Summer is breeding season for songbirds – but if you want to know how many birds will be bred in a given year, winter holds the biggest clues.

That’s according to researchers from the University of Guelph, who used tracking devices to follow birds to and from their winter homes in the United States.

The study came out of a desire to learn more about why populations of certain migratory birds have been falling.

“What’s prevented us from learning has been lack of knowing where these birds go and what they do after leaving the breeding grounds,” Guelph professor Ryan Norris, one of three study authors from the school, said in a news release.

The researchers found that the ground temperature at savannah sparrows’ winter home plays a major role in determining how many of the birds return to Canada to breed in the spring and summer.

In a nutshell, colder ground temperatures mean harsher conditions and fewer birds surviving the winter.

The problem could worsen in the coming years if predictions of more severe and extreme weather come to pass.

By understanding these patterns, biologists hope savannah sparrows and other migratory birds seeing population decreases can be better protected.

Specifically, they say, conservation groups looking to protect birds’ habitats might want to start paying more attention to where the birds are during the winter.

“For many species like savannah sparrows, the non-breeding grounds might matter more,” Norris said.

Researchers in Maine and Switzerland also took part in the project, findings from which can be found in the journal Nature Communications.

They say their findings might also be able to applied to whales, caribou and other migratory animals.