A quarter of the world's bird species will likely be extinct or critically endangered by the end of the century, according to a new study by U.S. researchers.

This projected extinction wave has implications beyond the fate of individual bird species, the researchers said, as the loss of birds will have negative impacts on the environment and may encourage the spread of human disease.

The findings add to growing concern about the planet's biodiversity and echo several other recent studies that indicate conservation efforts are failing.

The most recent Red List of Threatened Species, released late last month by IUCN-The World Conservation Union indicates that 12 percent of all bird species, 23 percent of all mammal species, one-third of all amphibian species and 42 percent of all turtles and tortoises are already threatened with extinction.

This latest study, considered one of the largest ever of avian biodiversity, centers on analysis of conservation, distribution, ecological function and life history data for all 9,789 living and 129 extinct bird species.

"The result is one of the most comprehensive databases of a class of organisms ever compiled," said lead author Cagan Sekercioglu, a researcher at the Stanford Center for Conservation Biology (CCB).

The black-browed albatross is considered endangered, in part due to declines attributed to longline fishing. (Photo courtesy Cagan Sekercioglu)

"Given the momentum of climate change, widespread habitat loss and increasing numbers of invasive species, avian declines and extinctions are predicted to continue unabated in the near future," the authors said.

Published online Monday in the "Proceedings of the National Academy of Sciences," the study used computer modeling to simulate best case, intermediate case and worst case scenarios for the future of bird species.

The forecast is a worrying one for birds even under the best case scenario, which was based on improved conservation methods. It predicted six percent of all bird species would be extinct by 2100 and another eight percent on the brink.

For the intermediate case scenario, the scientists assumed present trends would continue - that scenario projected one in 10 species will disappear by 2100, with 15 percent critically endangered.

Of even graver concern to conservationists is the projection made under the worst case scenario, which assumed the number of threatened species would increase by about one percent per decade.

"These assumptions are conservative, since it is estimated that, every year, natural habitats and dependent vertebrate populations decrease by an average of 1.1 percent," the authors wrote.

The worst case prediction found 14 percent of all bird species extinct and another 25 percent critically endangered or extinct in the wild.

The impacts from the predicted loss of bird species will ripple across ecosystems, the authors said.

Biologists fear last month's death of the only known po'ouli, a small Hawaiian forest bird, has pushed the species into extinction. (Photo by Paul Baker courtesy U.S. Fish and Wildlife Service)

More insect eating bird species are prone to extinction than any other group, the authors said, and it unlikely that other organisms will be able to take up their role in controlling pests.

Island birds and birds with highly specialized diets are also predicted to experience more extinctions than average, according to the report, which notes their loss could doom some plants that depend on individual species for pollination and seed dispersal.

The researchers found more than a third of all scavengers and fish eaters are prone to extinction.

The disappearance of scavengers is of particular concern and could have a serious impact on both the environment and human health.

"These birds are important in the recycling of nutrients, leading other scavengers to dead animals and limiting the spread of diseases to human communities as a result of slowly decomposing carcasses," the authors said.

The researchers cited the ramifications from the collapse of the vulture population in India over the last decade.

In a decline linked to widespread veterinary use of a pharmaceutical called diclofenac, vulture populations in India and other areas of South Asia plummeted by some 95 percent in the 1990s.

White-backed vulture populations have crashed 99 percent since the late 1980s, with the loss of tens of millions of birds. (Photo by Guy Shorrock courtesy BirdLife International)

History offers another lesson in the impact of bird extinction on human health, said coauthor Gretchen Daily, an associate professor in Stanford's Department of Biological Sciences and director of the CCB Tropical Research Program.

Daily pointed to the example of the passenger pigeon - a North American bird wiped out early in the 20th century by hunting and habitat loss.

"Its loss is thought to have made Lyme disease the huge problem it is today," Daily explained. "When passenger pigeons were abundant - and they used to occur in unimaginably large flocks of hundreds of millions of birds - the acorns on which they specialized would have been too scarce to support large populations of deer mice, the main reservoir of Lyme disease, that thrive on them today."