Two U.S. conservation groups say more than a quarter of bird species in the United States are vulnerable to extinction.

The National Audubon Society and the American Bird Conservancy issued a joint research project called 'Watchlist 2007' Wednesday, detailing which species in the continental United States and Hawaii are most threatened.

Ninety eight bird species are considered most endangered, with their small populations declining rapidly as they face major conservation threats. Another 119 species face a less serious threat, but are considered declining or rare.

The National Audubon Society's website says global warming and loss of habitat due to urban and suburban sprawl are among the reasons the survival of these birds is tenuous.

The researchers say identifying birds at the greatest risk helps focus the scope of public policies and the funding of conservation initiatives to help save the species.

Some information for this report was provided by Reuters.