That's because ordinary birds play a more vital role in their habitats than rare birds, so losing these species could reverberate through the ecosystem, said Blackburn, who was not involved in the report.
For instance, millions of white-rumped vultures once flocked Asian skies. But since surveys began in the 1990s, their numbers have sunk by 99.9 percent, in part from ingesting a deadly anti-inflammatory drug, called diclofenac, found in dead livestock.
Without these scavenging animals, the rotting carcasses they used to feed on may pose serious disease risks to humans, Blackburn said.
(See related: "Many Asian Vultures Close to Extinction, Survey Finds" [May 1, 2008].)
Another common species, the European turtle dove, has fallen 65 percent in the past 25 years.
And in Argentina the household pet market, combined with logging, has now endangered the once-widespread yellow cardinal.
Squeezing Out Biodiversity
Birds are also suffering as human demand for farmland intensifies—especially with a soaring demand for biofuels decimating bird habitat.
"Humans are co-opting more and more natural areas for our own uses," Blackburn said. "The more we do that, the less room there is for native biodiversity."
The collapse of the world's fisheries due to overfishing, for example, has devastated seabird populations that have lost their main food source.
(See related: "Still Waters, The Global Fish Crisis" in National Geographic magazine [April 2007].)
What's more, fishing industries have "severely knocked back" numbers of albatross, which drown when they try to grab bait from fishers' longline hooks and get pulled underwater, according to the BirdLife report.
The huge seabirds are long-lived and reproduce slowly, making them especially vulnerable, BirdLife's Bennun said.
Yet human-induced climate change "may be the biggest threat of all," the report said.
For instance, the azure-winged magpie may lose 95 percent of its habitat range in Spain and Portugal as warming temperatures displace populations northward.
Some conservation actions can be relatively simple, such as helping vultures by banning diclofenac for both human and animal uses, Blackburn, of the Zoological Society of London, said.
But most situations are complex, interwoven with human activities across the globe.
BirdLife recently created the Preventing Extinctions Programme, which relies on a ground-based, volunteer network of "species guardians" to keep tabs on the 180 critically endangered bird species worldwide.
These individuals or groups observe the birds and their habitats and offer tailored suggestions, such as controlling predators or keeping nest sites safe.
Ultimately, though, the most crucial hurdle is to convince world leaders to value biodiversity as an asset, Bennun said.
"We need to hold governments accountable," he said. "It can't be business as usual."
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