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Birds of Spring
March 29, 2015
Blackpoll Warbler
Wood warblers come north as the leaves unfold. They feed on the forest caterpillars that feed on the new green leaves. Their northward flight keeps pace with the unfolding bud and expanding leaf….Buds burst, new leaves unfurl, larvae hatch, and warblers appear.
—EDWIN WAY TEALE, North with the Spring
In the eastern and central United States during the spring songbird migration, new colors and sounds flood northward across the landscape. Although generally overlooked by most people, the songbirds travel through woods, cities, and suburban neighborhoods for a few weeks in large numbers, showing their bright breeding plumages in backyards as the sap is rising, the leaves are flushing out, and the woodland wildflowers are blooming. Everything is fresh and green, and the migrant birds are vibrant window dressing to this most seductive of nature’s seasons.
We birders love the sounds of spring song, the visitors’ resplendent plumages, and the fecund, damp-loam aroma of the new season in the woods. Males of many songbird species sing their territorial songs and forage greedily to refuel for their next move northward. Birding most months of the year is pleasant enough, but birding on certain days of spring is transporting. Within a half-hour’s drive of my home in Bethesda, Maryland, I can track down nearly a hundred species of birds—including more than twenty species of wood warblers, five vireos, five flycatchers, five thrushes, two tanagers, two orioles, and others—on a single good day in May.
THE NATURAL HISTORY OF MIGRATION
The birds we glimpse passing northward in spring are conducting a strategic relocation exercise to reproduce and to enhance the long-term survival of their genes. The birds cross seas and continents in their travels in spring, just as they do in autumn, to situate themselves in just the right place at the right season. Migration, then, is the purposeful and directed seasonal movement from one geography to another in a repeated annual cycle—a life-history solution to the ecological challenges posed by living in a seasonally variable world.
Many animals migrate: butterflies, dragonflies, salamanders, birds, bats, and whales, to name a few. Migration is most prevalent in environments that exhibit predictable changes in environmental conditions with the seasons: typically from wet to dry or from warm to cold. Animals migrate to escape from the cold, to find food, and, in some instances, to reach places where they can reproduce safely. Think of female Gray Whales, traveling from Alaska to the warm and secluded waters of the Gulf of California each year to give birth. Some migrations are grand, such as the seasonal movement of wildebeest across the Serengeti. Other migrations are less substantial, as when our local populations of American Robins join into flocks and wander within the Mid-Atlantic region in search of winter fruit after the ground has frozen and they can no longer find earthworms. Some migrations are just a matter of a few hundred meters, as when Spotted Salamanders move from their forest floor home ranges to vernal pools to reproduce. Migratory movements come in so many forms that whole books have been written to describe the breadth of the phenomenon.
In the Mid-Atlantic, some birds migrate and some do not, and the new birdwatcher might wonder why. Those that do not migrate seem to have the capacity to provision themselves on their home range year-round, even during the leanest months of winter. Those that cannot find adequate food during the winter move to another location where they can. What advantages accrue to species that do not migrate? To answer that, first let’s acknowledge that in nature, long-distance travel is dangerous. The migrant might fall prey to predators, get lost, or have difficulty finding food along the way. The migrant also might have trouble finding an ideal wintering environment that is not already occupied by competitors, or it might encounter bad weather en route. Staying at home, by contrast, has advantages. The resident bird is familiar with the landscape, is aware of local threats, and knows where to find food. Typical nonmigratory birds have a broad and seasonally adapting diet that allows them to meet the challenges of winter. An overwintering chickadee, for example, might visit the same backyard feeder each winter. It probably inhabits a territorial patch of no more than fifty acres over a full year. By staying put, the bird has a reliable habitat in which to feed and reproduce. It gets to know the nooks and crannies of its own patch and, year by year, knowledge of its home range grows, and accumulated information about local threats helps it to avoid them. Staying at home is the best bet for certain species.
What about advantages accruing to the migratory bird? First, let’s remind ourselves that the spring migration of birds is ultimately motivated by the physics of the earth’s orbit around the sun. Spring happens because the earth’s axis of rotation is tilted 23.5 degrees away from vertical with respect to its annual orbit of the sun. As the earth circles the sun, its poles gradually tilt toward and then away from the sun. As the earth moves from its northern winter solstice to spring equinox, more direct sunlight hits the earth’s surface in the Northern Hemisphere and day length increases. These two physical changes to our north-temperate sector of the planet produce the procession of spring. Longer days and increased daily solar radiation encourage the rapid growth of plant life and the invertebrates that subsist on plants. Birds move north to follow this burst of plant and insect life as the sun moves northward with the season. The migrants time their travel to arrive at their breeding habitat at a point of maximum seasonal abundance.
Tropical forest, where many songbird species spend the winter, is benign but lacks the very long days of northern summer that give birds plenty of time to forage, and it also lacks the seasonal flush of biotic growth that accompanies springtime in northern climes. During the nesting season, most migrant songbirds eat mainly insects, and thus their annual movements are partly tied to tracking the abundance and availability of insect prey. Insects are abundant in boreal forests in late spring and summer but essentially absent during the cold and dark time of the year, from late autumn to early spring, so the songbirds migrate to the Tropics to escape the long season of privation. In both autumn and spring, the signal to migrate comes from the sun—day length and the sun’s track across the sky.
