NASHVILLE — It was 18 degrees in Nashville on Jan. 30, the day I made up my mind to participate this year in the Great Backyard Bird Count, which runs Feb. 15-18. By the time I got around to signing up, it was 70 degrees outside and raining torrentially, breaking a rainfall record that had stood since 1884. Tornado sirens were wailing for hours, setting back all my efforts to rehabilitate our traumatized little rescue dog by what I’m guessing will be weeks.

But right now I’m more concerned about the birds. North American birds evolved to withstand normal North American weather, and normal North American weather includes cold snaps, blizzards, floods and tornadoes. But tornadoes in February are not normal, even in the American South, and a truly severe cold snap can be devastating.

It’s too early to say what effect this year’s polar vortex and extreme flooding have had on bird populations, but in January 1977 an ongoing survey of the bird population in southern Illinois inadvertently became a case study in avian survival rates during brutal weather. Researchers conducting a bird census in the area were forced by heavy snowfall and extreme cold to stop collecting data. What they found when they were able to resume the count nearly a month later was astonishing: Whole species of birds had simply disappeared from the survey area. Carolina wrens, gone. Eastern bluebirds, gone. Hermit thrushes, gone. Two different species of kinglets, gone. Many other species were decimated, with populations reduced by up to 80 percent.

The Great Backyard Bird Count, a citizen-science project of the Cornell Lab of Ornithology and the National Audubon Society, relies on thousands of volunteers around the world to report the number of birds they see during the annual four-day event. It’s easy to participate, requiring very little time and no special expertise, though I’ve never paid much attention to the program before now. I love birds, but I’m not a real birder. I don’t keep a life list of the species I’ve encountered, and I can never tell the difference between a Cooper’s hawk and a sharp-shinned hawk without looking them up.

But during the recent polar vortex, I thought about the Great Backyard Bird Count because I remembered reading about that study from the winter of 1977. It was only an accident of timing that made it possible to identify the terrible songbird losses: The census was already underway when bad weather hit, providing an opportunity for researchers to compare bird counts immediately before and immediately after the blizzard. Otherwise, no one would have known how many birds died in the extreme cold.

“It’s really hard to see the big effects of anything on bird populations because they’re hard to keep track of,” Kevin McGowan, an ornithologist at the Cornell lab told me. “We’ve been looking for ways for a long time to try to monitor birds and get some idea of what’s happening because, let’s face it, most of the birds in the woods could disappear and you might not notice. They could literally all fall dead, and it would take you a while to figure out what’s going on.” With the Great Backyard Bird Count, volunteers become a vast data-gathering resource for ornithologists, reporting on which birds are where, and how many birds there are.

Growing evidence suggests that extreme weather events like the polar vortex will occur more often as a warming climate affects the jet stream. “Weather fluctuations have always occurred to some extent,” Dr. McGowan said, “but if things get to a point where this becomes the new normal, then it’s hard to know what to expect in the influence on animal populations.”

To give us that information, we can’t depend on accidents of timing like the bird census in 1977 that just happened to coincide with a major weather event. Getting thousands and thousands of backyard bird-watchers to count the birds they see during the same time period each year, over and over again across the years, can show patterns that no individual study is likely to detect. “The internet makes participation so easy — people just log in and put in their numbers,” said Dr. McGowan. “We can capture these data so much faster now and analyze them so much better.”

And it’s not just the data on birds. For nearly 30 years, volunteers have been counting the Western monarch butterfly population, which overwinters in California. Thanks to their efforts, we know that this year’s total — 28,429 butterflies, dramatically lower than the 1980s average of 4.5 million — has dipped below the critical number believed to be needed for the population to survive.