The report reviewed data from the International Union for Conservation of Nature Red List, which is an inventory of the conservation status of the world's organisms. Analysis showed that 5,412 of bird species worldwide, or 49 percent, have declining populations, while 38 percent are stable, 6 percent are increasing, and 6 percent have unknown trends.

The report found that one in eight species is threatened with extinction, and also highlighted the massive scale of population loss in several well-surveyed areas. For example, there has been a net loss of 2.9 billion individual birds on the North American continent since 1970, and 600 million have been lost since 1980 across European Union countries.