Data gathered over decades by the thousands of volunteers who participate in the North American Breeding Bird Survey have yielded a vivid portrait of trends in the abundance of birds in eastern North America. In an article in the April 2007 issue of BioScience, Ivan Valiela, of the Boston University Marine Program, and Paulina Martinetto, at Argentina's Universidad Nacional de Mar del Plata, describe how they analyzed data from 40 years of observations made by volunteers who each year drive set routes. Valiela and Martinetto classified the observations by species' habitat preference and migratory habit.
Although the total number of birds recorded as nesting in the eastern and central United States has steadily decreased, the losses were heavily concentrated among species that either resided or migrated within the United States and Canada. The numbers of those that migrated farther south after the nesting season mostly did not decline, and some Neotropical migrants increased in abundance. The trends varied, however, between birds preferring different habitat.
The North American Breeding Bird Survey is susceptible to a variety of possible biases. For example, new road construction since the 1960s may have some bearing on the number of birds that volunteers notice. Yet the differences in how the number of observations changed over time for birds preferring different habitats add up to a strong case for further research.
The researchers found that decreases were especially common among birds preferring open, edge, and wetland habitat, a fact they tentatively ascribe to the spread of industrial, suburban, and other human-affected land cover in North America--loosely, "urban sprawl." Forest-loving species, in contrast, often increased in abundance, an observation that finds a likely explanation in the expansion of northern forests during much of the 20th century.
The increases among forest-loving birds were most pronounced, however, among species that migrate south of the United States--a surprise, given the well-known loss of Neotropical forests. Likewise, numbers of open-habitat birds that migrate south for the northern winter did not, by and large, increase as a result of the growing expanse of pasture in the Neotropics. A possibly related surprise is the relative lack of declines among wetland-loving birds that migrate south of the United States.
All told, the patterns prompt Valiela and Martinetto to suggest that alterations in the northern part of the ranges of migrant birds dominate over the effects of changes further south, an effect that is so far unexplained.
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