In North America, 20 common bird species have suffered population declines of over 50% in the last 40 years in response to changes in human land-use and climate.
In 2007, the National Audubon Society (BirdLife in the US) published the results of the first-ever analysis of combined annual sighting data from the two major censuses of bird populations in North America—the Christmas Bird Count (CBC) and the Breeding Bird Survey (BBS) (Butcher and Niven 2007, Butcher 2007). The analysis highlighted the plight of 20 common bird species, all of which have lost over half of their continental population since 1967. Common birds are defined here as species with a global population of over 500,000 and with a range of one million square kilometres or more.
Northern Bobwhite Colinus virginianus has declined the most dramatically, with population reductions of 82% occurring over the past 40 years. Consequently this species has been uplisted from the Least Concern category to Near Threatened on the IUCN Red List (BirdLife International 2008). Other widespread species found to have suffered significant declines include Evening Grosbeak Coccothraustes vespertinus (78%), Northern Pintail Anas acuta (77%) and Boreal Chickadee Parus hudsonica (73%).
The dramatic population declines are largely attributed to the loss of valuable habitats such as grasslands, forests and wetlands through a range of threats including infrastructure and housing development, energy development (e.g. mining or drilling for fossil fuels) and the spread and intensification of agriculture. Climate change is also a significant threat, especially for those species that breed in the tundra of Alaska and northern Canada such as Snow Bunting Plectrophenax nivalis and Greater Scaup Aythya marila. Both these species have declined by 64% and 75% respectively since 1967, partly due to the progressively earlier thawing of the permafrost leading to changes in tundra vegetation and stimulating the northward expansion of formerly southern species.
BirdLife International (2008) Common birds are declining in North America. Presented as part of the BirdLife State of the world's birds website. Available from: . Checked: