The Effect of Climate Change on Birds

by David Leech

1. Overview

• Mean global temperatures have risen significantly over the last two centuries. Further increases are predicted during the next 100 years as the concentration of greenhouse gases in the Earth's atmosphere continues to rise. Global warming will not only influence temperatures, but may also affect other climatic variables due to shifts in patterns of atmospheric and oceanic circulation

• Using long-term data sets, such as those collected by the BTO, relationships between survival rates, breeding success, abundance, distribution and phenology of bird species and a range of climatic variables can be investigated. Once established, these relationships may then be used to predict the effects of future climatic change on avian populations.

• The survival rates and breeding productivity of many British bird species are related to ambient temperature and rainfall, either directly through increases in energetic demands imposed on individuals, or indirectly through changes in food availability or the incidence of disease.

• Changes in survival rates and/or breeding productivity of a population due to climatic change may affect species abundance, but this will depend on the degree to which other factors controlling the size of the population are density-dependent.

• Climatic change may also influence the distribution of bird populations due to changes in settlement patterns favouring areas of increased quality or to local extinctions resulting from habitat deterioration. Species particularly affected by such changes will be those inhabiting the polar regions that cannot move to higher latitudes in response to climatic amelioration, and high altitude species unable to retreat further upslope. Increasing temperatures may also lead to sea level rises as polar ice sheets begin to melt, potentially resulting in the submergence of areas of lowland coastal habitat such as mudflats and salt marshes.

• Increasing global temperatures may also influence the phenology of avian populations in temperate areas as milder springs lead to the seasonal advancement of leaf, and therefore caterpillar, emergence. In general, the dates at which birds commence egg laying and the dates at which migrant birds arrive in Britain to breed in spring are becoming progressively earlier, enabling populations to track changes in the timing of prey species availability. However, advancement of both laying and arrival dates may be constrained by energetic demand or by unreliable environmental cues, causing a lack of synchrony between peak offspring demand and peak food availability, which may reduce breeding success.

• In addition to the continuation of long-term population monitoring under the Common Birds Census (CBC), the National Ringing Scheme and the Nest Record Scheme, the BTO has recently implemented two additional studies, the Retrapping Adults for Survival (RAS) scheme and Migration Watch, which will provide more detailed information on the influence of climatic effects on survival rates and the timing of migration respectively.

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