularly at risk.
BirdLife International (2008) Climate change is driving poleward shifts in the distributions of species. Presented as part of the BirdLife State of the world's birds website. Available from: http://birdlife.org/datazone/sowb/casestudy/171. Checked: 23/07/2014
|Key message: Climate change is already affecting birds in diverse ways|