Vermont meadow in September
Fiona Lee MacLean
As the weather begins to shift, we turn our attention to southward migration. From Green Darner Dragonflies to Bicknell's Thrush, VCE scientists are uncovering the mysteries of migrating species and using this knowledge to help conservation efforts.
By Michael T. Hallworth
The early, lucky dragons develop fast and face a long, fall migration to a destination they have never seen. The later ones get locked under the ice, enter a state of diapause, and wait for their time to shine above the water next year.
I’m talking about dragonflies. In late April and early May, Common Green Darners (Anax junius) arrive in Vermont after migrating north for several weeks from the southern ponds where they eclosed (emerged). Even after a long migration of 680 to 2,000 kilometers (about 425 to 1,200 miles), they don’t waste any time finding mates and laying eggs—one of the earliest observations this spring was on May 26th of a pair laying eggs soon after arriving in Vermont.
Over the last several months, as the water temperatures in our ponds have risen, the nymphs have developed into one of the ponds’ top predators, eating anything that can fit in their mouths, from mosquito larvae to small tadpoles. September is when the nymphs hatched from eggs laid in May climb up out of the water, shed their keratinous exoskeleton that served them well in an aquatic environment, spread their wings, and take to the skies.
It doesn’t take long for them to find other dragons. Large swarms of Common Green Darners can be observed aggregating along coastlines in the hundreds of thousands as they start their migration south. In fact, these large swarms provided the first evidence that Common Green Darners were migratory. Some of the first reports of dragon swarms date back to the 1800s.
Most are headed to the deep South, including Florida, Louisiana, Texas, Mexico, and the Caribbean. We documented their epic migrations using the chemical composition of their wings, which allowed us to estimate where they developed as nymphs. Every Common Green Darner captured in the South in September was from northern latitudes such as southern Canada, northern New York, and northern New England.
The southward migrants play a critical role in their migratory circuit. They are the second of at least three generations required to complete their annual cycle. Once they arrive at their destination in the South, which they have never seen, their final act will be to lay eggs in the subtropical ponds. Their young will become a non-migratory generation that stays in the South. Their ‘grand-dragons’ will undertake a similar, yet opposite northbound migration beginning next March, following the rising temperatures northward. As they arrive next spring, they’ll lay eggs and start the cycle anew.
By Kevin Tolan
On most nights during spring and fall migration, as humans sleep, birds fly overhead in numbers that would thrill any daytime birder. In the last few weeks of summer, species ranging from warblers to thrushes to shorebirds take flight as the sun sets and temperatures drop.
Migrating at night provides several advantages over migrating during the day, such as cooler temperatures, fewer avian predators, and the heightened visibility of celestial navigation cues. While their tiny bodies are invisible against the night sky, they let their presence be known through nocturnal flight calls (NFCs): short vocalizations given primarily during flight.
NFCs are species-specific vocalizations which tend to consist of a single whistled or buzzed note, and have been of interest to researchers since the late 1800s. Our understanding of NFCs has increased greatly in the past decade due to the advancements of small, inexpensive, high-quality recording equipment and large-scale data storage. Specialized detectors such as Nighthawk, akin to the Cornell Lab of Ornithology’s popular Merlin app, also greatly reduces the time needed to comb through months’ worth of night-time audio.
Though specialty microphones have been developed to maximize the number of NFCs detected, many can be heard by human ears. Certain species, such as Catharus thrushes, Indigo Buntings, and Killdeer, produce loud and distinctive NFCs that are readily identifiable in the field. Other species, like many of our sparrows and warblers, produce calls that are virtually indistinguishable without computer software. Surprisingly, the mystery of how to accurately differentiate Bicknell’s Thrush NFCs from those of the closely related Gray-cheeked Thrush has only recently been solved.
While the purpose of nocturnal flight calls isn’t fully understood, research suggests that NFCs may function to identify other individuals undergoing migration and act to stimulate Zugunruhe, the feeling of migratory restlessness. Combined, these may help maintain social cohesion within a flock.
NFCs can be effective at documenting species underdetected by birders during migration, like skulky species such as Wood Thrush and Veery, and Grasshopper Sparrow. They can also detect rare migratory birds, such as Black-bellied Plover and Upland Sandpiper, that may be easily missed during traditional bird surveys. The ability to identify species based on their NFCs can also improve weather radar-derived migratory estimations, such as those used by BirdCast, which can be referenced for lights out initiatives.
