There are many examples of the effects of climate change on birds from around the world, which taken together, provide compelling evidence that climate change is already affecting birds in diverse ways. It is these proximate responses that drive the ultimate impacts of climate change on species—the significant changes to ranges that will be catastrophic for many species. Examples of responses include:

Distribution and density

• In Costa Rica, lowland and foothill species such as Keel-billed Toucan Ramphastos sulfuratus have extended their ranges up mountain slopes to at least 1,540 m in response to elevated cloud-base levels between 1979 and 1998 (Pounds et al. 1999).
• In the UK, breeding birds extended their ranges north by 19 km on average between 1968 and 1988, in association with increasing temperatures (Thomas and Lennon 1999).
• In the USA, the number of Sooty Shearwater Puffinus griseus found off the west coast during the non-breeding season declined by 90% between 1987 and 1994. The decline was attributed to changes in ocean surface temperatures and ocean currents, associated with climate change. It appears unlikely that the birds simply moved to new feeding grounds, because the declines have occurred over huge areas (Veit et al. 1997).
• In Germany, the proportion of long-distance migrants decreased and the number and proportion of short-distance migrants and residents increased between 1980 and 1992. This may be because increased winter temperatures benefit residents and increase the competitive pressure they impose on long-distance migrants (Lemoine and Bohning-Gaese 2003).

• In the UK between 1971 and 1995, 63% of 65 breeding bird species tended to nest earlier—by nine days on average for those showing significant trends (Crick et al. 1997)
• In Europe, the migration of birds that winter south of the Sahara has advanced by an average of 2.5 days in the last 40 years, possibly so that they can cross the Sahel before the seasonal dry period. By contrast, migrants wintering north of the Sahara have delayed autumn passage by 3.4 days on average over the same period (Jenni and Kéry 2003).
• In New York and Massachusetts, USA, migrants arrived significantly earlier during the period 1951–993 than 1903–950. From 1951 to 1993, birds that winter in southern USA arrived on average 13 days earlier, while birds wintering in South America arrived four days earlier (Butler 2003).
• In the UK, increasing spring temperatures over the past decades have led to changes in vegetation phenology. The food supply for Great Tit Parus major chicks now peaks earlier. However, egg-laying by the tits has not advanced, presumably because the cues to which tits respond during reproductive decision-making have not shifted in synchrony with changes in vegetation phenology. Hence, there is now a mismatch between food supply and timing of breeding (Visser et al. 1998).

In Finland, breeding Pied Flycatcher Ficedula hypoleuca laid progressively larger eggs through the period 1975–1993, correlating with rises in mean spring temperatures. Since larger eggs enjoy improved hatching success, global warming may allow females to alter their reproductive strategy and invest more resources in reproduction (Jarvinen 1994).