The North American continent has lost 29% of its bird population – some 3 billion – since the 1970s, paralleling bird losses elsewhere in the world, a study published Thursday in the journal Science indicates.

Even the most common species like blackbirds and sparrows are in decline.

"It's staggering," said Ken Rosenberg, a conservation scientist at the Cornell University Laboratory of Ornithology, in releasing what is being described as “the most comprehensive inventory” of North American birds ever completed. "We want this to be the real wake-up call."

David Yarnold, president and CEO of the National Audubon Society, said the findings point to “a full-blown crisis.”

Though study did not pinpoint causes of the declines, previous studies have blamed feral cats, collisions with buildings and other structures, and pesticides. Climate change is expected to compound the problem.

"We expected to see continuing declines of threatened species. But for the first time, the results also showed pervasive losses among common birds across all habitats, including backyard birds," Rosenberg said.

Co-author Peter Marra, senior scientist emeritus and former head of the Smithsonian Migratory Bird Center and now director of the Georgetown Environment Initiative at Georgetown University, said the data “are consistent with what we’re seeing elsewhere” in terms of declining insect and amphibian populations.

“When you lose a common species, the impact will be much more massive on the ecosystem and ecosystem services," said Gerardo Ceballos, an ecologist and conservation biologist at the National Autonomous University of Mexico in Mexico City. "It's showing the magnitude of the problem."

The analysis included data collected by non-scientists.

“The crisis reaches far beyond our individual borders. Many of the birds that breed in Canadian backyards migrate through or spend the winter in the U.S. and places farther south -- from Mexico and the Caribbean to Central and South America,” said coauthor Adam Smith of Environment and Climate Change Canada. “What our birds need now is an historic, hemispheric effort that unites people and organizations with one common goal: bringing our birds back."

The study was organized by the American Bird Conservancy, Bird Conservancy of the Rockies, the Cornell Lab of Ornithology, Environment and Climate Change Canada, the Georgetown Environment Initiative and the Smithsonian Migratory Bird Center.

Even common species like sparrows are seeing declines in population. National Park Service