Aeroecology

As the days shorten, birds begin to migrate long before cold weather sets in. I notice the first flush of migratory birds on our mountain sometime in mid-to-late August. But September and early October are the peak months here for bird migration.

Migrating snow geese near the Middle Creek Wildlife Management Area

Migrating snow geese near the Middle Creek Wildlife Management Area (Photo by the Chesapeake Bay Program on Flickr, Creative Commons license)

Now that they are on the move, birds enter the most dangerous phase of their lives, especially the immature birds that have never migrated before. They need safe places to land and forage, what ecologists call “habitat,” both water and land-based, depending on whether they are songbirds, raptors, waterfowl, or shorebirds.

Although researchers have made progress in studying how birds migrate, using the stars at night and landforms during the day, for instance, as well as what they eat and where they forage on land, they know less about what happens when the birds are airborne, especially when they encounter human activities.

A barn swallow in flight at the John Heinz National Wildlife Refuge

A barn swallow in flight at the John Heinz National Wildlife Refuge (Photo by Stan Lupo on Flickr, Creative Commons license)/

But in the last decade some ecologists maintain that since birds spend much of their lives in the air foraging, migrating and sometimes even mating, especially swallows and swifts, airspace should also be considered habitat, not only for birds, but bats and a wide variety of flying and even migrating insects such as butterflies and dragonflies.

While there was some discussion of airspace as habitat early in this century, it was a paper written in 2013 entitled “The Airspace is Habitat” by research ecologist Robert H. Diehl that launched the new study called “aeroecology.” Diehl, who studies migratory birds at the United States Geological Survey’s Northern Rocky Mountain Science Center, cited numerous definitions of habitat by several well-known ecologists. He summarized their contention “that occupancy, and perhaps resource use, are essential attributes of habitat,” and concluded that, ”airspace satisfies these criteria.”

Since Diehl’s ground-breaking but brief paper, numerous studies have been done on all aspects of airspace and its importance in the conservation of birds, bats and flying insects.

A wood thrush killed by striking a window

A wood thrush killed by striking a window (Photo by Bob on Flickr, Creative Commons license)

Studies show that up to a billion birds a year die in the United States when they fly into the windows of homes and office buildings, the most common and numerous obstacles in their airspace, according to the American Bird Conservancy. In addition, seven million fly into communication towers, although the latter number may decline since the Federal Communications Commission issued guidelines suggesting that tower operations switch from steady-burning to blinking lights that are less confusing to birds.

Then there are the wind turbines that kill many species of birds and bats that fly into the huge, rapidly-spinning blades. Unfortunately, many of those turbines have been built in the very places where birds and bats migrate, taking advantage of the wind. Here in our area, the leading Allegheny Ridge, a proven migratory highway, has dozens of wind turbines erected on it and more to come in the future. To its credit, the Pennsylvania Game Commission does not allow wind turbines on game lands but plenty of private and local government-owned landowners in Pennsylvania do.

Wildlife biologists collecting and recording migrating song birds killed by wind turbines

Wildlife biologists collecting and recording migrating song birds killed by wind turbines (Photo by kqedquest on Flickr, Creative Commons license)

Well-known birder Kenn Kaufman, in his new book A Season on the Wind, relates an on-going battle by his wife, Kimberly, executive director of the Black Swamp Bird Observatory, to keep wind turbines off Camp Perry, a military-owned property along Lake Erie in northwestern Ohio, which is next to Magee Marsh and other famous migrating bird stopovers. He writes that “wind turbines placed in the middle of essential stopover habitat” [such as the shores of Lake Erie] “are likely to kill disproportionate numbers of long-distance migrants, birds that are already pushing the limits of survival.” The same is true of Pennsylvania’s own shoreline of Lake Erie.

Furthermore, Kaufman insists, the numbers of birds and bats the wind industry claims are killed by the turbines are vast under estimates. While using the wind as a source of carbon-free electricity is necessary, it would be better for the birds and bats if the installations were smaller and local instead of massive and requiring powerlines, another threat to birds and bats, to transmit the energy.

