Bird diversity and environmental gradients in Britain: a test of the species-energy hypothesis

1. We tested the species diversity-energy hypothesis using the British bird fauna. This predicts that temperature patterns should match diversity patt erns. We also tested the hypothesis that the mechanism operates directly th rough effects of temperature on thermoregulatory loads; this further predic ts that seasonal changes in temperature cause matching changes in patterns of diversity, and that species' body mass is influential. 2. We defined four assemblages using migration status (residents or visitor s) and season (summer or winter distribution). Records of species' presence /absence in a total of 2362, 10 x 10-km, quadrats covering most of Britain were used, together with a wide selection of habitat, topographic and seaso nal climatic data. 3. We fitted a logistic regression model to each species' distribution usin g the environmental data. We then combined these individual species models mathematically to form a diversity model. Analysis of this composite model revealed that summer temperature was the factor most strongly associated wi th diversity. 4. Although the species-energy hypothesis was supported, the direct mechani sm, predicting an important role for body mass and matching seasonal patter ns of change between diversity and temperature, was not supported. 5. However, summer temperature is the best overall explanation for bird div ersity patterns in Britain. It is a better predictor of winter diversity th an winter temperature. Winter diversity is predicted more precisely from en vironmental factors than summer diversity. 6. Climate change is likely to influence the diversity of different areas t o different extents; for resident species, low diversity areas may respond more strongly as climate change progresses. For winter visitors, higher div ersity areas may respond more strongly, while summer visitors are approxima tely neutral.