North America has lost nearly three billion birds since 1970, according to a new report, which also details widespread population declines among hundreds of North American bird species, including those once considered abundant. The results signal a long-developing yet largely overlooked biodiversity crisis occurring in avifaunal habitats across North America. Human impacts have contributed to an increase in global extinctions. Research focused on understanding extinction is underway, but much of it fails to recognize ongoing declines in "abundance" within still-common species, even as such declines can have significant ecological, evolutionary and economic impacts. "Given the current pace of global environmental change, quantifying change in species abundances is essential to assess ecosystem impacts," Kenneth V. Rosenberg and colleagues say. Evaluating these declines requires large and long-term datasets, which do not exist for most animals. However, long-term, detailed records do exist for bird populations. Using multiple standardized bird-monitoring datasets, Rosenberg et al. analyzed the net change over recent decades in numbers of birds for 529 species in the continental United States and Canada. Their results show a loss of nearly one in four birds since 1970 - a net loss of 2.9 billion birds. According to the authors, more than 90% of this loss can be attributed to 12 bird families, including songbird species like sparrows and warblers. However, not all species are on the decline; some bird species, including raptors and waterfowl, showed population gains - likely due to focused conservation efforts and Endangered Species legislation. Similar strategies for other species could avert the potential collapse of North American avifauna, the authors say. To expand their analysis, the authors used migration data from the NEXRAD radar network to estimate long-term changes in nocturnal migratory passage. The results, similar to those from the ground-based bird-monitoring datasets, reveal a steep decline for migrating birds over a recent 10-year period, particularly in the eastern U.S.