Night flyers: Estimated 40 million migrating birds have passed over Savannah this year

Another springtime avian migration period has largely passed, but not before an estimated 40 million birds crossed over Savannah in their semiannual rite.

The northward flow peaked on May 1, when nearly 2 million nighttime travelers passed through the area, according to the BirdCast dashboard managed by the Cornell Lab at Cornell University, Colorado State University and the University of Massachusetts Amherst.

A quartet of warblers – the yellow-rumped, blue-throated, hooded and prothonotary – accounted for more than one-third of those peak-period passages, BirdCast projected.

The yellow-rumped variety is “often the core member of mixed warbler flocks during migration,” according to the Cornell Lab of Ornithology.

Like most seasonal flyers, the warblers went unseen.

Birds prefer to migrate at night, when the stars and moon aid in their navigation. The atmosphere also is more stable when the sun isn’t shining, so it's easier to maintain a steady course, especially for smaller birds such as warblers that might fly as slowly as 15 miles per hour, the National Audubon Society says.

Despite their shared fondness for darkness, migratory flocks include a mixture of non-stop flyers and those electing to lay over at least once.

“Some of the species on the move, the expected nocturnal migrants, are arriving to breed and also just passing,” said Andrew Farnsworth, a visiting scientist at Cornell Lab. “The great crested flycatcher happens to be a common breeder in your area, with the birds migrating into the region early in the spring, breeding and then departing.”

In fact, the flycatcher currently accounts for nearly half of the more than 50,000 birds in flight nightly over Savannah, BirdCast estimates.

However, “this species also breeds much farther north, so while some birds stop migrating to breed in your area, other individuals of the same species continue passing through,” Farnsworth explained.

For other birds, the coasts of Georgia and South Carolina are destinations, particularly in winter. But loss of habitat due to climate change, development and other factors impact where they choose to settle.

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Return of the whistling duck

Staff members at the 32,000-acre Savannah National Wildlife Refuge experienced that phenomenon firsthand.

In 2017, storm surge from the remnants of Hurricane Irma sent torrents of saltwater over levees and into expanses of carefully protected freshwater wetlands, leaving an estimated $18 million in damage at the refuge.

The system of canals and earthen barriers meant to keep saltwater out and freshwater in was severely compromised. As a result, the refuge was unable to effectively control water flow in 70% of its 3,000-acre impoundment area, leading to a widespread loss of freshwater vegetation that serves as valuable habitat for migratory birds and other species.

That included black-bellied whistling ducks, which spend winters at the refuge but whose numbers eventually fell to fewer than a dozen.

That changed after Ducks Unlimited completed restoration work at the refuge last year.

The project “put the (freshwater area) back almost better than it was,” U.S. Fish and Wildlife Service Deputy Director Siva Sundaresen said during a February event commemorating completion of the work as waves of whistling ducks meandered over the marsh behind him. “And that’s something we all have to recognize that we have to do. ... We have to build resilience into these landscapes, and we have to build resilience into our management of them.”

With their habitat restored, as many as 2,500 whistling ducks were observed at one time at the refuge this past winter, said U.S. Fish and Wildlife spokesman Jennifer Koches.

The agency attributed the rebound to a “combination of plentiful food and water resources following the initial re-flooding of (freshwater) impoundments.”

The restoration also buttressed the refuge against sea-level rise and extreme weather tied to climate change, as well as saltwater intrusion fueled by expansion of the nearby Port of Savannah.

Freshwater marshes in the Savannah River Estuary once totaled 12,000 acres but now make up less than 5,000 acres, according to the U.S. Army Corps of Engineers.

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‘Climate is almost certainly a factor’

Like humans, birds are impacted by rising temperatures linked to heat-trapping pollution from the burning of fossil fuels.

“Climate change is affecting birds in a multitude of ways, some I am sure we aren't properly monitoring yet,” said Adam Betuel, director of conservation at Birds Georgia

Factors that have been identified include changes in natural cycles like the transition of seasons, the leafing out of trees and hatching of insects, Betual said.

“The plants and insects normally key in on local conditions that migratory birds can't perceive from hundreds or thousands of miles away,” he explained.

For coastal nesting birds, rising sea levels and tides pose threats.

“High tides can wash away the nests of birds that are breeding just above the (normal high-water) line or in marsh grasses just above historic tidal levels,” Betual noted.

Shifting ranges also offer a preview of expected future trends.

“We are seeing some birds start to pop up more and more frequently in new locations, Betual said. “But it is really hard to determine the precise causes, though climate is almost certainly a driver.”

Warmer waters also change the water's acidity and the depths at which small beach creatures hide in the sand.

"I have heard how both issues can hurt prey availability for coastal species,” Betual added. “Drought, fire and temperature changes also all can negatively impact birds.”

By the numbers

Here are spring bird migration statistics for Chatham County, based on BirdCast estimates:

Total migrating birds: 40 million

Most in flight at one time: 228,000 at 2:10 a.m. on May 2

Most in one night: 1.92 million from 8:10 p.m. May 1 to 6:30 a.m. May 2

John Deem covers climate change and the environment in coastal Georgia. He can be reached at 980-355-2995.

This article originally appeared on Savannah Morning News: BirdCast puts Savannah's migration total at 40 million for season