Abstract from BTO Research Report No 369:

Leech, D.I., Crick, H.Q.P. & Rehfisch, M.M. (2004, published 2006)

The effect of climate change on bird species in the UK.
ISBN: 1-904870-85-6

Executive summary

1. Mean annual global temperatures have increased by 0.6°C since 1900, making the 1990s the warmest decade in the millennium. The Intergovernmental Panel on Climate Change (IPCC), in its 3rd Assessment report (2002), stated that "most of the observed warming over the last 50 years is likely to have been due to the increase in greenhouse gas concentrations" and went on to say that there was “new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities."

2. Temperature and precipitation levels can influence survival rates and productivity and the movement of individuals into and out of a population either directly, for example through their actions on the energetic costs of thermoregulation, or indirectly, for example by determining the availability or accessibility of food supplies.

3. Changes in survival rates, productivity and immigration/emigration rates may lead to changes in species abundance in some cases. However, density-dependent processes, such as competition for resources, may act to maintain numbers, such that they fluctuate about a certain level. For example, as the size of a population is reduced, the remaining individuals may benefit from the increased availability of food or nesting places, resulting in higher survival rates and/or greater productivity.

4. Responses of birds to climate change in the UK during the previous century have been well documented.

· Changes in sea temperatures have affected food availability for some seabird species, including Common Guillemot (Uria aalge) and Kittiwake (Rissa tridactyla), resulting in reduced breeding success.
· An increasing number of short-distance (intra-European) migrants from continental breeding populations, including many Blackcap (Sylvia atricapilla), are now wintering in the UK rather than moving to their more traditional wintering grounds in the Mediterranean Basin. An easterly and northerly shift in the distribution of waders such as Ringed Plover (Charadrius hiaticula) and Dunlin (Calidris alpina) wintering in Britain has also been demonstrated.
· Laying dates of a wide range of UK breeding species have advanced seasonally since the 1970s.
· Summer migrants have advanced their arrival time and, at least for short-distance migrants, may also have delayed their departure to the wintering grounds.
· Changes in the distribution of some European species, for example Black Redstart (Phoenicurus ochruros) and Cetti’s Warbler (Cettia cetti), may have resulted in their colonisation of the UK. Breeding ranges of UK avifauna have, on average, moved northwards since the 1970s.

5. Climatic models for the next 75 years predict that winters in the UK will become warmer and wetter, whilst summers will become warmer and drier, leading to a reduction in the moisture content of the soil. The frequency of extreme events such as storms, droughts and floods, is also predicted to increase. Sea level is predicted to continue to rise.

6. Climatic warming may increase over-winter survival by reducing thermoregulatory costs and increasing food availability for many terrestrial species. However, increased winter rainfall may reduce the survival rates of wader species, particularly first-year birds, and of terrestrial species such as Treecreeper (Certhia familiaris) that are particularly susceptible to wetting.

7. Warmer, drier summers may also increase productivity, particularly for insectivorous species and aerial predators that may benefit from the increasing abundance and/or activity of their prey species. However, the reduction in soil moisture content in summer may reduce food availability, and therefore productivity, for wader (e.g. Snipe (Gallinago gallinago), Curlew (Numenius arquata)) and passerine (e.g. Blackbird (Turdus merula), Song Thrush (Turdus philomelos)) species feeding on soil invertebrates.

8. Shifts in the North Atlantic Oscillation (NAO) are likely to influence oceanic currents and sea surface temperatures. These in turn will affect the abundance of prey species available to breeding seabirds in the UK.

9. Increasing spring temperatures are likely to result in the continued seasonal advancement of leaf, and therefore caterpillar, emergence dates. Further advancement of laying dates is therefore predicted. However, there is increasing evidence to suggest that constraints preventing sufficient advancement of laying dates may lead to increasing asynchrony between peak offspring demand and peak food availability for some species, significantly reducing productivity for species such as Great Tit (Parus major). Long-distance summer migrants such as Common Redstart (Phoenicurus phoenicurus) and Pied Flycatcher (Ficedula hypoleuca) may be particularly susceptible to such asynchrony if environmental cues used to initiate departure form the wintering grounds do not advance at the same rate as the phenology of the resources on the breeding grounds.

10. Northward and westward shifts in species distribution may lead to the colonisation of the UK by novel species from the continent, but may also lead to the loss of species restricted to breeding either at high altitudes (e.g. Ptarmigan (Lagopus mutus), Snow Bunting (Plectrophenax nivalis)) or in northern Scotland (e.g. Whooper Swan (Cygnus cygnus), Arctic Skua (Stercorarius parasiticus), Greenshank (Tringa nebularia)). Reductions in internationally important non-breeding wader populations resulting from an eastwards shift in the preferred wintering grounds might also be predicted for species such as Sanderling (Calidris alba) and Purple Sandpiper (Calidris maritima). Even under conservative scenarios, populations are predicted to decrease by up to 60%. Indeed, declines in populations of eight out of 14 common wader species have already been recorded. These distributional shifts may have important impacts on the effectiveness of current conservation policy in the UK.

11. Inter-tidal environments, such as mudflats and salt marshes, would naturally respond to increases in sea level by retreating inland. However, the presence of coastal defences and human habitation may prevent retreat in many areas, resulting in inundation and habitat loss. Other coastal habitats, such as reedbeds, may also be affected by increasing salinity. The consequences of these changes could be severe for breeding species such as Bittern (Botaurus stellaris) and wintering species such as Twite (Carduelis flavirostris). The creation of new habitat through managed retreat schemes may potentially increase the area of some inter-tidal habitats and therefore the size of the bird populations that they are able to support. This may be particularly beneficial in terms of global conservation, as internationally important numbers of waders and wildfowl utilise these habitats during the winter months.

12. Climate change outside the UK could have severe repercussions for migrant species. Reduced rainfall, particularly on the margins of the Sahara, and an increase in the incidence of extreme weather events may reduce food availability and therefore population sizes of long-distance migrants wintering in Africa such as Swallow (Hirundo rustica), Common Whitethroat (Sylvia communis) and Sedge Warbler (Acrocephalus schoenobaenus). Northward movement of temperate habitats in the high Arctic is likely to greatly reduce the availability of breeding habitat for many wader and geese species that winter in the UK, including Knot (Calidris canuta and Bean Goose (Anser fabalis), in the latter case by as much as 93%. However, warmer wintering conditions may permit more short-distance migrants to remain on, or close to, their breeding grounds (e.g. Chiffchaff (Phylloscopus collybita), Woodlark (Lullula arborea)), which may be beneficial for their survival rates.

13. Although most evidence for the influence of climate change on ecosystems is predominantly correlational, the complexity of the relationships reported and their consistency across regions and taxa suggest that the interactions observed are not the result of confounding factors such as habitat destruction.

14. There is increasing evidence to suggest that climatic change has already impacted significantly on the population dynamics and distribution of a wide range of taxa, with between 17% and 35% of species globally predicted to become extinct by 2050, dependent on levels of greenhouse gas emissions over the next 50 years. Bird populations in the UK are no exception. Although birds are a highly mobile group and are therefore theoretically able to move to areas in which conditions were more favourable, the rapidity of changes in climatic condition coupled with the effects of habitat destruction and other direct impacts of human activity may leave many unable to do so. While generalist species may be able to cope with rapidly changing conditions, species unable to adapt quickly are likely to decline in abundance or, in severe cases, to become extinct, leading to a loss in avian biodiversity.

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