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The planet's forests are shrinking, and it's playing out to the tune of massive species losses, a new study shows. And birds could be one of the biggest fatalities.

In the landmark study, published in , researchers across the globe used results from seven separate experiments carried out in five continents to posit that habitat fragmentation is rife. Ultimately, they found that 70 percent of global forests lie within just half a mile of their edges, exposing woodland species to human developments and agriculture. Even worse, almost 20 percent of forested land has just 100 meters (the equivalent length of a football field) to buffer it from the outside world. “That means almost no forest can really be considered wilderness,” said study leader Dr. Nick Haddad from North Carolina State University, in a .

When raveled ecosystems are whittled down, they're less able to support the species that comprise them. As a result, habitats around the world are losing 13 to 75 percent of their biodiversity. This study shows that forest ecosystems are particularly at risk of losing their flora and fauna.

In the United States, Լ����Ƶ and other conservation groups recently signed on to a campaign to shield the country's largest stretch of forest, the Boreal forest, from this very fate. The initiative seeks to protect the hundreds of bird species that are seasonal residents of this 3,500-mile-long tract of largely pristine forest that runs across from Alaska to the eastern seaboard. The habitat is vital to almost half of North American birds, 300 of which use it as a breeding base in the spring. “That Boreal forest is incredibly important, especially since the abundance of forest birds is at its peak there,” says Curtis Smalling, director of land bird conservation for Լ����Ƶ North Carolina. Logging and energy development are the biggest threats, causing millions of boreal acres to be cleared on an annual basis.

For birds, a football field’s worth of forest just doesn’t cut it: larger swathes promote breeding success and buoy bird populations. “In forests in particular, it's not just about their own particular territory, but a lot of species are also looking for a specific 'patch size',” Smalling says.

For instance, “if you have less than 70 percent forest cover at a landscape scale, Golden-winged Warblers don't like it.” The same is true for songbirds like Vesper Sparrows and Cerulean Warblers, who respectively seek territory that's at least 40 hectares and 200 hectares in size. There's also the entire “edge effect” to consider: The more habitat shrinkage that occurs, the more birds will be exposed to predators and contaminants lurking on the outskirts.  

Just as Haddad and his co-authors emphasized in the study, Smalling says it's important to look after fragmented habitat, even if it's not idyllic to wildlife. Smalling is part of the team that runs Լ����Ƶ's to protect birds in the eastern United States, which is mostly designated as a forest biome. “The effects of fragmentation are probably felt the largest here,” Smalling says. By training foresters and landowners to be bird-friendly, the team aims to uphold what’s left of forests and maximize their value to birds.

The new numbers on global habitat degradation may be shocking, but they also reveal what needs to be done, says Smalling. “Working to protect and conserve these big forest blocks is really critical and needs to continue. As for fragmented forests, “it really points to the fact that we shouldn't give up on those habitats.”