TORONTO -- Bird species once considered “abundant” are becoming more scarce in Canada and the U.S. after decades of decline that amounts to a net loss of almost 3 billion birds since 1970, according to new analysis.

The research, published in the journal Science on Thursday, points to a “largely overlooked” crisis affecting North American bird habitats. In 1970, the bird population in Canada and the U.S. at roughly 10 billion, a number that should have remained stable among breeding adult birds, but instead it has dropped by 2.9 billion, according to the analysis by Cornell University researchers.

As some of the 529 species of bird analyzed grow in number thanks to conservation and legislation efforts, some of the most common species are on a path toward extinction. The new research sheds light on how habitat degradation is affecting even “backyard birds” familiar to most people, including sparrows, blackbirds, cardinals and finches. Scientists point to human impact such as agricultural intensification as a main driver for the die-off, but everything from skyscrapers to cats may be contributing.

“It’s death by a thousand cuts,” said Ken Rosenberg, a conservation scientist with the Cornell Lab of Ornithology and the American Bird Conservancy, in an interview with “A lot of this loss is associated with changes in land use for agriculture. This is something we’re seeing around the world. Creating more and more hostile environments in places that used to support quite a few birds.”

The new report is not the first to quantify the threat. Earlier this year, the United Nations released its first comprehensive report on biodiversity in which it said extinction threatens more than 1 million species of plants and animals. Nearly one quarter of birds around the globe live in habitats already affected by climate change, the report said. Research by the International Union for the Conservation of Nature, or IUCN, said in March that nearly 1,500 bird species are threatened, endangered or extinct in the wild.

The new report is not about the loss of species but the loss of “abundance.” It makes it clear that a shrinking bird population is a particular concern, perhaps more than the extinction of uncommon birds. “It’s still important to focus on threatened species, but those tend to be rare species that a lot of people are not very familiar with,” said Rosenberg.

“If we can’t sustain populations of birds like starlings and house sparrows, then that is the ‘canary in the coal mine’ indicator.”

The loss of common abundant birds could have a negative domino effect on the environment since those birds are integral to the ecosystem, Rosenberg added. They play a role in the food chain and contribute to pest control, seed dispersal and pollination. “It’s difficult to measure, but the loss of abundance is causing other things to happen than the loss of a species perhaps,” he said.

Though the scale of the declines may be staggering, some species -- including raptors and waterfowl -- are actually making gains. That growth is likely thanks to conservation efforts and legislation protecting endangered species. Efforts such as the banning of pesticides and ending raptor hunting have had a measurable impact, said Rosenberg. Even hunters have been involved with efforts to grow populations of certain birds, with the intention of preserving their hobby.

“People who care about birds and nature who are not hunters, it’s time for that very large group of people to pay attention, to raise their voices, to try to affect change in society and politically,” he said. “Until that voice is raised as a constituency, it’s really the only way we’re going to make the big changes that have to happen.”