Light pollution from human-related sources such as office towers in cities is especially dangerous to night-flying and/or migrating birds. How light pollution in general affects birds is still another branch of aeroecology research. According to one scientist, artificial light can be seen as far as 224 miles on the horizon. Artificial light at night may change the ability of night-migrating birds to orient and navigate, and also change the way birds communicate and avoid predation in addition to possibly screwing up their geomagnetic sense that helps them find their way.

Tribute in Lights 9-11-2017

Tribute in Lights 9-11-2017 (Photo by Kim Carpenter on Flickr, Creative Commons license)

One study of the National September 11 Memorial and Museum’s “Tribute in Light” in New York City by a team of ornithologists over seven years found that the light changed the behavior of migrating birds at altitudes up to almost two-and-a-half miles. The birds gathered in high numbers, decreased their flight speeds, and flew in circles, all the while calling. But when the lights went out, the birds resumed their migration. The researchers agreed with other studies “highlighting disorientation due to artificial lighting” on many structures such as sport stadiums, construction sites, and offshore oil rigs. In addition, they found, ”There is mounting evidence that migratory bird populations are more likely to occur in urban areas during migration, especially in the autumn.”

Still, a variety of stakeholders in the Tribute of Light event, including the NYC Audubon and the Cornell Lab of Ornithology, agreed to keep the lights on despite its effect on the birds unless conditions were particularly hazardous and then they would shut off the lights for short periods. These shut-offs were enough for birds to continue on their way.

In Pittsburgh, every time the University of Pittsburgh football team wins a game, powerful lights are aimed high in the sky from the tallest campus building. These “Victory Lights” were on for the weekend of October 7-8, 2018 when Pitt won its game against Syracuse. At that time, nature writer and bird blogger Kate St. John was monitoring weather radar and noticed that there were large numbers of migrating birds around Pittsburgh. After she wrote about it, Pitt board members, the Pitt chancellor, and the Executive Director of Audubon Society of Western Pennsylvania agreed that the university would dim the lights for 15 minutes out of every illuminated hour so that trapped migrants could continue their journeys.

A Season on the WindSt. John was able to use weather radar to figure out the movements of migrating birds and how they react to human-related obstacles, because in 2004 the National Oceanic and Atmospheric Administration made its national weather radar data freely available to every interested person. Kaufman, for example, writes about how he and other birders check radar information on their computers every day and night during spring and fall migrations so they know when and where large pulses of migrating birds will land to rest and forage.

A wide range of other researchers, such as Jeffrey J. Buler, who runs an Aeroecology Program at the University of Delaware, uses radar data to study the effects of lighting on migrating birds and how such brightly-lit urban areas attract greater densities of migrating land birds. Such information has led to Lights Out and Dark Skies movements in Toronto, Chicago, and other major North American cities.

Still another study using radar found that migrating birds look for favorable winds and then fly in the atmospheric layers that have them. For instance, many migrating songbirds fly at night because of cooler air and less turbulence. But several studies have also been able to estimate how high most migrating birds fly in relation to human-caused collision threats.

A turkey vulture flies near an office tower

A turkey vulture flies near an office tower (Photo by Finn Maxwell Cousineau on Flickr, Creative Commons license)

The cruising altitude of songbirds averages 500 to 2,500 feet above the ground, raptors, taking advantage of the daytime thermals 700 to 3,000 feet, and waterfowl 200 to 4000 feet. In addition, tall buildings and their windows at 340 million and private residences at 253 million are the largest annual killers of migrating birds followed by powerlines (22.8 million), communication towers (6.6 million) and wind turbines (366,000).

This may be why some studies maintain that “a songbird is 15 times more likely to die during migration than during its long stays in its summer and winter territories,” Kaufman writes. And why some researchers in aeroecology are moving into what they call “aeroconservation” in order to keep not only the birds but the bats and the flying insects they eat safe as they navigate our crowded skies.

 

Moosic Mountain

Last May seven of us stood atop Moosic Mountain listening to the thin, quick, ascending notes of a singing prairie warbler. It was mid-afternoon after hours of pouring rain and the mountain was still swathed in fog.

A heath barren in the Eales Preserve on Moosic Mt.A heath barren in the Eales Preserve on Moosic Mt.

