Over half of the Neotropical migratory birds in North America have suffered substantial declines over the past 40 years, particularly since the 1980s, for largely uncertain reasons.
In the Americas, declines in landbird populations have been reported from studies of individual species, geographical areas and migration sites, and also from the results of continent-wide monitoring surveys. Using data from the North American Breeding Bird Survey (BBS), Robbins et al. (1989) examined the population trends of migratory and resident bird species in North America between 1966 and 1987. They found that, over this 22 year period, a general decline in the abundance of Nearctic–Neotropical migratory bird species (those birds breeding in North America and wintering in Latin America) had occurred in the east of the continent. Worryingly, over just ten years in the second half of the study period (1978–1987), 44 (71 %) of 62 Neotropical migrants exhibited substantial declines. Subsequent re-analyses of the BBS data (Sauer and Droege 1992, Peterjohn et al. 1995) have confirmed that the rate of decline of Neotropical migrants was pronounced in eastern North America during the 1980s, exceeding that documented in both the central and western regions of the continent.
Recent BBS census data have provided a longer time-series of population trend data, allowing the conclusions of Robbins et al. (1989) to be revisited. There are worrying indications that the declines in Neotropical migrant abundance seen in the 1980s have continued to the present day, and have spread in their geographical extent. Between 1980 and 2006, 62 % of Neotropical migrants in the eastern BBS region have exhibited negative population trends, while in the western BBS region, an area not previously recognised for its dwindling migrant populations, 63% of migratory bird species were categorised as declining. Over the entire 40 years of BBS data (1966 to 2006) 57 % of Neotropical migrants across the whole of North America have suffered from population declines (Sauer et al. 2007; also summarised in Kirby et al. 2008).
The reason for these long-term declines in migrant species is unclear. Many of the affected species are forest birds and forest fragmentation in breeding areas has been shown to be an important factor. However, tropical deforestation in the non-breeding areas of Central America and on the Caribbean islands may also be important.
BirdLife International (2008) North American monitoring schemes are revealing declines in migratory species. Downloaded from http://www.birdlife.org on 16/02/2019