In addition to species of high tri-national concern due to high or severe threats, the PIF assessment identified 42 other common bird species whose populations have declined by 50% or more in the past 40 years (Appendix C). These species are found in nearly every habitat type, breeding primarily in Canada and the United States. It is likely that many common Mexican birds are steeply declining as well, but we lack long-term monitoring data for most Mexican species. Population declines of common species are an important indicator of deteriorating environmental health.
The combined loss of 42 steeply declining species is conservatively estimated at a staggering 800 million breeding birds, about two-thirds of those present 40 years ago. In total, more than half of the 882 landbird species show evidence of declines of at least 15%. This loss of bird abundance is especially troubling in light of the vital ecosystem services that these birds provide. To reverse population declines, we need to address the underlying causes of declines in every habitat, rather than manage for each species.
Declining species are found in every terrestrial habitat. Many of these species are migrants and depend on a variety of habitats throughout their lives. For example, tropical evergreen forests are important for residents as well as wintering populations of migrants.
Common birds are declining by 50% or more over much of the North American continent, with the largest number of species breeding in the northern United States and southern Canada (top). In winter, these species are concentrated in the southern United States and Mexico (bottom).
Several widespread speciesof tropical evergreen forest, including Blackfaced Antthrush (left), Gray-headed Kite, and Plain Xenops, are estimated to have declined by more than 50% in Mexico, based on the reduction of their primary habitat over the last halfcentury. Tropical habitats are also probably home to other common species in steep decline, but we lack long-term monitoring data to identify these species in Mexico.
Among the most steeply declining species in temperate forests are birds dependent on disturbed and early successional habitat, including Ruffed Grouse (left), Whip-poorwill, Rufous Hummingbird,and Prairie Warbler. Managing a mosaic of age classes of forests, as well as maintaining natural disturbance regimes such as fire, will be necessary to reverse declines of many forest birds.
Many common aridland birds, such as Loggerhead Shrike (left), Verdin, Rock Wren, and Brewer’s Sparrow, have lost more than half of their breeding populations over the past 40 years. Sagebrush, chaparral, and desert shrublands have been severely degraded and are threatened by the spread of exotic plants, energy development, and urban sprawl.
Grassland birds in this habitat have suffered among the steepest declines of any North American landbirds. These include many familiar birds of rural landscapes including Grasshopper Sparrow (left), Eastern Meadowlark, Bobolink, Lark Bunting, and Horned Lark. Incentives for bird-friendly agricultural practices and protection of native prairie are essential for reversing declines of grassland birds.
Steep declines are occurring in permanent residents such as Boreal Chickadee, temperate migrants such as Rusty Blackbird (left), and long distance migrants such as Wilson's Warbler. The current rate and extent of industrial resource extraction threatens the integrity of our continent’s boreal nursery. Yet vast areas of remaining boreal forest present large-scale opportunities for conservation.
and reducing bird mortality from manmade structures and pesticides will benefit generalist breeders and migrants.