A heath barren in the Eales Preserve on Moosic Mt. (Photo by Bruce Bonta)

Six of us, Mike and Laura Jackson, George Mahon, Sam Dietz, Bruce and I, had traveled three and a half hours from west-central Pennsylvania to explore at least a portion of this 32-mile-long mountain northeast of Scranton in Lackawanna and Wayne counties. At its tallest point–2,323 feet– it is the highest place on the Pocono Plateau. Described by biologists as having “one of the best examples of ridgetop heath barrens in the northeastern United States,” much of it is a mixed forest of small tree species–primarily pitch pine and scrub oak–with an understory of huckleberry, blueberry, rhodora, sheep laurel and other small shrubs.

Outside nearby Jessup, thousands of acres of this unique landscape are preserved at The Nature Conservancy’s 2,250-acre Dick and Nancy Eales Preserve, the 5,756-acre State Game Lands # 300, and the connecting 2000-acre Moosic Mountain Tract in Pinchot State Forest.

Rhodora in bloom at the Eales Preserve

Rhodora in bloom at the Eales Preserve (Photo by Bruce Bonta)

We spent what was left of the afternoon walking a small portion of the Eales Preserve, listening to and watching the many bird species as well as enjoying the flora under the direction of David Trently, a local birding guide and expert on the nature of this unique mountain. It was the third weekend in May and the lovely, rose-purple flowers of the rhodora shrub were in bloom. Another small shrub, sheep laurel, which is in the same genus as mountain laurel, was still in deep pink bud, while the lowbush blueberry shrubs displayed their white and pinkish, bell-shaped blossoms. Here and there small shadbush trees, which in our area are usually blooming by the second or third week in April, were still covered with white flowers, at the same time that clumps of pink lady’s slipper orchids were in bud.

Beside a small pool, we knelt to look closely at the dark red, sticky, opening sundew leaves that this carnivorous plant uses to close and trap insects for food. Then we stopped to admire the bright orange of a red eft, the terrestrial, sub-adult of the red-spotted newt, resting on a lichen-covered rock. We also heard the rattling trill of a calling gray tree frog. When we reached a meadow, Trently told us that one of the rare species at the Eales Preserve was a Leonard’s skipper, a large, reddish-brown butterfly with bright white spots that appears in late August and feeds on grasses.

But there was no doubt that the Eales Preserve was mostly bursting with birds during their peak of migration, and as members of the Juniata Valley Audubon Society, we were pleased at the variety we saw and heard despite the misty, moisty day. Since prairie warblers like scrubby, successional habitats, we were not surprised to hear them, but we were surprised to find one perched in a tree with three scarlet tanagers and a black-and-white warbler, the latter two species common breeding birds in the mature deciduous forests of central Pennsylvania.

A chestnut-sided warbler

A chestnut-sided warbler (Photo by Yankech gary on Flickr Creative Commons license)

Chestnut-sided warblers, also breeders in scrub and low second growth forests, were expected when we heard and then saw their golden-topped heads and chestnut-colored sides, and we were delighted to learn that the yellow-rumped warblers, with golden crowns in addition to their bright yellow rumps, nested in the Preserve.

In fact, almost all the birds we saw there were breeding including a singing, black-necklaced Canada warbler, a buffy-spectacled Swainson’s thrush, nine black-masked, pointed crested, cedar waxwings, a flashy, black and white, rose-breasted grosbeak, a black and orange American redstart, a black masked, common yellowthroat, a black-throated green warbler, and a black and orange Baltimore oriole, two magnolia warblers, two rusty-brown veery thrushes singing their breezy, ethereal, downward scale song in the surrounding forest and too many eastern towhees to count. It may have been a chilly 56 degrees, but the humidity and the brilliant colors and songs of so many birds seemed reminiscent of their southern winter homes in tropical and subtropical forests.

A bay-breasted warbler

A bay-breasted warbler (Photo by Andy Reago & Chrissy McClarren on Flickr, Creative Commons license)

In addition to the singing of the veeries, I was thrilled by the sight of a bay-breasted warbler, the only non-breeding bird species we saw that day. This large, handsome male warbler has a black face, a buff patch on either side of his neck, and a reddish-brown patch on the top of his head and another on his throat and either side of his white breast. This species breeds across the boreal regions of Canada. I used to see these warblers during migration on our mountain but I hadn’t seen one in years. Apparently, their numbers fluctuate from year to year depending on outbreaks of spruce budworms and other caterpillars.

