Perspectives on Wildlife: Heading South for the Winter

The palm warbler passes through our area to and from breeding grounds further north. Photo: John Burk Photography.

For birders anywhere, spring and fall migrations provide an opportunity to encounter an enormous variety of birds, including some seldom, if ever, seen at other times of year. And there is no better place to see them than the Connecticut River watershed and its valley in particular, which is internationally recognized as an especially important corridor for migrating birds, equivalent to an avian interstate highway. The annual fall staging of up to a half million tree swallows at Goose Island near the river’s mouth punctuates this fact.

Scientists have figured out some of the mysteries about bird migration but not all, and there are some common misconceptions that are folklore rather than truth. For one, the old saw about robins being a sign of spring is bona fide hokum. They handle cold well enough for some to stay here year round, although the robin you see in your backyard in January may not necessarily be the same one you hear chirruping in June. Many do head south in fall, but the vacancy they leave behind is often filled by others migrating from breeding grounds as far north as the shores of the Arctic Ocean.

The common yellowthroat is a breeding-season visitor in New England. Winter sends it to the southeastern states, Mexico, Central America, and the Caribbean. Photo: John Burk Photography.

Those robins that winter over switch to berries when they cannot yank earthworms from frozen or snow-covered ground. Birds that, unlike our robins, can’t find something to eat migrate to where they can. Disappearing food resources are at least as much a cause for avian migration as climate.

Among the triggers that give birds the migratory jitters are rising or falling temperatures, along with all-important changes in daylight. These environmental cues signal it is time to get moving.

Along with Arctic robins, myriad other birds that breed at high latitudes are passing through our region about now as they migrate to warmer climates, as far as South America. Notable among them are warblers, including a species named after the Nutmeg State, the Connecticut warbler. Ironically, although so named because it was discovered in Connecticut, it does not breed nor is especially common there, even during migration.

As they have prepped for their journey, migratory birds that grace our area from mid-spring through summer have been in a feeding frenzy, storing fat to fuel their migration to warm-climate wintering grounds. Even before summer’s end, the semiannual seasonal changing of the backyard avian guard is well underway.

Baltimore and orchard orioles, veeries, wood thrushes, hummingbirds, various warblers, and a host of other breeding species are going south, to be replaced by dark-eyed juncos, redpolls, snow buntings, pine siskins, evening grosbeaks, tree sparrows, white-throated sparrows, and horned larks coming down from further north.

During winter, when the variety of backyard species declines even with the presence of northern migrants, year-round birds such as the northern cardinal, Carolina wren, tufted titmouse, and black-capped chickadee grab the limelight they had to share during spring and summer. They seem much more obvious without the competition.

During irruptions, the pine grosbeak, like this one in Massachusetts’s Wendell State Forest, ventures well south of wintering grounds in northernmost New England and Canada. Photo: John Burk Photography.

Many birds that arrive here in spring and leave by fall and that we think of as “ours,” in fact, are virtually transients, seemingly here today, gone tomorrow. The haunting ee-oh-lay call of the wood thrush that starts echoing through our woods about the beginning of May fades even while summer peaks. A recent study by New Hampshire Audubon found that some adult wood thrushes in the Granite State had packed their bags and lifted off for southern Mexico and Central America by late July.

Fall migration can be a leisurely affair compared to that in spring when, spurred by the urge to breed, migratory songbirds often cover thousands of miles in just a couple of weeks. The wood thrush that jets 2,000 miles in about two weeks heading north may spend three times as long in the opposite direction in fall.

Migratory birds may wait several days before setting out, gathering in large groups—a behavior called “staging”—before starting the long fall haul. Some really take their time, visiting for several days at various spots along the way.

Be that as it may, not all of our winter birds are present in similar numbers every year. Some, such as pine siskins and evening grosbeaks, undergo what are called irruptions, that is, explosive, sporadic mass migrations south when food is short in Canada. Juncos have two forms of migration: altitudinal and latitudinal. They move in from the north but also down from higher elevations in our region, where some stay to breed. That makes them a top candidate for the most abundant backyard winter bird.