I’ve already set up one recorder behind the VCE office in White River Junction, and a second unit at my house 55 miles northwest in Roxbury. With peak migration in Burlington running from September 15th through October 12th, there’s still time for you to get set up, too. To learn more about recording NFCs, some good online primers are available at Nemisis Bird, Oldbird, and LycoBirds.
While NFCs have a fairly steep learning curve, documenting them is a rewarding experience that gives you a unique view into the nightlife of transient migratory birds.
By Alden Wicker
You never know what kind of crucial habitat your backyard—yes, even backyards surrounded by housing developments or within smelling distance of a large animal farming operation—could provide to a migrating bird.
Take Bicknell’s Thrush (alpha code BITH). Right now, this little brown bird is up on New England and eastern Canada’s high peaks in the rarified spruce-fir forests, molting and refreshing plumage in preparation for an epic, southbound migration. At the end of this month or the beginning of October, the wind will change and they’ll take to the air.
Before they cross the Atlantic to Hispaniola, however, the thrushes will touch down somewhere on the East Coast to rest and refuel for a few weeks, building up fat and muscle for a trans-Atlantic flight. “It’s not until recently that we realized, through Motus radio telemetry and GPS tracking, that almost the entire population of Bicknell’s Thrush from Vermont to Maine to Canada seem to be stopping over in the same Mid-atlantic locations in the fall,” says VCE biologist Desirée Narango, Ph.D, whose research before she arrived at VCE focused on stopover habitats in urban areas.
Like road-tripping humans traveling the Eastern Seaboard, they congregate at rest stops in the Delmarva Peninsula, and in some parts of coastal North Carolina. “When we think about Bicknell’s Thrush, we think about northern boreal forest and high-elevation rainforest in the Caribbean, but fragmented forests in an agricultural landscape in Maryland is not what immediately comes to mind,” Narango says.
Narango is from Maryland herself, and she thinks that if local birders like her did observe a grayish Catharus thrush in the area, they would be more likely to have logged it as a Gray-Cheeked Thrush, a nearly identical-looking species from which Bicknell’s was split off in 1995. Now it looks like they might be having a mixer in the mid-Atlantic Region.
That means the wealth of eBird or iNaturalist data, which scientists typically use to look at habitat use, probably underrepresents Bicknell’s Thrush in the area. However, it’s likely that Bicknell’s and Gray-cheeked Thrushes use similar habitat, and crowd-sourced observations of migrating Gray-cheeked Thrushes have put them in both heavy agricultural areas such as the Eastern Shore of Maryland, and along the highly developed I-95 corridor, which runs through the Mid-Atlantic’s largest cities like Washington D.C. “It’s either urbanization or it’s agriculture, and neither of those are good for habitat,” Narango says.
VCE has been studying Bicknell’s Thrush on Mount Mansfield and in the Dominican Republic for decades, but the species’ use of this stopover area—in particular, its habitat and diet preferences while there—remains a mystery. This is an especially important area of study, because VCE’s recent research on female thrushes suggests that something is working against their survival during fall migration. Evidence suggests that an equal proportion of male and female fledglings hatch in the Northeast, but in the adult population the following year, there are two or even four males for every female.
In short, the team wants to figure out: “Where are these birds dying and what can we do to help?” Narango says. It’s important to fill in gaps in our understanding, and according to the Bicknell’s Thrush Conservation Action Plan, stopover habitat is a major unknown. “We have to use more sophisticated techniques to figure out where they are and what they’re doing.”
The BITH team at VCE, including Narango, Michael Hallworth, Ph.D., and Jim Goetz, is partnering with bird banders in the Mid-Atlantic Region who will collect blood samples and precise body measurements of Gray-cheeked/Bicknell’s Thrushes to determine which bird species they’ve caught. They will also record characteristics of the habitat at the location of capture. With funding from the USFS International Program, VCE’s BITH team will also test out some new methods of detecting Bicknell’s and Gray-cheeked Thrushes during migration, including using audioplayback and autonomous recording units.
So this September and October, even if you’re in a place you might not typically think of as habitat for threatened birds—perhaps a bird feeder in a suburban yard or a forest patch beside a highway—keep your eyes peeled for unexpected birds.
“During fall migration, birds end up in all kinds of weird places,” Narango says. “You might get birds that breed much farther north than you, and your habitat could be super important for their survival to make it south.”