Altogether, we counted 33 bird species on the Eales Preserve during only a mile or so of walking atop the high, relatively flat, mountaintop terrain. This property has 12 miles of trails open to hiking and biking, and we regretted that we couldn’t explore more of it. Still, we were glad that in 2001 the Pennsylvania chapter of The Nature Conservancy had stepped in, with the help of local activists, to purchase this unique area that had been slated to become a business park. Since then, the Eales generosity has allowed the Conservancy to continue to expand the preserve and conduct prescribed burning that is necessary to maintain the fire-dependent natural community.

A prairie warbler

A prairie warbler (Photo by Kenneth Cole Schneider on Flickr, Creative Commons license)

Early the following foggy, windy morning, we drove to SGL # 300. On this gamelands there were many singing prairie warblers, and we had an excellent view of one singing from the top of a dead snag. In the scrub oak habitat interrupted and cinnamon ferns were unfurling and Canada mayflower was in flower. It was still red eft weather and we found four on the gravel road and two more on rocks. Spring peepers called and spotted salamander egg masses floated in Robinson Pond, a manmade wetland/marsh impoundment of about 50 acres with cattails, phragmites, pole stage birch trees, and steeplebush shrubs.

Watching and listening for birds in the fog at SGL 300, left to right, Mike Jackson, David Trently, Laura Jackson, the author

Watching and listening for birds in the fog at SGL 300, left to right, Mike Jackson, David Trently, Laura Jackson, the author (Photo by Bruce Bonta)

As we walked along the network of gated roads, we found starflower, wild oats, and dwarf ginseng in bloom. But it was the birds that delighted and amazed us, singing vigorously in a habitat that shifted from patches of more mature trees to shrubby scrub oak and pitch pine. We saw and heard many of the same species we had seen at the Eales Preserve—black-and-white warbler, veery, yellow-rumped warbler, rose-breasted grosbeak, black-throated green warbler, for instance—but in a large planting of small birches field sparrows sang, a black and yellow magnolia warbler, which prefers to breed in low conifers, perched and posed in a red pine tree, and a black-throated blue warbler sang in the mature forest along with a scarlet tanager and an ovenbird.

Once again the bay-breasted warblers were my birds of the day. This time four were perched together in gray birches and were close enough that we could clearly see that three were either the duller colored immatures or females and the fourth was a brightly-hued male.

Even with seven pairs of eyes and ears alert to every bird species, we were grateful for Trently’s superior birding skills, especially when we heard what sounded like a scolding red squirrel but turned out to be an uncommon call of a common yellowthroat.

The tracks of a black bear

The tracks of a black bear (Photo by K Young in Wikimedia, Creative Commons license)

In fact, we saw no mammals on Moosic Mountain during our two days, although it is known to have a population of snowshoe hares as well as the usual black bears, bobcats, coyotes, and deer. We did spot fresh tracks of a black bear in the muddy road of SGL # 300 and those of deer. We also had none of the views at 2240 feet that visitors on clear days rave about.

Only when we dropped down to Lake Ariel did the sun break through and deliver a new suite of birds in the brushy trail near the lake and the lake itself, for example, bald eagle, red-shouldered hawk, ruby-throated hummingbird, belted kingfisher, yellow-bellied sapsucker, alder and least flycatchers, blue-gray gnatcatcher, warbling vireo, and barn swallow.

Then it was time to leave. In spite of the weather we had enjoyed our time on Moosic Mountain and our intimate looks at 53 bird species including 12 species of warblers.

 

Songbird Journeys

For those of us who appreciate songbirds, September is the saddest month. That’s when most of them start their long journeys south. Gone are the songs of spring and early summer, the raising of youngsters, even, in some cases, their bright spring colors.

A yellow-rumped warbler in winter plumage photographed at the Wakodahatchee Wetlands, South Florida, Feb. 7, 2016

A yellow-rumped warbler in winter plumage photographed at the Wakodahatchee Wetlands, South Florida, Feb. 7, 2016 (Photo by Don Burkett on Flickr, Creative Commons license)

A few songbirds, such as eastern towhees and yellow-rumped warblers, migrate no farther than the southern United States. Others head for Mexico and Central America. Still others spend their winters in the Amazon basin—Peru, Brazil, Venezuela, Ecuador—of South America.

