The number of birds in the United States and Canada has fallen by 2.9 billion since 1970. In a mere half-century, 29 percent of avian populations have vanished from our skies, including birds in every ecosystem – from backyard birds and meadowlarks to songsters and swallows. The study was conducted by researchers from seven institutions and published in Science.

The full-tilt biodiversity loss on our planet has been pinned as one of the great environmental crises of our time. It has been called "sobering", "unprecedented", and "tragic" over the years, with the loss echoed across species and lands.

"The results even stunned those of us who were doing the research. We knew some bird species populations were declining – some drastically – but we thought that increases in other species would balance everything out overall. That’s not the case at all," lead author Ken Rosenberg, from the Cornell Lab of Ornithology and American Bird Conservancy, told IFLScience.

But why care?

"Given their ubiquity, a dramatic decline in birds indicates not only a problem for birds; it also indicates a larger problem with the natural world as a whole," said Rosenberg.

Aaron Bernstein wrote in the International Encyclopedia of Public Health that "biodiversity maintains the availability of basic human necessities such as clean water, clean air, and food. It also serves as a resource for the discovery of new medicines and as the resource for the plant, animal, and microbial experimental models that scientists rely upon every day to better understand how cells and organs function in health and disease."

The new study is based on 50 years of data,143 weather radar stations, and a survey of 529 species in the United States and Canada. The hardest-hit avians have been grassland species with more than 715 million birds lost since 1970. Shorebirds have been wiped of one-third of their population numbers. 

A dozen bird families accounted for 90 percent of the total loss, including finches, warblers, and blackbirds. The bulk of the losses were not rare species, but rather large declines in common birds.

Some species have seen an uptick in numbers, such as bald eagles after DDT pesticides were banned and legislation was passed to help protect the birds. Waterfowl management has allowed ducks and geese to thrive, while vireos, a mostly insect-feeding bird, have grown by more than 85 million, with scientists not entirely certain why their numbers are booming.

"These are important examples that show, when we choose to make changes and actively manage the threats birds face, we can make positive changes for birds and the environment as a whole," said Rosenberg. "Nature is resilient, and our study indicates that action is urgently needed, but it is not too late to save birds and our natural world.

So what can you do?

Make glass windows more visible if birds are flying into them – millions die each year from window collisions. You can also help conservation efforts of bird-rich habitats and support policy such as strengthening the Migratory Bird Treaty Act, says Michael Parr, president of American Bird Conservancy.

"Birds don't recognize political boundaries – migrating birds especially cross many countries and to protect them for their entire life cycle we need to address problems on breeding grounds, migratory routes, and wintering grounds as well as the crucial stopover habitats they use along the way," said Rosenberg. There needs to be international cooperation that makes preserving nature a priority." 

"The story is not over," Parr added. "There are so many ways to help save birds."