Purple Martins

One mid-July afternoon near our barn, our son Steve watched two purple martins insect-catching high in the sky. Several days before, down in nearby Sinking Valley, Steve had driven up a private road called Purple Martin Lane and found two large purple martin houses set up by a local Amish family.

The entrance to Purple Martin Lane in Sinking Valley

The entrance to Purple Martin Lane in Sinking Valley (Photo by Bruce Bonta)

“There were at least 50 purple martins using them,” he told me.

The martins he identified here were the 173rd bird species for our mountaintop property, and Steve thought they had come from the Amish farm he had visited because it was only a few miles away as the crow (or martin) flies.

The next time I drove to the valley I looked more carefully for purple martin houses and found two more Amish farms, out in the open, with two active martin houses.  Only one of each had four gourds hanging below, but I learned from Amish friends that they were growing more gourds for those Amish and other interested farm families to use as purple martin housing.

Purple martin housing

Purple martin housing (Photo by Adam Woodis on Flickr, Creative Commons license)

Purple martins live throughout North America, but those in the eastern part of the continent are wholly dependent on housing provided by humans. That’s because Native Americans fashioned homes for them from hanging gourds and pioneering farm families did the same. Both were interested in these large swallows that specialize in flying insects especially the flies that swarmed over their game and other food. This thousand-year human/martin relationship led to a remarkable trust on the part of purple martins toward humans, so much so that researchers believe purple martin housing should be erected within 100 feet of a human dwelling and 40 feet from the nearest tree.

Pennsylvania is close to the northeastern edge of the contiguous range of purple martins even though they nest as far north as southern Canada. Between Pennsylvania’s first and second atlasing periods, the martin population “contracted dramatically” according to John Tautin, writing in the Second Atlas of Breeding Birds in Pennsylvania, with losses most notably in northeastern and southwestern Pennsylvania, and central Pennsylvania’s Mifflin and Juniata counties. However, as I discovered, martins are more common in Pennsylvania where there are Amish farms.

A pair of purple martins on an Amish farm in Pennsylvania

A pair of purple martins on an Amish farm in Pennsylvania (Photo by fishhawk on Flickr, Creative Commons license)

While these cavity nesters have adopted apartment and gourd nesting in the East, their ancestral nest sites were abandoned woodpecker holes in dead snags. Yet, by 1900, the eastern purple martins had converted almost entirely to human-made houses.

Purple martins are the largest swallows in North America. The males are entirely purple but appear black in the field while female purple martins are pale below with a pale collar and forehead. They arrive in northwestern Pennsylvania from their winter homes in Amazonia Brazil near Manaus in late April, according to studies by Dr. Bridget Stutchbury and her students. By putting geolocators on five purple martins, those researchers found that it took the birds on average 23 days at 180 miles a day including a nonstop Gulf of Mexico crossing to reach northwestern Pennsylvania. However, one martin made it in 13 days which meant that it flew 300 miles a day.

Male purple martins arrive first and try to claim as many compartments in a birdhouse as possible and sometimes multiple gourds too. If each porch in a birdhouse is divided by a barrier that prevents a male seeing into the adjacent apartments, a single male defends less territory. This enables more martins to occupy a house.

A pair of purple martins, the male on the right

A pair of purple martins, the male on the right (Photo by Andrew Reding on Flickr, Creative Commons license)

To attract a female to his nest, a male flies from his nest cavity, sailing in a wide arc, and then returns to his nest site, ending in a steep dive. As he lands, he enters his nest, turns around, pokes his head out and sings. A female, more interested in evaluating the nest cavity instead of the male, visits it repeatedly over several hours. Finally, he treats her as his mate and starts accompanying her (mate-guarding) when she leaves the nest site. After several days they move in together and greet each other whenever they are separated.

Purple martins have a wide variety of calls and songs, but the male uses a series of clicks called the “croak song” and the female a “chortle song” during courtship. Most famous is the very loud “dawn song,” which is used before daylight perhaps to attract other martins to the bird house.

Studies of nest preferences for purple martins indicate that they like the higher compartments in bird houses and the deeper they are, the more fledglings they can produce. Best of all are the gourds that are so deep that the martins can raise seven young in them instead of the more usual four in the birdhouse compartments.

