The sharp decline, described in a study in the journal Science, is not just bad for birds. It also bodes ill for the ecosystems those birds inhabit, and points to a need for action to halt and perhaps reverse the drop, scientists said.

"Three billion was a pretty astounding number for us," said lead author Kenneth Rosenberg, a conservation scientist at Cornell University and American Bird Conservancy.

Steven Beissinger, a conservation biologist at the University of California, Berkeley, called the results and their implications "dizzying.""

Of those lost birds, 90 per cent come from just 12 bird families that include common and widespread species such as sparrows, swallows, warblers and finches.

Declines in the abundance of common species may not seem as dramatic as the endangerment of rare ones, but it is a very serious form of ecosystem erosion, the scientists said.

That's because abundant species often play important roles in their habitats, whether they control pests, pollinate flowers, disperse seeds, provide food for other animals and even draw tourists who support local economies.

"When you're losing abundance, you're losing the fabric of the food chains, the fabric of the ecosystems - more perhaps than losing one rare species," Rosenberg said.

Across ecosystems, grassland birds - a group that includes sparrows and meadowlarks - were hit the hardest, the researchers said. Since 1970, their numbers have fallen by more than 720 million, representing 53 per cent of the initial population.

Shorebirds, long threatened by the draining of coastal wetlands and urbanisation, saw declines of more than 37 per cent.

However, there were a few success stories in the data that could offer a road map for aiding other bird populations, Rosenberg said.

Wetland birds, such as ducks and geese, have increased, primarily because of conservation efforts that protected wetland habitats over the last few decades.

And raptors such as bald eagles, ospreys and peregrine falcons have also improved since the 1970s, when their numbers had been decimated by the use of the now banned pesticide DDT.

Australian Associated Press