Despite a century or more of migration studies by ornithologists and citizen scientists, using bird-banding, radar images, and even small airplanes, as well as on the ground field work both here and on the wintering grounds, much more research needs to be done, especially here late in the summer, when most songbirds moult, during their fall migration, and on their wintering grounds.

Recently, Bridget Stutchbury and her team at the Hemlock Hill Biological Research Area in northwest Crawford County have pioneered the use of geolocators to track long-distant songbird migrations of purple martins and wood thrushes.

A Kirtland’s warbler with a geolocator mounted on its rump

A Kirtland’s warbler with a geolocator mounted on its rump (Photo by Dan Elbert/USFWS on Flickr, Creative Commons license)

Weighing a mere 1.5 grams—that of a dime—a geolocator is carried on a bird like a backpack and is looped around the bird’s legs. Because a geolocator can detect light levels, it is able to show the cycles of sunrise and sunset so that during good weather, a bird’s geographical location can be calculated by the timing of sunrise and sunset in that area.

Working with the Purple Martin Conservation Association’s main colony in Edinboro, Pennsylvania in the summer of 2007, Stutchbury and her associates spent two mornings attaching geolocators to the birds. The martins seemed undisturbed by their “backpacks” and continued feeding and raising their offspring.

On August 31 one of the female martins flew south on the way to her Brazilian wintering grounds. In five days she made it across the Gulf of Mexico to Mexico’s Yucatan peninsula—1,440 miles. By the 13th of October, she had arrived at Manaus, Brazil and spent the winter in the Amazonian rainforest. She left Brazil on April 12 and was back in Edinboro at her breeding colony on 25th of April flying 4,200 miles in 13 days.

A purple martin taken at the Horicon National Wildlife Refuge, Wisconsin, June 28, 2009

A purple martin taken at the Horicon National Wildlife Refuge, Wisconsin, June 28, 2009 (Photo by Dori in Wikimedia, Creative Commons license)

Five days later Emily Pifer of the Purple Martin Conservation Association found that female with her geolocator still attached and, as Stutchbury wrote in her book The Bird Detective, “Emily was looking at the first migratory songbird, anywhere in the world, for whom we would know its arrival time on the wintering grounds, where it had spent the winter, and how quickly it had come home.”

Later a second female martin arrived with her geolocator and the following year three more were recovered. All indicated the same fast flight over the Gulf of Mexico from northwest Pennsylvania and similar arrival times in Brazil, in which they took more than a month migrating through Central and northern South America.

But all five of their martins averaged 23 days from Brazil to Pennsylvania in the spring, flying about 180 miles a day, thus proving that spring migration is faster than fall’s, most likely because the birds are eager to claim breeding territories and mates.

A wood thrush on its breeding ground in Chester County, PA, June 20, 2010

A wood thrush on its breeding ground in Chester County, PA, June 20, 2010 (Photo by Kelly Colgan Azar on Flickr, Creative Commons license)

Stutchbury also put geolocators on 47 adult wood thrushes in 2007 and 2008 because their numbers are declining probably due to deforestation both on their breeding and wintering grounds. In two years, they retrieved 14 wood thrushes with geolocators. They learned from them that wood thrush fall migration, mostly to Honduras and Nicaragua, is relatively slow and the arrival time varied from mid-October to early December. They too mostly crossed the Gulf of Mexico, especially in the spring when they flew on average 2,160 miles in two weeks.

However, one female did not cross the Gulf, and instead she flew an extra 600 miles overland, arriving much later on her nesting grounds. Why she did this is anyone’s guess, although Stutchbury wondered if she had left her wintering territory in poor condition and hadn’t the strength to cross the Gulf.

Stutchbury further questioned if wood thrushes that double-brooded and thus moulted their feathers late in the summer, would postpone their migration and subsequently arrive too late to acquire territory on their wintering grounds. But she learned through her geolocator-wearing wood thrushes that even though the late moulting birds crossed later into the tropics, they did not arrive later on their winter territories, contrary to the expectations of Stutchbury and her associates.

