Robert Brown|Climate change leaves some migrating birds 'out of sync' and hungry

When Logan Parker first sees a male,Robert Brown black-throated blue warbler near his Maine home each spring, two things strike him about the tiny bird – its stunning appearance and the thousands of miles it traveled from its winter home in Central America or the Caribbean.

The birds, with their striking black masks and contrasting midnight blue and white feathers, “look so delicate,” said Parker, a birdwatcher and an ecologist with the Maine Natural History Observatory. “It’s hard to imagine they make these long-distance movements to come here to raise young.”

For centuries, these birds and many others completed long-distance migrations in tune with other ancient spring rhythms, such as the trees budding out into leaves in myriad shades of green. But that’s changing.

Spring – with its “pulse of food and pulse of life” – arrives earlier than it used to, driven by warmer temperatures, said Ellen Robertson, who co-authored a recent bird migration study while doing post-doctoral research at Oklahoma State University. The study found a mismatch between earlier spring green up and the timing of migration for some long-distance travelers, leaving birds such as the black-throated blue, “out of sync." For example, they might arrive in their spring nesting locations after the peak period for insects to emerge.

Robertson is one of many scientists working to unravel more of the mystery around migrating birds and their potential long-term survival as climate change multiplies the threats they face.

To do so, they are using a growing body of information that offers the most complete picture in history of when, where and how birds migrate. Recently published studies have tapped more than three decades of data from the nation’s radar network, decades of satellite information and remote sensing and citizen science projects. Mountains of long-term data are stacking up from bird watchers, bird banding stations, breeding bird surveys and more.

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The technological advances in radar, acoustic remote sensing and machine learning have been “incredible,” according to Andrew Farnsworth, a visiting scientist at Cornell University.

The ability to analyze, gather, and visualize enormous amounts of data are essential for making sense of patterns associated with the changing climate, Farnsworth said.

Warmer springs bring changes in bird migration

Spring’s arrival has always shown variation. It may be early. It may be late. But researchers say the data studied over several decades shows warmer temperatures are arriving earlier and earlier, especially in the more northern latitudes.

This year, many parts of the U.S., especially in the Eastern half, saw spring temperatures 10 to 15 days sooner than the long-term average, said Scott Loss, a professor of natural resource ecology and management at Oklahoma State and one of Robertson’s four co-authors. The peak of the famed cherry tree blossoms in Washington, D.C., for instance, was the earliest on record.

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Changes also are seen in other seasons, Loss said. “Our winters are getting shorter. Our summers are getting longer.”

By comparing more than two decades of radar data showing bird movements at night across the U.S., one study documented the shifting migration patterns.

“We’re not just sort of seeing one location or one species shift in a directional way, but we’re seeing the whole mass of migratory birds showing up earlier,” said co-author Kyle Horton, an assistant professor at Colorado State University. “It’s not just the fringes at the beginning or the end of migration.”

“That’s quite telling to us ‒ birds are responding to these changes,” he said. Similar earlier patterns have been noted in the Caribbean and the Mediterranean.

An international report released earlier this year by the United Nations Convention on the Conservation of Migratory Species of Wild Animals listed climate change as one of the top three threats for migrating birds. It's also a threat multiplier for other migratory species, such as the wildebeest.

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Some birds left 'out of sync'

Warming triggers some bird species to change their migration patterns and arrive in their spring breeding grounds days or weeks earlier than they used to.

Other species are less adaptable. They time their migration to length of day rather than temperatures, which means they migrate by the calendar, despite the warming world around them.

When flowers bloom, trees leaf out and insects emerge earlier than in the past, these birds miss out.

“You get to your breeding grounds and you didn't arrive at the right time to have the big insect hatch that you need while you're raising your fledglings,” said Melanie Smith, director of digital science and data products for the National Audubon Society.

Robertson and Loss were two of four co-authors on a study that looked at the migrations of 150 bird species along flyways from South America to the high Arctic, using remote sensing and satellite data to look at how bird migration coincided with spring green up.

The study, published in March in the Proceedings of the National Academy of Sciences, found migration and timing for some species were more closely related to the long-term average date for peak spring greening and leaf out rather than its current timing. That suggests the birds are stuck on calendar dates, Loss said.

Their study found reason to be “most worried” about long-distance migrants, like the black-throated blue warbler, Robertson said. Migratory journeys are “really costly” in terms of energy and calories for the species like the warbler that weighs only about a third of an ounce, the same as about two nickels.

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Migration already has been heavily altered by building and development and the loss of green spaces along the major migratory flyways, Smith said. Layer climate change on top of that, with its changes in weather patterns, rain, heat and wind and “we’ve made it even harder for them.”

The study by the team Robertson was working with found the longer distance migrants – birds traveling from southern Central America or South America to higher latitude boreal forests in the U.S. and Canada – are less responsive to change than shorter-distance migrants like American robins and red-winged blackbirds, who may travel only a few hundred miles.

The birds that travel the farthest have no way of knowing the local weather conditions, so they can't respond to them, Robertson said.

Cold snaps still arrive despite warmer springs

Warmer temperatures are driving changing patterns among the birds and the insects they eat, studies show. A study of tree swallows and the emergence of aquatic insects at Cornell University in New York over 30 years found multifaceted threats posed by climate change

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As May temperatures warmed, the birds shifted their nesting earlier, said David Winkler, a California-based senior research biologist, and retired Cornell University professor. A study he co-authored with Ryan Shipley found that tree swallows advanced their egg laying about 3 days per decade.

Now those chicks are exposed to inclement weather events twice as often as they were in the 1970s, because despite the overall warming trend, late cold blasts of air still arrive around the same calendar dates as before, the study found. Even a single inclement weather event can reduce offspring survival by more than 50%.

For at least 25 years, Winkler and others tallied the available flying insects at their field site every day using what is basically an aerial vacuum cleaner. They found the timing and variety of emerging insects is changing in response to warmer temperatures.

Cold-adapted species such as "big juicy (aquatic) flies are being replaced by tiny things like midges and gnats that give the birds less nutrients," Shipley previously told USA TODAY. The availability of insects, especially nutritious ones, has a considerable impact on chick survival.

An adult tree swallow sitting on her eggs can go a day or two with fewer insects, but it's a different story for growing chicks generating their own heat to stay warm, Winkler said. When adult swallows couldn't find enough food for themselves, they would sometimes temporarily abandon the nests for one or two days, and sometimes permanently, to search farther afield for food.

More changes are likely with time as birds and the ecosystems that support them experience the more extreme conditions expected, Farnsworth said.

While Parker birds for fun in Maine, for work he monitors two migratory bird species, the Eastern whippoorwill and the common nighthawk, one of the longest distance migratory birds in North America.

The key to saving these migratory birds, he said, is to show people the amazing feats these creatures accomplish every year and enlist the public's help in saving their habitats, breeding grounds and flyways. “What we are trying to foster is being a responsible steward.”

Dinah Voyles Pulver covers climate change, wildlife and the environment for USA TODAY. Reach her at [email protected] or @dinahvp.