The Connecticut River Valley Migration Superhighway
So valuable is the Connecticut River Valley flyway to migratory birds that it has several spots designated by the National Audubon Society as Important Bird Areas (IBAs). Among them are the “Middle Connecticut River IBA” which stretches from the Massachusetts border up to Charlestown, New Hampshire, and Springfield, Vermont; “Barton’s Cove IBA” in Gill, Massachusetts; and “Mouth of the Connecticut River IBA” between Old Lyme and Old Saybrook, Connecticut.

New Hampshire Audubon and Audubon Vermont attribute the valley’s importance to its north-south orientation. Massachusetts bird migration expert Aaron Bagg wrote in 1923 about “The Connecticut Valley—A Highway for Bird Migration” in The Auk, the journal of the American Ornithological Society. He suggested that “this Valley, like others, was used for centuries, if not ages, by the birds in their annual spring invasion and autumn exodus.” Before and since Bragg made that statement, researchers concerned with the importance of migratory patterns to the survival of bird species that trek between north and south every year have focused on the Connecticut River corridor.

Studies in recent years have focused in and around the US Fish and Wildlife Service’s Silvio O. Conte National Fish and Wildlife Refuge, established in 1997 “to conserve, protect, and enhance the abundance and diversity of native plant, fish, and wildlife species and the ecosystems on which they depend throughout the 7.2 million acre Connecticut River watershed.” The refuge covers almost 40,000 conserved acres within parts of the four watershed states of New Hampshire, Vermont, Massachusetts, and Connecticut.

In one study, researchers used mist nets to catch woodland birds during spring and fall migrations. They took various morphological measurements and blood samples to determine the birds’ overall health and whether they were eating well enough to fuel their journey.

In fall, particularly, the estuarine habitats where the river meets Long Island Sound make for a critical rest area. There, birds can take a much-needed break and feed before heading south over the open waters of the Atlantic Ocean.

Scientists have focused on stopover areas all along the river where migratory birds rest and gas up during their travels. Outside of protected areas like the Conte refuge, many resting spots have been eliminated or, at least, damaged by habitat loss and fragmentation.

Researchers have categorized three different stopover habitats: sites adjacent to the Connecticut River’s main trunk; those in smaller tributaries, or so-called sub-watersheds; and uplands at sub-watershed margins not associated with any stream.

Studies have shown that larger numbers of spring migrants stop close to the Connecticut River than at the other two types of rest areas. Those rest areas along its shores probably were sheltered and warmer than others, and perhaps had more food. As the migrating squadrons press north, birds tend to fan out from the river corridor and disperse into the other habitat sites.

The dark-eyed junco migrates here from further north and also from higher elevations, making it an abundant backyard winter bird. Photo: Courtney Celley/USFWS.

The north-south delineation of the river makes it a clear-cut geographical marker that helps migrating birds navigate as they fly. Visual landmarks, however, are but one of the many navigational tools used by migrating birds. Celestial objects, magnetic fields, and a host of other cues are orchestrated into a complex system that scientists have studied intensely and still do not fully understand but are slowly unravelling.

Recently, for instance, researchers at Germany’s University of Oldenburg and the Institute of Avian Research in Wilhelmshaven found that a protein in the bird’s eye called cryptochrome 4, which senses magnetic fields, functions at a much higher level in birds that migrate than in those that don’t. They found that the sequence for the gene that encodes chryptochrome 4 is highly optimized for sensing magnetic fields in migrating birds while some species that do not migrate have lost it entirely. The process demonstrates evolutionary adaption for migration.

Increasingly, scientists look to quantum physics to describe how chryptochrome 4 works. One theory gaining credence is that when light activates the protein a quantum operation enables the bird’s eye to see magnetic fields.

All well and good, but much about bird migration still remains as mysterious to scientists versed in quantum biology as it was to Aristotle, who thought that in winter redstarts turned into robins—European robins, that is, which are not closely related to American robins but have a reddish breast.

Ed Ricciuti has been writing since the 1960s, including about science, nature, and conservation around the globe. He was a curator at the New York Zoological Society (now Wildlife Conservation Society) and a consultant for the World Conservation Strategy of the International Union for Conservation of Nature and Natural Resources.