For some people, spring doesn't start until they hear robins cheerfully chirping in the backyard. In many parts of the U.S., that day now comes sooner than it did even a few decades ago.

Some populations of the ubiquitous worm-pulling birds are starting their northward migration about five days earlier per decade, in a race to keep up with the rapid changes that global warming is bringing to their breeding grounds in northern Canada and Alaska. New research published April 1 in the journal Environmental Research Letters shows that their flights follow trails of melting snow.

When they arrive in Canada and Alaska in May, the robins only have a few short weeks to find a mate, breed and fatten up for the return flight. But seasons and vegetation in the far north are shifting fast. The Arctic is warming twice as fast as the rest of the world, but so far the birds have been able to adjust by responding to changed conditions along their migration path.

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Robins start heading north earlier when winters are warm and dry, the new study found, affirming findings from earlier studies that, even 20 years ago, documented climate effects on migration. Declining snow cover, a well-documented impact of global warming, seems to be the main environmental cue, said the new report's lead author Ruth Oliver, who did the research as a doctoral student at Columbia University's Lamont-Doherty Earth Observatory, and is now a postdoctoral associate at Yale University.

"We've generally felt like birds must be responding to when food is available," Oliver said. "When the snow melts, there are insects to get at, but we've never had data like this before. We were interested in looking at how songbirds navigate these dynamic landscapes. We know the environment is changing, but it's been puzzling to researchers how songbirds are anticipating these changes."

The scientists studied the movement of robins over 25 years of migration, beginning in 1994, at the Great Slave Lake in Northwestern Canada. To understand exactly why the robins are migrating earlier, the researchers put tiny GPS devices on 55 birds and linked their movements with weather data like air temperature, snow depth, winds and precipitation, as well as other conditions that affect migration.

When they mapped out their findings, they could see how the robins approached the Rocky Mountains, then waited for the snow to melt across the mountain passes before rushing northward to complete their migration, Oliver said.

"They go faster when conditions are nice and slow down when it gets stormy and cold," she said. "You can think of them as a sentinel species, showing how changes in remote ecosystems, like melting snow in the Arctic, can change things in your backyard."

Other studies have examined how charismatic species like eagles or snow geese respond to climate change. But, said Oliver, the worm-tugging robins, with their friendly backyard chirping, are also strands binding the global web of life together. Bird migrations connect ecosystems across thousands of miles through exchanges of nutrients and energy, she said.

Detailed knowledge of how robins respond to earlier spring snowmelt and warmer temperatures can help identify when the birds might go over a biological cliff, beyond which they can no longer adjust their schedule. That's important because the timing of the migration can directly affect reproductive success, said co-author Natalie Boelman, a Lamont associate research professor.

The new study is part of a broader NASA-funded research and outreach project, called the Arctic-Boreal Vulnerability Experiment (ABoVE), that is tracking how the rapid warming of the far north affects wildlife. Studies of caribou and Dall sheep, for example, showed they are also sensitive to changes in spring snow cover.

The robin study and other related projects provide data that can be scaled up and used together with satellite and aerial data to show how changes in snow cover and other global warming impacts affect ecosystems, said ABoVE project manager Peter Griffith.

A broad overview of Arctic landscape and ecosystem changes may be able to help Indigenous communities, fisheries and energy planners prepare for changes ahead. Several of this summer's ABoVE missions have been canceled because of the coronavirus pandemic, he added.

Not All Species can Adapt

Researchers have been documenting climate-driven shifts in the migration patterns of robins and other birds for decades. A study of robins in 2000 showed they had moved up their migration to the Rocky Mountains by two weeks over the previous two decades.

Between 1974 and 1980, the date of the first robin sightings in Gothic, Colorado, was always the first week of April. By 2019, that date had advanced to mid-March.

Biologist David Inouye, lead author of the 2000 study, said the robins were flying to the mountains earlier because of warmer spring temperatures in their overwintering areas in Arizona and Mexico.

But in some cases they started arriving in Colorado too early, when the ground was still covered with snow, making it tough to find food. From 1980 to 2000, the gap between the robins' arrival date and the first day of snow-free ground had grown by 18 days.

Inouye said the results of the new study are in line with what other research shows and that many other birds, like snow geese, also use snowpack cues to time their migration. Since the 2000 study, the "trend toward earlier arrival of robins in Gothic (at the Rocky Mountain Biological Lab) has continued," he said.

A 2012 study underlined the effect of the shifting climate on the food supply: Earlier snowmelt in the Rockies, the researchers found, has reduced the amount of nectar for migrating hummingbirds.

Spring is warming faster than any other season in the Rockies, and yellow glacier lilies are blooming two to three weeks earlier than in the 1980s. But hummingbirds that rely on the nectar from the flowers have not shifted their migration to match the flowers, so they may face food shortages after their long flight from Central America.

Growing disturbances in biological cycles have been documented in other ecosystems important for birds. At Mono Lake, California, for example, changes in the lake water's temperature and salt concentration linked with global warming affect the reproduction of aquatic flies and tiny crustaceans.

In the past, millions of migratory birds have timed their stops at Mono Lake to coincide with the peak abundance of food. But in recent years, its timing hasn't always been reliable. That can be a big problem for birds that only have a short window to make trans-hemispheric flights from summer breeding grounds in the Arctic to their overwintering grounds in South America.

Similar disruptions are under way in the oceans, where warmer temperatures and rapid decline of sea ice have resulted in unseasonal blooms of plankton and krill. Those changes ripple up the food chain to birds—as well as fish and marine mammals—which can no longer rely on the relatively stable climate cycles of the past 10,000 years.

"Marine species near ice shelves are disappearing, and others whose environments are changing so dramatically that they can't respond quickly enough in ecological or evolutionary time, will go extinct," said Inouye.