Nest-building in Pennsylvania begins in early to mid-May and is mostly done by the female purple martin, although the male may collect green leaves and sometimes starts nest-building by gathering a few twigs or leaves. But once the female begins, he either accompanies her while she gathers nesting material or sits near the nest cavity and watches.

A purple martin nest with eggs

A purple martin nest with eggs (Photo by OakleyOriginals on Flickr, Creative Commons license)

She builds her nest out of twigs, stems of herbaceous plants, leaves, and mud, but each pair designs their own nest so they vary widely in shape. The depressed nest bowl is filled up with green leaves that often hide the three to seven white eggs the female lays. In Pennsylvania, returning martins reuse the same nest cavities and fledge more young than those that use clean compartments.

The female has a brood patch and is thought to be the incubator during the 15 to 18 days of incubation, but the male often relieves the female for long periods and sits on the eggs.

Purple martin fledglings

Purple martin fledglings (Photo by OakleyOriginals on Flickr, Creative Commons license)

The altricial young hatch and are brooded by the female until their eyes open in 10 days. Both parents feed the nestlings and when they are 12 days old, they cluster together near the entrance hole to greet their parents. By the time they are 17 to 21 days of age, they have their feathers and are exercising their wings by stretching and flapping. Finally, they fledge when they are 28 to 29 days old. In northwestern Pennsylvania that is mid-July on average.

Their parents continue to feed them for several days, but the young begin catching insects on their own and are fully independent seven to 10 days after they fledge. Then they join other fledglings in roosts.

James Hill at the Purple Martin Conservation Association in Edinboro, PA

James Hill at the Purple Martin Conservation Association in Edinboro, PA (Photo by Bruce Bonta)

Back in 1990, my husband Bruce and I visited James Hill, who had founded the membership and research-based Purple Martin Conservation Association in Edinboro, Pennsylvania. Already, Hill had set up a variety of nesting apartments and assemblages of gourds to figure out the kinds of nests purple martins preferred. He also published an excellent quarterly periodical called Purple Martin Update, which is still being published.

Today the PMCA is housed at the Tom Ridge Environmental Center under the leadership of Joe Siegrist. In late summer he takes people on pontoon boat tours near sunset to view the huge martin roost of 20,000 at Presque isle Bay, which draws in birds from at least a 250-mile radius. Under their program called Project Martin Roost they’ve discovered at least 350 premigratory roosts in eastern North America and Canada. Most of these roosts are so large they show up on weather radar.

Usually the roosts are over large bodies of water with cover from reed beds and long brushy islands, but in urban and suburban settings they roost in trees or on manmade structures such as bridges and pipes. The farther south these roosts are, the longer the martins remain. Some are used from eight to 12 weeks.

A purple martin flying at the Codorus State Park in Pennsylvania

A purple martin flying at the Codorus State Park in Pennsylvania (Photo by Henry T. McLin on Flickr, Creative Commons license)

But Stutchbury’s geolocator studies show that once the Pennsylvania birds set out for the south, they fly almost nonstop until they reach Central America as soon as five days later. Then they rest before continuing on to Brazil more slowly than when they head north in the spring.

Other research projects the PMCA engages in with citizen scientists are a Scout-
Arrival Study that tracks the northward migration of the first martins to be sighted at individual colonies and Project Martin Watch, a continent-wide effort to study the nesting dates, number of eggs laid and hatched and how many nestlings survived. They’ve also been banding martins since 1987.

Wet weather is the largest threat to purple martins. If there is a long, cold, rainy spell in June and July, most of the year’s progeny will perish of cold, drown in their nests, or die of starvation since martins don’t fly in wet weather. European starlings and house sparrows may take over martin nests, so martins depend on their human landlords to remove the non-native invaders.

Owls and snakes are also a threat and deeper cavities make nests harder for owls and other avian predators to reach into. Snake guards installed on birdhouse poles can keep them away from the birdhouses.

With enough folks willing to be purple martin landlords, the eastern population of this beautiful bird should grow and thrive.

 

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.