The world is getting smaller, which is not necessarily a good thing if you are a bird.
Of the 485 species of animals that have become certifiably extinct since 1600, 116 of them -- nearly 25 percent -- are birds. Of the 9,500 or so listed as threatened by the World Conservation Union's Red List in 2000, more than 2,100 -- about 22 percent -- are birds.
Some of these extinctions are famous, such as the dodo and passenger pigeon. Some are relatively obscure, such as the Guam flycatcher and Carolina parakeet. Some are having near-death experiences with varying degrees of recovery success. The California condor, bald eagle and whooping crane come to mind.
Except for certain kinds of plants, bird species are showing the worst survival record in the natural world. This observation carries some caveats, however. The disaster rate may look higher because we know more about birds than we do about lots of other animals. People have been fascinated by birds for millennia and have tracked their biology closely. Today, birdwatchers are among the most avid and dedicated of conservationists, and among the best informed.
Despite this symphony of interest, many bird species remain on the edge of oblivion and increasing scientific evidence suggests habitat fragmentation is the culprit. The longer people have lived in a given place, more bird species sharing the same locale become extinct.
A study last summer by U.S. Geological Survey biologist Rosalind Renfrew and colleagues from the University of Wisconsin found North American grassland habitats are being decimated, killing off birds species. Many grassland species nest in the open. If they do not have large open areas available, they have decreased nesting success because they are more vulnerable to predators.
Animals that prey on these birds' nests prefer the woody edges of grassland -- opossums, raccoons, squirrels and snakes. When the large areas are carved up, it creates more "edges" for predators to live in. Because these areas are often carved up for housing developments or shopping malls, the effect tends to favor predators that do well around people, such as like raccoons, opossum and maybe coyotes.
The scientists concluded habitat fragmentation makes it easier for more predators to find the nests, resulting in declining nesting success.
Virtually the same results were found among southern songbirds. A 1998 study by Amber Keyser, a geneticist from the University of Georgia, found forest cutting was allowing "large predators like raccoons and opossums access to ground-nesting songbirds."
Keyser concluded some 20 percent of songbirds species are ground-nesters, from ovenbirds to black-and-white warblers. "Many are in decline," she wrote, adding as continuous forests are broken up, the trend creates "avenues in which predators are virtually funneled to nesting sites."
More bad news can be found in a study from Tom Brooks of the University of Arkansas and Lisa Manne and Stuart Pimm of the University of Tennessee. Pimm is the godfather of bird extinction studies. The trio found, contrary to long-held beliefs, island bird species no longer may be more vulnerable to extinction than their continental counterparts.
"Historically," they said, "a higher proportion of species extinctions have occurred on islands rather than on continents. So scientists have assumed that island species inherently are more vulnerable to the threat of extinction." Mann, Brooks and Pimm found the opposite was true: "Lowland continental areas have a higher proportion of threatened bird species within similarly sized breeding ranges."
The group concluded, "The added and unexpected vulnerability of range-restricted lowland bird species gives conservation organizations and governmental organizations another priority: To proactively protect these ecosystems now so bird species won't disappear later."
It probably is futile to suggest humans discontinue invading forests and prairies in order to save vulnerable bird species. But it may be by planning these invasions better, to preserve preferred habitat for birds and other species, we can help their long-term survival.
The work of University of Colorado biologist Carl Bock indicates the size and shape of open spaces may help reduce the vulnerability of species. Development that leaves large prairie-like open spaces, with a minimum of "edge" habitat to hide predators, may offer the best hope for lowland birds.
Birds are unlikely to disappear entirely in the modern extinction crisis, but the mix of species almost certainly will be affected if we fail to start paying attention to this issue. The "suburban gang of five" bird species -- robins, house sparrows, house finches, starlings and grackles, those birds that commonly appear at backyard feeders -- will out-compete native nesting birds such as meadowlarks, grasshopper sparrows, savanna sparrows, vesper sparrows, lark sparrows, bobolinks and horned larks.
It now seems -- to turn an old expression inside out and on its head -- every continent is an island.
Stuart Pimm told United Press International, "The Oxford English Dictionary says of the dodo" -- a former resident of the island of Mauritius -- "that it 'went extinct,' as if it was the dodo's own stupid fault." In fact, he said, the dodo met its fate at the hands of humans, and humans now are shrinking the continents.
"When you look around at what is happening on continents, a huge number of species are teetering on the brink of extinction," he said. "It is not just a matter of island species. The short-grass prairies have been hammered by human impacts."