In spring, most migrant songbirds make an effort to return to their natal territory, settling quite close to where they were raised. In addition, in their second autumn, they return south to the same area where they successfully wintered the preceding year. How they achieve the GPS-like adjustments to locate these pinpoints on the globe remains a mystery, as we will discuss at length later in the book, but we do know that many birds accomplish this remarkable feat of orientation year after year.
SONGBIRDS, NEOTROPICAL MIGRATORY BIRDS, AND WARBLERS
As the days lengthen in the United States, most of the small land birds that migrate here from south of the border in springtime are songbirds. Even non-birdwatchers know plenty of these creatures; examples include the Northern Mockingbird, Blue Jay, Carolina Chickadee, Yellow Warbler, Northern Cardinal, House Wren, and American Crow. Their lineage—the suborder Passeri within the order Passeriformes (the perching birds)—globally comprises about four thousand species of land birds that evolved in the Australian region fifty million years ago, when it was part of Gondwanaland. Today songbirds inhabit every continent except Antarctica. As their name suggests, they are accomplished singers: they alone among the birds possess a highly evolved syrinx, a complex vocal organ that allows them to make their elaborate and distinctive sounds. That said, most birds are regular vocalists, but those without this anatomic specialization cannot match the singing capabilities of the songbirds, which make memorable and affecting music.
Most familiar Neotropical migrants are songbirds, but there are exceptions. One group, the tyrant flycatchers (such as the Eastern Kingbird and Great Crested Flycatcher), are passerines but not songbirds. For simplicity in this book, however, I use the term migrant songbird as a convenient shorthand to refer to all migrant perching birds, including the flycatchers.
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br /> In this book, we shall encounter many kinds of Neotropical migratory birds, meaning those that breed in Canada and the United States and migrate south in autumn to spend the nonbreeding winter season in Mexico, Central America, South America, and the Caribbean. They include more than two hundred species of land birds, plus many other groups: waterfowl, raptors, shorebirds, and terns. Some of these species, such as the Red Knot, travel as much as ten thousand miles on their southbound journey in fall and do it again on their return in spring. All these birds are border-crossing international travelers.
Special among the Neotropical migrants are the New World wood warblers (the Parulidae), a unique Western Hemisphere family of 118 species of small songbirds. As a family, they are widespread and well known, but individual species often are tantalizingly difficult to locate on their breeding grounds. Forty-seven of the wood warbler species nest in the United States and Canada and migrate to winter in the Subtropics and Tropics. Warblers that nest north of the Mexican border are the passionate focus of many birders, especially during spring migration; among these are the thirty-seven eastern wood warbler species that I hoped to see on their breeding territories on the journey described in this book. Part of their allure is their elusiveness—how many of us have seen all thirty-seven in a spring season? How many of us have seen all of them on their breeding grounds?
THE TENNESSEE WARBLER
Often in this book, I will highlight a particular wood warbler when describing an aspect of the life history of the Neotropical migrant songbirds. The Tennessee Warbler, for example, displays the migratory pattern and life cycle of a prototypical Neotropical migratory songbird. This bird—a four-inch-long warbler wearing a subtle plumage of gray, olive green, and white—winters in southern Central America and northern South America and breeds in the boreal forests of the northernmost United States and Canada. In late June, the Tennessee Warbler hatches from its egg in a small cup nest hidden amid low vegetation on the ground in a northern forest clearing. It starts its life in a time of long days, warm temperatures, and abundant insect prey. It is provisioned by its two parents while in the nest, which it departs after only eleven days. Its parents continue to bring it meals of small insects as it learns to fly and begins to move about the environs of its local forest patch with its siblings. By mid-August, it can fly well and is able to forage for itself—quite an accomplishment for a bird less than two months old.
Tennessee Warbler
In late August, feeling the tug of the innate impulse that scientists call migratory restlessness, our Tennessee Warbler takes to the sky after dark, heading southeastward in a series of night flights. Each flight might last as long as ten hours and cover more than two hundred miles, punctuated by stops to rest and refuel. After several weeks, it reaches the Gulf states. There it halts to build up stores of fat for its overwater trek to the Tropics. One evening after dark, it heads out over the Gulf to Cuba or another Caribbean stopover. After a few days of refueling, the bird once again takes off after nightfall and heads to northern South America or southern Central America, where it will spend the winter. In its chosen winter home, it seeks out a large patch of habitat to pass the next seven months of its sociable life, joining other Tennessee Warblers as well as other warbler species in flocks that forage for food each day. This flocking behavior probably helps the birds to find food more efficiently and avoid predators.
With the lengthening of the days in March, the warbler, in its tropical retreat, becomes restless once again. In early April, on a clear, warm evening with light southerly winds, it departs its forest patch, climbs into the air, and heads northward in stages to its breeding habitat in the Great North Woods. Its main challenges are crossing the Caribbean and then the Gulf of Mexico, which it presumably achieves in separate night flights.
