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Songbird Journeys

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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.

Minstrel of the Woods

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I don’t have to leave this planet to hear the music of the spheres. Surely, listening to wood thrushes singing is as ethereal an experience as any mortal can hope for on earth. Many evenings, when I step outside, wood thrush song envelops me and it seems as if all the world’s wood thrushes are singing on our mountain.

Most years I can count on the first wood thrush song between April 30 and May 5 and the absolute last wood thrush song near the end of July. But, in 1997, I heard a wood thrush singing on August 5. The following summer a wood thrush broke that record by singing for a few brief moments at dawn on August 12. Then, last summer, the final date was August 3.

No matter when it happens, though, I listen with an impending sense of loss throughout July. Once again, time is running out, and I have not yet had my fill of wood thrush music. They are with us too short a time, singing like what I imagine angels would sound like, then leaving me bereft the rest of the year. Yet, I wonder if I would be as enchanted by wood thrush song if I heard it all year long or if familiarity would breed, if not contempt, inattention?

Of course, what is exquisite music to me is serious business for wood thrushes. As soon as the males return from their fall and winter homes in the lowland tropical forests from southern Mexico to Panama, they sing loudly from elevated, exposed perches, intent on attracting females and defending their one-quarter to two-acre territories from other males.

Song duels between males, called “countersinging” by ornithologists, in which two males come to within ten yards of each other and alternate songs, are common during territorial disputes. When females arrive, males chase them and sing aggressively. Each male tries to attract a female to settle in his territory. Once he does so, the male sings in lower, hidden sites.

But there are singers and then there are the outstanding virtuosos. The flutelike ee-oh-lay is learned from adult wood thrush males during the fledgling stage. But both his mostly inaudible, short note introduction and trill-like ending are either innate or invented. Each part of his song has several variations and every male has his own repertoire. For instance, one Ohio male sang 18 patterns out of 90 possible ones based on his variations, according to a study conducted by D.J. Borror and C.R. Reese entitled “Vocal gymnastics in Wood Thrush Songs” back in the early 1950s. Another study of 115 wood thrush songs by Aretas Saunders showed ranges as wide as two octaves with the average thrush’s range slightly over an octave.

I well remember the organlike tones of a particularly varied wood thrush song I listened to one evening in a darkening wood. To my ears, the singer seemed to be singing for the pure joy of it. Whether birds have a musical sense is still debatable among scientists, but there is no doubt that they use musical devices, such as crescendo and diminuendo, as well as complex musical phrases, and that they continue to develop their songs long after they have mated and established a territory. In fact, after a lull in their singing in early June, when they are busy feeding nestlings, wood thrushes begin singing again in July with almost as much intensity as in May, and I often hear them countersinging.

One July evening, along the Short Circuit Trail, I counted three wood thrushes singing at the same time, each song coming from a different direction. On other summer evenings my walks take me from one singing wood thrush to another as I move in and out of a succession of thrush territories. Long after I return home, my ears ring with their tremulous echoes and vibratos and, like Henry David Thoreau, the wood thrush’s song “touches a depth in me which no other bird’s song does…it lifts and exhilarates me.” The wood thrush, he writes, is the “minstrel of the woods,” the “master of a finer-toned instrument…a Shakespeare among birds…”

Also known as “bellbird,” “song thrush,” and “swamp angel,” the wood thrush’s genus name Hylocichla is Greek for “wood thrush” while its species name mustelina means “weasel-like” in Latin, referring to its tawny head, wings and back which are supposed to resemble the color of a weasel. Otherwise, the wood thrush has a white breast and belly, liberally sprinkled with larger round or oval black spots.

The nicknames, “swamp robin” and “wood robin,” refer to its membership in the Thrush family along with other outstanding singers such as the American robin. Like robins, wood thrushes forage on the ground by hopping and then pausing to search. They also toss leaves aside and probe the earth with their bills. Once a male attracts a female, they engage in sexual chasing in which the female leads the way during silent, circular flights, interspersed by perching together. Copulation occurs on branches and most pairs are monogamous through the nesting season, which includes the raising of two broods. A small percentage nest with the same mate a second season. One female was unusually loyal, mating with the same male five years in a row and then with another male for three years.

The female chooses the nest site with some input from the male who indicates his preference by “pit-pit” calls and by bringing nesting material to the spot even though the female does the nest-building. It takes three to six days to fashion the cup-shaped nest anchored in a hidden tree crotch or shrub or on a branch five to 20 feet from the ground. Resembling a robin’s nest in shape, it is distinguished by a lining of rootlets and often has a piece of paper or white cloth hanging from the base. The ones I have found here incorporate a piece of plastic from the old farm dump instead of the cloth or paper. During the 1970s, when we had a large population of wood thrushes, I once found three nests within a couple hundred feet constructed in mountain laurel shrubs.

The female usually lays three to four, blue-green eggs, and incubates them from 12 to 13 days while the male perches on trees 20 to 30 yards from the nest and feeds, sings, or preens. He also guards the nest by standing on or next to the nest rim and sometimes singing while the female forages. He occasionally brings her food and freezes if an intruder approaches the nest.

Once the eggs hatch, the male does two-thirds of the feeding of nestlings until they fledge at 12 to 15 days of age. Then the parents divide up the brood and continue feeding them until they become independent and leave their parents’ territory at the age of 21 to 31 days. By then the female has begun laying her second clutch of eggs.

Since the 1970s, wood thrush numbers have fallen, both on their breeding and wintering grounds. Researchers in the Midwest pointed to excessive parasitism by brown-headed cowbirds, which lay their eggs in other birds’ nests for them to raise. The cowbirds grow faster and are more aggressive than the nest-builder’s young so usually the host species raises less of its own offspring. But in Pennsylvania, Jeffrey Hoover and Margaret Brittingham found that despite a nine-percent cowbird parasitism rate on wood thrush nests, there were no nest failures.

On the other hand, in a study done at Hawk Mountain Sanctuary by Hoover, Brittingham and Laurie Goodrich in 1990, fragmented forest habitat was the more critical issue. Wood thrush nests in small forest patches were more heavily preyed upon than those in larger forests.

Researchers have found that the primary predators on wood thrush eggs, nestlings, and/or fledglings are blue jays, common grackles, American crows, gray and southern flying squirrels, chipmunks, least weasels, white-footed mice, black rat snakes, sharp-shinned hawks, raccoons and pet and feral cats. But large, mature forests with well-developed understories provide better protection from predators.

Such forests are also an excellent source of the beetles, ants, caterpillars, moths, flies, bugs, spiders, sow bugs, snails, and earthworms that wood thrushes eat in the summer and feed their young. Later, after the fledglings have dispersed, and during migration, wood thrushes switch to fruits. Then they prefer more open areas that support fruit-bearing shrubs, vines and trees such as spicebush, fox grapes, blueberry, blackberry, mulberry, holly, elderberry, Virginia creeper, pokeweed, dogwood, black cherry and black gum trees.

Here in Pennsylvania the United States Fish and Wildlife Service’s annual Breeding Bird Survey recorded its highest number of wood thrushes per route in 1977 (22). Since then there has been a slow downward trend, but during the Pennsylvania breeding bird atlasing project–from 1983 to 1990–atlas volunteers found wood thrushes throughout the state in every county. Pennsylvania also holds the record for the longest-lived wood thrush–eight years and ten months old.

As long as suitable woodland habitat remains for wood thrushes here, along their migration route, and on their wintering grounds, wood thrushes will survive. As Thoreau wrote on July 5, 1952, “The thrush alone declares the immortal wealth and vigor that is in the forest…Whenever a man hears it, he is young, and Nature is in her Spring.”