In much of the world, many of the familiar bird species that we value are in decline. These declines are taking place in both temperate and tropical regions and in a variety of different habitats, such as farmlands, forests and wetlands, alerting us to wider environmental problems. There are exceptions: some bird populations are stable and a few are increasing—a reflection of conservation efforts, but also because they can thrive in altered habitats.
The majority of the world’s bird species are not currently considered threatened with extinction, but even amongst relatively widespread and abundant species there may still be cause for concern. In Europe, the combined data from a number of national monitoring schemes shows an alarming overall decline in common farmland birds (). In North America, data from the Breeding Bird Survey, which has been monitoring population trends of more than 400 species for over 40 years shows similar declines in open-country birds in agricultural landscapes in eastern and central USA (). In 2007, a study combining Breeding Bird Survey and Christmas Bird Count data identified 20 common bird species which have lost over half of their populations since 1967, largely attributed to the loss of valuable habitats such as grasslands, forests and wetlands () Declines in seabird populations off southern California suggest that climate change may already be having a profound effect here, and by implication on marine systems around the world (). Analyses of the North America Breeding Bird Survey data also show substantial declines in long-distance migrants in the Americas (). Similar analyses of European data indicate declines in species that migrate between Europe and Africa ().
Declines in common birds are not just confined to Europe and North America. A worldwide review of trends in waterbird populations, for example, shows widespread declines, with Asia particularly affected (). However, most of the world lacks the extensive, long-term monitoring schemes found in Europe and North America so that it is necessary to estimate population trends using indirect information, such as national distribution atlases and species-mapping schemes. This is the case in Australia, where analysis of changes in bird distribution over 20 years shows declines in species richness in areas where there has been recent habitat clearance (). Interpretation of site-based census data and land-use changes also provide strong evidence that many common tropical and subtropical bird species are declining. For example, in a number of African countries, survey data indicate notable declines in the abundance of raptors over the past 30 years with increasing distance from protected areas (, ).
Not all bird species are declining. For example, some temperate forest birds in Europe and North America have shown positive trends over the past few decades, thanks to increasing tree cover in these areas and a shift to more sustainable forestry management practices. Some birds benefit from human activities and thrive with increasing urbanisation, spreading into new areas. The net result, however, is fewer, ‘weedy’ species overall.