 Global warming and its impact on the world's birdlife
by Paul Stancliffe, British Trust for Ornithology
"It is no longer a question of whether the earth's climate will change but rather when, where and by how much." Robert T. Watson, Chairman of the United Nations Inter-Governmental Panel on Climate Change.
"Climate change will reduce, and force shifts in, the ranges of many species. Many will not be able to move fast enough, or in concert with other species. This will result in extinctions. Whether we lose a few species or huge numbers will depend critically on the degree of warming. We must act now to minimise this." Birdlife International
The world's wildlife is under pressure, that much we know, the human race is continuing to grow and expand across the planet, leading to an increase in the burning of fossil fuels, CO2 emissions and an acceleration of global warming. What impact will this have on the world's birds?
Mean global temperatures have increased by 0.6°C since 1900, and the rate of warming since 1976 has been greater than at any other time in the last 1,000 years. Indeed, historical data suggest that the 1990's were the warmest decade in the millennium, with 1998 the warmest year, at least in the Northern Hemisphere.
The projected global temperature increase is expected to be somewhere in the region of 1.4 to 5.8°C over the period 1990 to 2100. Elevated temperatures and levels of atmospheric CO2 have increased the length of the growing season in many regions; in Europe, leaf emergence has advanced by an average of six days between 1959 and 1963.
Changes in the seasonal timing of reproductive events have not been limited to plants - the emergence dates of some butterfly species and the spawning dates of some amphibians have been affected. A growing body of evidence suggests that the direct and indirect effects of global climate change may also influence the size, distribution and reproductive timing of some bird populations.
Several warbler species, most notably Blackcap and Chiffchaff, which traditionally winter in Iberia and Africa, have become increasingly frequent winter visitors to the UK over the past decade. There is good evidence to suggest that the majority of these are continental birds taking advantage of the increasingly warm winter conditions.
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Black-throated Blue Warbler, Yakima County, WA, Near Wapato October 17, 2005 © Denny Granstrand
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In a study of the Black-throated Blue Warbler in North America, it was estimated that 85% of the study population's annual mortality occurred during either the spring or autumn migration. By reducing the distance travelled between their wintering and breeding grounds, warblers wintering in Britain are not only likely to increase their probability of survival in transit, but also have the potential to arrive back on the breeding grounds before those birds that have wintered further south. It is also possible that a small proportion of the British warbler populations may even have ceased to migrate completely and become resident.
Analysis of arrival dates for summer migrants in the Western Palaearctic suggests that many species are arriving earlier on the breeding grounds. Using data from bird observatories, collected over the last forty years, a trend was identified towards earlier migration in seven of the 27 species studied, including Willow Warbler, Chiffchaff, Blackcap and Pied Flycatcher. Arrival date advancement has also been reported from North America, with migrants reaching their breeding grounds on average one day earlier for each 1°C rise in mean temperatures. In areas where temperatures have remained constant, no change in arrival dates has been observed, and in regions where temperatures have fallen, arrival has been delayed, suggesting that climate change is indeed playing a causal role.
Range shifts may also occur on a smaller scale than geographical expansion. The altitudinal limits of vegetation are broadly climatically determined, particularly in relation to thermal characteristics, and altitudinal shifts in these will affect the distribution of animals dependent on these habitats. The best demonstration of such effects comes from the montane rainforest of Costa Rica, where the average altitude of the cloud base has been raised through climate warming, resulting in the colonisation of previously cloud-forest areas by bird species from lower altitudes.
Coastal species are likely to suffer from rising sea levels, which are predicted to have a huge impact on lowland coastal habitats in the UK, and low-lying areas elsewhere. Increases in sea levels could result in the permanent inundation of mudflats with severe implications for both wintering and breeding wildfowl and wader species. Some areas, particularly those in the tropics, will face an additional threat in the form of elevated rates of erosion that have resulted from increases in the strength and frequency of winds and storm events.
Predicted climate change scenarios are likely to impact on birds in a number of different ways, these include:
· Increased storm frequency: this is likely to affect the productivity of species nesting in low-lying coastal areas, particularly on tropical islands.
For more information visit, http://www.bto.org/research/advice/ecc
· Lowered water tables: will reduce habitat availability for aquatic species, and reduce food availability for terrestrial species that forage in such areas. Additionally, such wetland areas provide critical staging habitat for many migratory species, and habitat loss may compromise their ability to complete their migratory journeys.
This will affect species such as Nightingale, to see the results of the BTO Nightingale survey visit, http://www.bto.org/survey/complete/nightingale.htm
· Higher drought frequency: will particularly affect those species that use seasonal wetlands, such as many waterfowl. Increased droughts are likely to extend the widths of deserts that present significant barriers to some migrants.
Visit http://www.bto.org/birdtrends2005/wcrsedwa.htm
· Sea level rise: will reduce habitat availability of species that nest or forage in low-lying coastal areas. This may be particularly problematic for migratory species as many important stopover areas are in such coastal habitat.
See www.bto.org/survey/webs
· Habitat shifts: changing climate will lead to shifts in vegetation zones; this will affect many bird species that nest and winter in such habitats. Arctic and high montane species, many of which are migratory, are the most likely to be affected in this way.
· Altered prey distribution: changing climate will impact on species that are prey for birds as much as birds themselves. Such changes have perhaps been best demonstrated for seabirds, reliant on nearby fish stocks in the breeding season.
There is an increasing body of evidence to suggest that the global climate is changing rapidly in response to the increased output of greenhouse gasses as a result of human activity over the last 200 years. These climatic changes have already impacted on the population dynamics and distribution of a huge range of taxa across a broad range of geographical locations and habitats, and are likely to have even more severe consequences over the coming century.
You can help to measure the effects of climate change by taking part in the British Trust for Ornithology surveys, for more information on how to get involved in surveys such as BirdTrack, the Breeding Bird Survey, Bird Atlases, the Nest Record Scheme, the Wetland Bird Survey or for further information on any of these, visit the BTO website.
www.bto.org
Or write to the: British Trust for Ornithology, The Nunnery, Thetford, Norfolk, IP24 2PU
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