In a paper they wrote for the Proceedings of the Royal Society B they concluded, “We suggest the possibility that some individuals prepare to migrate more rapidly than others by investing more heavily in fat storage during the early stages of moult.”

A veery photographed in Chester County, PA, on June 2, 2011

A veery photographed in Chester County, PA, on June 2, 2011 (Photo by Kelly Colgan Azar on Flickr, Creative Commons license)

Other researchers have taken up the challenges and rewards of geolocators including Christopher M. Heckscher and associates of Delaware State University who attached geolocators on 24 veeries in White Clay Creek State Park in Delaware near the southeastern Pennsylvania border. Like the purple martins, veeries also migrate to the tropical forests of South America.

While they wanted to find out whether each veery spent its winter in two different areas in southern Brazil as another ornithologist had proposed, they also wished to discover veeries’ migration routes and timing. Furthermore, in a paper in The Auk, they wrote, “Building on the work of Stutchbury et al…” they wanted “to determine whether geolocator technology can successfully track a terrestrial forest-dwelling songbird from its North American breeding site through dense tropical forests of equatorial South America where day length and night length are equal.”

They proved that point by tracking the veeries to multiple wintering sites first south of the Amazon River in Brazil at five separate locations from November 2 to December 2 and then to second wintering sites as far north as Venezuela and as far south as east-central Bolivia with the other three in widely separated areas in Brazil. They suspect that the “predictable seasonal flooding of lowland forests in Amazonia may be the ultimate factor that prompted the Veeries to relocate.”

From those five birds they “documented three different migratory routes between South and North America and three different routes from the Gulf Coast to Delaware.” And like Stutchbury’s purple martins and wood thrushes, veeries took their time going south but left their wintering grounds in mid-April and returned to Delaware in 17 days.

A gray catbird in Washington, D.C.

A gray catbird in Washington, D.C. (Photo by Steve in Wikimedia, Creative Commons license)

Still another geolocator study, this one of gray catbirds, was by Thomas B. Ryder et al. of the Smithsonian Conservation Biology Institution’s Migratory Bird Center. They pointed out that although geolocators can estimate longitude fairly accurately, latitudinal error can be large—108 miles for purple martins and between 132 and 192 miles for wood thrushes. For this reason, they used both geolocators and bird-banding records “to estimate the migratory connectivity of breeding and nonbreeding populations of Gray Catbirds,” according to their paper in The Auk.

In July of 2009 they put 22 geolocators on gray catbirds in two forest parks near Washington, D.C. These birds left their breeding territory in late August and early September and arrived on wintering grounds in south Florida or Cuba in mid-October. They left those grounds in April and arrived back in the D.C. area in early to mid-May.

Looking at recovered bird-banding data that showed Midwestern gray catbirds overwintered exclusively in Central America and our birds from the mid-Atlantic overwintered in Florida and the Caribbean and combining it with their geolocator studies, they concluded that their research “underscores the importance of geolocators, as well as other tools, to advance our understanding of migratory connectivity.”

A common cuckoo

A common cuckoo (Photo by Ron Knight in Wikimedia, Creative Commons license)

With all this research and much more both here and in Europe using geolocators, bird migration is proving to be more complex and varied than we could have imagined. A recent study of the European common cuckoo using geolocators found them 600 miles away from their usual departure area in northern Europe. Then each cuckoo flew by itself back to its normal route and on to its wintering grounds in central Africa.

In an interview with a National Wildlife reporter, researcher Mikkel Willemoes said that, “They [cuckoos] evaluate their own conditions and adjust their reactions to it, displaying a complicated behavior that we were able to document for the first time in migratory birds.”

He concluded that, “This tells us that bird migration in general is far more complex than previously assumed”—a point we can ponder as we watch our songbirds head south, knowing that only an estimated half of them will survive their migratory journeys and return to us next spring.

Watch a video of Dr. Bridget Stutchbury and associates at the Purple Martin Conservation Association attaching geolocators to purple martins before they set out on their fall migration from Presque Isle State Park, Pennsylvania.

