One approach was to investigate how the proportion of warmth-loving and cold-loving species has changed in Sweden. To start with, all bird species in the study were classified in relation to the average temperature of the species’ breeding distribution in Europe (STI, Species Temperature Index). The Siberian tit, for example, bredding in northernmost Europe only, get a STI of +6.9° C. It is a ”cold” species. The Goldfinch, with a STI of +15.3° C, is a ”warm” species. The average STI of all birds in a given site is the ”Community Temperature Index” (CTI). The CTI was calculated for a large number of sites around Sweden and compared over years (Lindström et al. 2012). Generally, CTI increased over time at most sites in Sweden, in parallel to increasing summer temperatures. Clearly, as Swedish summers get warmer, we get more and more warmth-loving birds. This is primarily because the “warm” birds of southern Sweden become more common and also move northwards, and to some extent that “cold” birds of northern Sweden retreat northwards.
However, the study also showed that, while a given summer temperature moved about 300km north during 1975-2009, the bird communities only moved 100 km north – thus lagging behind about 200 km! The birds are not moving as fast as the climate. Although the long term effects of this is not yet known, a potential problem could be that the birds get out of phase with important resources, such as food and habitat. A similar study on birds in Europe (Devictor et al. 2012), where Swedish data were included, showed a similar trend. It also revealed that butterflies were much better in following the changing temperature, possibly due to their much shorter life span.
Another European study, using data from the Swedish bird survey, showed that habitat generalists are doing better than habitat specialists (LeViol etal. 2012). This trend was visible throughout all of Europe, but most apparent in Sweden.
Based on several different forecasts of climate and landscape changes, the Swedish Bird Survey data also formed the basis of detailed scenarios on how the Swedish bird population will be distributed in 2050 (Jiguet et al. 2012). These scenarios show that the population changes the last decade follow the pattern of the predicted bird distribution in 2050!
Even though there are other factors affecting bird numbers, such as land use, the above studies clearly reveal that current climate change affects the Swedish bird populations here and now.