Bird Decline Tied to Deforestation

Populations of many species of migratory birds are rapidly and dramatically declining, and tropical deforestation may be the primary reason, according to scientists who have just completed an analysis of 20 years of bird surveys from across North America.

The severe declines of many songbirds, which spend their winters in South and Central America and their summers in the United States and Canada, have occurred in less than a decade, with some species diminishing by almost half.

“Something dramatic is happening,” said Russell Greenberg, a research scientist at the National Zoological Park who participated in the study. “I would call the decline severe and say it was of tremendous concern.”

Even some of the most common birds known to the casual back yard bird watcher have suffered a serious setback. In the past nine years, the wood thrush has been reduced by 30%, the Baltimore, or northern, oriole population has fallen by 23% and the scarlet tanager has declined by 10%.

“It’s tragic. We’re going to have fewer birds,” said Sam Droege of the U.S. Fish and Wildlife Service’s Patuxent Wildlife Research Center. Droege did the study with Greenberg and colleagues Chandler Robbins and John Sauer.

“It’s not at the point where we’re talking about extinction, but it should never get to that point,” said David Wilcove, an ornithologist and ecologist with the Wilderness Society.

The survey overcomes limitations that had led some to discount similar findings of earlier bird surveys. In recent years, a number of scientists have shown long-term declines in migrating songbirds, particularly those species that breed and raise their young in forests. But previous studies have been based on an examination of a small number of special plots that are surveyed for birds year after year. Some locales experienced more dramatic declines than others and some scientists had argued that these downward trends could be explained by the localized effects of development or increased urbanization.

One long-term study site, for example, is in Rock Creek Park in Washington. The site has experienced a 70% decline in migratory songbirds since the 1940s, a period that saw explosive development in the Washington area.

The current research by Greenberg and his colleagues is different from previous work because it is based on data gleaned from the North American Breeding Bird Survey. This annual survey covers North America from the Florida Keys to the North Slope of Alaska and involves hundreds of volunteers who survey 2,000 sectors. The bird watchers do the survey by car, driving about 50,000 miles on back roads, stopping precisely every half mile to record every bird they can hear or see in the three minutes alloted to each stop.

The extensive survey shows that the downward trends seen in fixed study sites are not just local phenomena but are happening across the entire continent, Greenberg said. “The size of the survey is extremely powerful,” he said. “A pattern emerges that you can believe.”

Scientists, however, have been debating vigorously the cause of the songbird decline. Are the birds being harmed, scientists have wondered, by changes in their summer homes in temperate North America or by changes to their winter grounds in the tropics? Many bird experts have argued that forest fragmentation in North America is most to blame since cutting the forests into smaller and smaller patches increases the access that predators and parasites have to the forest-dwelling birds.

But Greenberg and his colleagues argue in their study that the primary cause is more likely to be tropical deforestation.

“When you remove the habitat, you remove the birds,” Greenberg said. Their study has been accepted for publication in the Proceedings of the National Academy of Sciences.