Migrating Palm Warblers

It’s the middle of October and every day songbird migrants dwindle. Still, between rainstorms one morning the trees fill with warblers, especially yellow-rumps, which haven’t all that far to go. But I spot the pumping tail of a palm warbler too.

A yellow palm warbler in fall plumage

A yellow palm warbler in fall plumage, taken on October 15, 2013, in Chester County (Image by Kelly Colgan Azar on Flickr, Creative Commons license)

The following day three palm warblers sit and bob their tails in the one American elm beside our driveway. The day after that, our First Field plants shake with sparrows—song, field, chipping and at least two white-crowned sparrows.

In the midst of brown sparrow bodies, I pish in a yellow palm warbler, its yellow breast and belly making it the yellow palm warbler subspecies Setophaga palmarum hypochrysea, which nests in coniferous boreal forests east of Ottawa, Canada, and winters mostly along the Gulf Coast. The western palm warbler, S. p. palmarum, nests west of Ottawa and winters primarily along the southeastern coast of the United States and in the West Indies.

Their wintering grounds may account for their common name, but this is a species that nests on the ground or in short shrubs or trees in a black spruce bog environment from Newfoundland and southern Labrador to northeastern British Columbia in Canada, although occasional nests have been reported from northeastern Minnesota across to the northern New England states.

Still, according to the Boreal Songbird Initiative that is “committed to protecting the Canadian Boreal Forest—the largest intact forest on Earth—on behalf of the billions of migrating birds that rely on it,” according to their website, an estimated 98% of palm warblers breed in Canada’s boreal forest. This is the highest number of any warbler species and is followed by Tennessee warblers (97%), Connecticut warblers (91%), blackpoll warblers (82%) and Cape May warblers (83%).

Both subspecies of palm warblers have chestnut caps, giving them the nickname “redpoll warblers” and pump their tails more than any other warbler species so they are also called “tip-up” or “wag-tail” warblers. They also have yellow beneath their tails, but many have subdued colors in fall so it isn’t always possible for the casual observer to separate the subspecies.

A western palm warbler in fall plumage

A western palm warbler in fall plumage, taken on October 6, 2013, in Pymatuning State Park (Image by David Inman on Flickr, Creative Commons license)

However, the western subspecies has a whitish throat and belly and a grayish brown back and wings in contrast with the yellow throat and belly and yellow-green back and wings of the yellow subspecies.

Mostly the western palm warbler migrates west of the Appalachians in the spring, beginning later than the yellow palm warbler and southeastward to the Atlantic coast in the fall earlier than the yellow palm warbler, while the yellow palm warbler migrates earlier northeast along the Appalachian Mountains in spring and follows the Appalachians south later in fall.

In Pennsylvania it is possible to see either subspecies. According to bird banding records from the Powdermill Nature Reserve in western Pennsylvania from 1961 to 2000, westerns outnumbered yellow significantly during migration, but especially in autumn when their nets recorded 1,265 westerns versus 35 yellows. In spring they netted 24 westerns and 13 yellows.

W. Herbert Wilson, Jr., author of the definitive palm warbler account in Birds of North America, hypothesizes that “the earlier spring migration of yellows may reflect a more advanced phenology [blooming of plants] along their typical migration route, the Coastal Plain, compared to western, inland routes of western palm warblers…[and] maybe an earlier breeding season.”

During migration in southeastern Pennsylvania, for example, palm warblers feed on the ground in early spring but switch to feeding in trees as soon as they bud and flower and attract the insects palm warblers prefer.

A yellow palm warbler in spring plumage with a caterpillar in its bill

A yellow palm warbler in spring plumage with a caterpillar in its bill (Image by Kelly Colgan Azar on Flickr, Creative Commons license)

Recent studies in 2013 by Frank LaSorte, a research associate at the Cornell Lab of Ornithology, and colleagues, discovered that many small, insectivorous North American bird species fly looping, clockwise migration routes. Those eastern species use strong southerly tailwinds in spring over the Gulf of Mexico and less severe headwinds in the fall. But they also seem to be following the flush of green vegetation in the spring in search of insects, which may help to explain the difference in migration rates between western and yellow palm warbler species.

Even though scientists have been studying bird migration for many decades and have found so far that they orient using magnetic fields, stars that rotate around the North Star, patterns of polarized light near sunset, as well as learning their route through experience, they are not certain if the birds can easily switch from one navigation process to another during a night when, for instance, it is overcast.

