State of the world's birds

A thorough literature review has found that climate-driven changes—in distribution, phenology and interspecific interactions—have already been documented for over 600 bird species (BirdLife International unpublished data). Given the paucity of data available for South America, Africa and Asia, this figure is likely to be a considerable underestimation.

Climate is a critical factor in determining species’ geographical ranges (Pearson and Dawson 2003). Although it is difficult to causally link an observed shift in a single species’ range to changes in climate, the consistent patterns documented for avifauna around the world are compelling. To date, over 400 bird species globally have undergone range shifts that follow the predicted trajectory associated with climate change (BirdLife International unpublished data). Namely, as global temperatures have risen, species’ ranges have moved poleward in latitude and upwards in elevation. Taken collectively, these observations provide convincing evidence that climate change is already impacting the distribution of avian communities.

In North America, over 200 bird species have experienced northward range shifts consistent with climate change (e.g. La Sorte and Thompson 2007, Hitch and Leberg 2007, Zuckerberg et al. 2009). Similar findings have been reported in Europe (Thomas and Lennon 1999, Brommer 2004), where the largest climate change induced range shifts have been recorded (Maclean et al. 2008). However, despite this, the magnitude of these responses may still be insufficient to keep pace with climate change. In France, for example, the temperature increase since 1989 is equivalent to a northward shift of 273 km; however, over the same period there has only been a 91 km northward shift in bird community composition (Devictor et al. 2008). Effectively, birds are lagging behind climate warming, and the long-term implications of this discrepancy could be profound.

Climate warming has had a notable effect on the temperate seasons—with markedly earlier springs and a delayed onset of autumn (Menzel et al. 2006, Schwartz et al. 2006). To date, a phenological response to these changes has been documented in around 300 bird species (BirdLife International unpublished data). In general, there has been a trend towards earlier spring arrival of migratory species (Butler 2003, Gordo and Sanz 2006, Jonzén et al. 2006, Tøttrup et al. 2006, Beaumont et al. 2006) and earlier breeding (Forchhammer et al. 1998, Crick and Sparks 1999). However, these phenological responses have not been consistent across taxa. Many bird species—having evolved to synchronise the timing of their annual routines (e.g. migration and breeding) with the life cycles of other species (e.g. predators and prey)—are becoming increasingly ecologically mismatched (Visser et al. 1998, Both and Visser 2001). For example, whilst the brood-parasitic Common Cuckoo Cuculus canorus has advanced its migration only modestly, many of its host species are now arriving at the breeding grounds considerably earlier (Saino et al. 2009). Those species unable to adequately respond to climate change may find themselves at a considerable disadvantage (Ahola et al. 2007, Møller et al. 2008).

Given that the rise in global average temperature has been relatively modest to date, the number of documented impacts on the world’s avifauna is sobering. It suggests that the impact of future climate warming on biological communities, and consequently ecosystem integrity, could be severe.