Loss of wetlands and spreading urbanization in eastern and central North America are blamed for the dramatic decline over the last 40 years in the number of nesting birds. Bird populations from the Atlantic flyway dropped by 12% and the Mississippi flyway by 18% between 1966 and 2004.

The declines were largely among birds that remain in Canada and the US rather than those migrating to the tropics for winter. The total number of birds flying from Canada and the US to Mexico and beyond for winter did not decrease. Indeed, 20% more birds than before are travelling to Central America and northern South America. Meanwhile, there are 19% fewer birds that do not migrate and 30% less birds migrating within Canada and the US.

Not all species became less abundant, however, and many actually increased their numbers. Among those species that stay within Canada and the US, 38% of the populations expanded significantly, including mourning dove, Carolina wren and Canada goose. The 21% of species, 23 in all, experiencing significant losses include common grackle, red-winged blackbird and eastern meadowlark. Among the bird species wintering south of the US, 51% increased in number, while 25% decreased.

The reductions are concentrated among species associated with certain habitats. For those species that prefer open habitat such as agricultural land, the total number of birds confined to the US and Canada dropped by half over the 38 years. These include rock pigeon, common nighthawk, gray partridge and greater prairie chicken. As well, there are 22% fewer neotropical migrants using open habitat.

Other heavy losses occurred among birds staying year-round in Canada and the US that occupy wetlands and coastal marshes, amounting to a 34% decline. As well, the number of birds frequenting edge habitats and remaining north of Mexico, such as blue jay, yellow-shafted flicker and European starling, declined by 13%.

On the other hand, birds mainly living in forests increased significantly since 1966. There are 63% more resident forest birds including pine warbler and pileated woodpecker, and 37% more neotropical migrants using forests. The population boom may be due to forest cover expanding in areas such as New England. Meanwhile, tropical deforestation seems to not have dampened the number of forest birds migrating in spring to the US and Canada.

The study, which analyzes counts from North American Breeding Bird Surveys, leads researchers to conclude that habitat loss in North America, but not in Mexico, Central and South America has caused substantial reductions in the number of birds that rely on wetlands, open lands and edge habitats. The scientists note that the particularly large decline in birds that do not migrate south of the US points to land development in temperate, but not tropical regions, as the culprit harming breeding birds. Urbanization and draining of wetlands has progressed rapidly in eastern North America, to the point that some bird species have felt the effects to an alarming extent.