The Effect of Climate Change on Birds

by David Leech

5. Influence of climatic change on species distribution

Climatic changes may lead to shifts in regional or global patterns of species distribution as conditions in some areas, become increasingly favourable in terms of winter weather conditions and food availability whilst those in other areas become less so. Large-scale changes in distribution are unlikely to be due to the movement of breeding individuals between years, but rather to changes in the settlement patterns of juveniles (Rehfisch and Austin, in prep) or to local extinctions of breeding populations.

• Burton (1995) observed that the breeding ranges of 71% of European bird species have changed since 1900, and suggested that climatic changes during the last century are likely to be responsible for the distributional trends of 44% of these species.

• Over the period 1972-1991, British bird species extended their breeding ranges north by an average of 18.9km in response to increasing mean annual temperatures at the northern extent of their distribution (Thomas and Lennon, 1999).

• Increasingly mild winters since the 1960s have resulted in an eastwards shift in the distribution of wintering waders in the UK (Rehfisch and Austin, in prep). Wintering on the east coasts reduces the distance over which the birds must migrate to their breeding grounds and also allows them to utilise the higher quality feeding grounds found there in the muddy estuaries, which contrast with the poorer quality sandier estuaries of the west coast.

• Brown (1970) suggested that range expansions of fulmars (Fulmarus glacialis) since the 18th century are linked to increases in eastern Atlantic temperatures.

Changes in the distribution of fulmars in the Atlantic may be due to increasing ocean temperatures.

• In the last 10 years, individuals of some species that previously moved to Ibeira and Africa during winter after breeding in the UK, such as blackcaps (Sylvia atricapilla) and chiffchaffs, have been observed to over-winter in Britain. It is generally considered that these individuals are birds from central Europe which have taken advantage of garden bird feeding and, perhaps, of progressively warmer British winters, and thus have avoided having to make the relatively longer journey to the traditional African wintering grounds further south.

Traditionally summer migrants, some blackcaps now winter in the UK.

• Continental species such as little egrets (Egretta garzetta), which were previously occasional visitors to Britain, now winter and breed in the UK. Although such shifts are highly suggestive of climate change, they have yet to be investigated in a rigorous scientific manner.

• The trend towards increasing global temperatures may have severe effects for species that breed in the Polar regions, including many British wader species. Whilst birds inhabiting lower latitudes may be able to extend their ranges into higher latitudes in response to increasing annual mean temperatures, this is not possible for species at the poles. Zockler and Lysenko (2000) predicted that the northward advance of boreal forests into tundra regions would lead to a reduction in the breeding range of Arctic species of between 5% and 93%, dependent on the species (figures given for sanderling (Calidris alba) and tundra bean goose (Anser fabalis) respectively).

The tundra habitat in which sanderling breed is threatened by the northwards advance of boreal forests in response to increasing global temperatures.

• High altitude species found in the Cairngorms and elsewhere in Scotland, such as dotterel (Charadrius morinellus) and snow bunting (Plectrophenax nivalis), may also experience loss of habitat as temperatures increase. Warmer temperatures may result in altitudinal extension of grass and heather moorland at the expense of the lichen-dominated montane habitat on which these species breed.

• The melting of the polar ice sheets as a result of increased global temperatures has led to a rise in sea level. This process has been exacerbated in the south of England by a gradual sinking of the land as the north of the country rebounds from the weight of the ice sheets that covered it during the last glacial period (a process known as isostatic change). Sea-level rises may lead to the loss of areas of lowland coastal habitat, including salt marshes and mudflats, as coastal sea defences and housing/industrial developments often prevent extension of these habitats inland (Norris and Atkinson, 2000). Southern England's salt marshes are also under increased pressure due to elevated rates of erosion as climatic change has lead to an increase in the strength and frequency of winds and storm events hitting the coast. Species dependent on these areas, such as breeding redshank (Tringa totanus) and wintering twite (Carduelis flavirostris), may decrease in abundance as the area of available habitat decreases.

A decrease in the area of salt marsh in the south of England due to sea level rises and increased rates of erosion may affect redshank abundance.