Back in the United States, it rests, refuels, and starts a series of nocturnal flights northward up the Mississippi Valley before it faces one more physical hurdle: the Great Lakes. We know that songbirds consider crossing the lakes a substantial challenge because of evidence from Magee Marsh, an important bird reserve on the south shore of Lake Erie. Under ideal springtime conditions, large numbers of northbound Tennessee Warblers settle into the woods here on the south side of the lake rather than continuing on across the lake. Instead, they make the lake crossing after they have refueled and while they are fresh at the start of a new flight northward.
Once back in the Great North Woods, each migrant returns to the boreal forest where it was born. The males arrive about ten days before the females because they must win a breeding territory to attract a mate once the females arrive on the breeding grounds. To do this, a male must defend his territory from competing males. His song helps him: one of its functions is to declare ownership of his territory, while the other is to attract a mate. The male bird sings his loud and staccato series of chip notes more than a thousand times a day. Once mated, the Tennessee Warbler’s life cycle begins all over again.
The most important measure of any organism’s life is reproduction. For a warbler, this takes place during a relatively brief period in June and July. Success in lifetime reproductive output among individuals drives natural selection, which is the basis for organic evolution. We might call the reproductive drive the “life force.” For human societies, wealth, knowledge, and culture constitute major life goals, but for species living as a part of wild nature, the single objective is to leave on earth an abundance of fecund offspring, thereby ensuring the survival of their genes in following generations. Thus, when a male Tennessee Warbler gives its staccato song, it is not singing because it is happy (though it may be) but because it is trying to do three important things: stake a claim on a patch of territory, signal its presence to other, competing males, and attract a mate from among the females in the vicinity. Male migrant songbirds also sing during migration—less commonly far from their breeding range, but more commonly as they get close to their nesting grounds. It appears they are “practicing” their territorial song, so they can put it to good use immediately upon their arrival.
At the end of summer, adult Tennessee Warblers and their young migrate southward separately and probably follow different routes and timetables. The first-year birds get together and head south for the first time with nothing more than genetically programmed rules to guide them toward their tropical winter destination, where they have never been. That many of these yearlings arrive safely at their destination is testament to the fundamental soundness of their evolved migration system, as well as the mysterious nature of the phenomenon. To complete the roundtrip journey south and then back north to the breeding area, the warbler needs, in essence, a map, a compass, a calendar, a clock, and a good memory, all stashed away in a brain little bigger than a couple of peas. It is no wonder that biologists look upon migration as a miracle of evolution. That said, many individual warblers perish during the two legs of migration each year. Typically, Tennessee Warblers that successfully fledge from the nest live only three to four years, and only half the birds that head south return to their northern birthplace the following spring.
So what advantages accrue to the Tennessee Warbler that impel its risky twice-yearly migration to and from the Tropics? Key among them is food, of course. Seasonal dietary specialization by the warbler leads it to follow its preferred arthropod prey from tropical to northern temperate forests each year. In the northern summer, this warbler raises a brood of four or more nestlings in a boreal forest where the long days produce a prodigious flush of insect prey, which the warbler adults harvest to feed to their nestlings. Later, as the northern winter approaches, migrating allows them to find a habitat in the Tropics that provides insects as well as nectar sources.
Between September and April—for as long as eight months a year—the typical Tennessee Warbler resides in some patch of habitat in the Tropics. One might assume this is a time of relaxation for the bird, but that’s probably not so; instead it is a time of survival. This is evidenced by the fact that ma
ny of these wintering birds have not added fat stores by the end of their sojourn in the Tropics, indicating that they haven’t found abundant food there. This is the nonproductive season, when the bird is biding its time. But it is an important period nonetheless, for the bird must remain healthy and fit during this off-season if it is to succeed during its hectic breeding season in the North Woods.
THE CONSERVATION STORY
The life story of any migratory bird such as the Tennessee Warbler is incomplete without an account of the threats that these birds face during migration. Various human-caused threats were noted in the 1960s by Rachel Carson, Roger Tory Peterson, and other prescient naturalists. More than half a century later, we have measured the specific impacts of those threats. Radar-based studies of migration across the Gulf, as well as long-term studies of occupancy by breeding wood warblers in forests in New England and on their tropical wintering grounds, indicate that the populations of Neotropical songbirds have decreased by more than half over the past fifty years.
An array of forces, operating in different portions of the birds’ ranges, has brought about this serious decline. Loss of habitat has occurred mainly on wintering grounds and along migratory pathways, while fragmentation of breeding habitat has led to reduced nesting success for some species because of the increasing abundance of lesser predators such as opossums and raccoons and because of nest parasitism by the Brown-headed Cowbird. Cats, lighted towers and buildings, glass windows, and wind farms also take a mortal toll. And, without doubt, climate change has had an impact, disconnecting the timing of the birds’ movements from the leafing-out of canopy vegetation and the attendant peaking of populations of the leaf-eating insects that are the birds’ primary prey.
North on the Wing Page 2