Palm warblers are, like many songbirds, night time migrants. In spring they travel with other early migrants—ruby-crowned kinglets, hermit thrushes, and their closest relative—yellow-rumped warblers. On migration either way they also mingle with other early migrants—pine warblers, chipping sparrows, and eastern bluebirds.

In the fall, they are among the last warblers, along with yellow-rumps, to migrate through Pennsylvania. Then they are usually seen in or near the ground in woodland edges, thickets, brushy areas or on the grassy sand dunes in Presque Isle State Park, according to The Birds of Pennsylvania.

Palm warblers don’t sing much on migration or even on their nesting ground, uttering five buzzy notes per second and reminding one observer of a debilitated chipping sparrow.

Even though much has been learned about migrating palm warblers, only one observer, D. A. Welsh, back in 1971, has studied palm warblers (in a Nova Scotia spruce bog) over their entire breeding period, although earlier observers in Maine, southern Quebec, and New Brunswick described nests they found.

Nest of a Yellow Palm Warbler in Maine

Nest of a Yellow Palm Warbler in Maine, from Ora Knight, Birds of Maine, 1908 (In the public domain)

Maine ornithologist, Ora Willis Knight, in 1904, discovered a yellow palm warbler nesting in a bog consisting “of large open expanses thickly carpeted with sphagnum mosses and dotted with numerous small trees and shrubs” including black spruce, tamarack, Labrador tea, swamp laurel and sedges.

Knight wrote that it is difficult to find nests because the incubating bird won’t flush until you practically step on it and he was not willing to do the alternative—“visit the bog and spend day after day during the nesting season, fighting the voracious mosquitoes.” I know about those mosquitoes. We lived in rural Maine from 1966 until 1971, and even though we were raised in New Jersey, we could not believe the clouds of mosquitoes in our woods.

Apparently, D.A. Welsh was made of sterner stuff. He reported that the average territory in Nova Scotia was between 2.61 and 6.61 acres and included trees around the margin of a bog.

Males established territory around the ten nests he studied by singing from high song posts and chasing interlopers, including American redstarts, dark-eyed juncos, magnolia warblers and common yellowthroats in addition to other palm warblers.

Nest-building occurred between May 26 and June 7 and was constructed in the sphagnum of a peat bog at the base of a small conifer. Even Welsh, though, never saw nest building and so it isn’t known which sex chooses the nest site, if the male assists the female in constructing the nest, what time of day it is built or how long it takes.

He and other researchers discovered that the nest is cup-shaped and hidden well in sphagnum and includes weed stalks, grass and sedges, and woody stems of Labrador tea. It is often lined with feathers including those of spruce and ruffed grouse, American bitterns, barred owls, blue jays, and American robins.

Females brood four to five eggs 11 days and are fed by the males. Both parents feed the nestlings for the 12 days they are in the nest, but Welsh found that the females did most of the feeding, with the males watching closely while the females fed and vice versa. Possible nest predators were gray jays, short-tailed weasels, and garter snakes.

Western palm warbler juvenile

Western palm warbler juvenile (Image by Jeff Bryant on Flickr, Creative Commons license)

Once the young fledged, they stayed with their parents at least eight additional days. Presumably, after those eight days, the young are on their own, but no researcher has verified this.

From 1966 to 1994, Breeding Bird Surveys reported increasing numbers of palm warblers. Christmas Bird Counts from 1995 to 2010 found a modest increase in Florida’s wintering palm warblers. However, at Manomet Banding Station in eastern Massachusetts, they recorded a 76% decline in western palm warblers from 1970 to 2001 and a 26% increase in yellow palm warblers during the spring.

While migrating, palm warblers are often killed in collisions with tall, lighted structures. Increased mining in peatland also may impact palm warblers because they need undisturbed vegetation for nesting, and once disturbed by mining, the peat takes many decades for even a partial recovery.

Still, unlike many songbird migrants, palm warblers are not threatened or endangered. As long as their nesting and wintering ground remain reasonably secure, and they can find plenty of food wherever they travel and live, we can look forward to seeing these birds during migration.