Highways in the Sky
Instead of following a simple north-south migration strategy, many birds fly along complex routes to optimize their survival chances.
By Noreen Damude
Over time, birds have evolved myriad strategies and techniques to ensure they arrive safe and sound at their respective breeding and wintering grounds. Far from being a simple north to south and back again affair, routes of migration are as varied as the migrants themselves. The multiplicity of crisscrossing pathways reflects the histories of populations, their abilities to cross large barriers, the position of topographical obstructions, and the relative locations of summering and wintering grounds.
One of the marvels of migration is the distances traveled by different migrants. Some go only as far as necessary to escape the privations of winter (blue jays, American robins, song sparrows and American crows). Others travel much farther, flying to the tropics and beyond (arctic terns, American golden-plovers, red knots and buff-breasted sandpipers). The exact routes birds follow are almost impossible to plot with precision. Even within a single species, there are variations in timing and pathways. Complex labyrinths of migratory trails overlap and interweave.
Let’s meet a few of those doughty migratory frequent-fliers to compare the varying migratory strategies they put into play to assure survival of their kind over generations.
(Buteo swainsoni) — long-distance, Neotropical migrant
Soaring birds such as raptors frequently migrate along narrow corridors defined by mountains, on whose flanks updrafts provide the motive force for their travels. Broad-front migrants may also be funneled into narrow paths by the nature of the territory over which they pass. This is clearly the case with the slim and graceful Swainson’s hawk, a long-distance Neotropical migrant, second only to the peregrine falcon in distance traveled by a bird of prey. Averaging nearly 124 miles per day, they complete the 6,500-mile trip from Canada to southern Argentina in less than two months. Dubbed the “the harrier hawk” for its hunting style, this long-winged prairie buteo with a rounded head and long tail dines on small mammals, reptiles and birds, but has a special yen for large insects. Observed feasting greedily on crickets, grasshoppers and caterpillars in summer and during migration, Swainson’s hawks have earned another sobriquet, the “grasshopper hawk.” Come autumn, birds congregate in huge flocks sometimes numbering in the thousands as they prepare for “the grand passage.” Once aloft, they move across the landscape by sliding from one thermal to the next. Clouds of hawks swirling like cyclones make their way south on turbulent winds. Caught in a bottleneck south of Texas between the mountains of Veracruz and the Gulf of Mexico, thousands upon thousands of Swainsons squeeze into thick narrow bands like sands in an hourglass across the sky.
(Limosa haemastica) — long-distance migrant, elliptical pattern
Following the same migration route as the once incredibly abundant Eskimo curlew, the boldly patterned, rich chestnut Hudsonian godwit is a classic long-distance migrant. Sturdy and exquisitely built for flight, the “ring-tailed marlin” of early naturalists first migrates west then south nonstop from James Bay, Ontario, to northern South America, a daunting trip of several thousands of miles. From major staging areas in Alaska and western Canada, post-breeders converge along western lakeshores to feed on invertebrates before moving well east to the Canadian Maritimes. Gorging on berries and final fall flushes of insects, they lay down gobs of fat to fuel the nonstop, transoceanic trip. After pre-migratory drills, restless flocks take to the skies, find the right winds and stream inexorably toward their ancestral wintering grounds. Juveniles make the trip on their own. First-timers are not led by experienced elders to their southern cone destination. They rely, instead, on genetically programmed cues to get them to their final destination. Come spring, they fly overland through the midsection of both hemispheres, stopping regularly along the way, thus exploiting a much better food supply.
(Passerina ciris) — medium-distance, Neotropical migrant
Seed eaters are much less likely to migrate long distances than insect eaters and tend not to go as far when they do. Painted buntings are seed eaters par excellence, and find the brushy, grassy habitats of Mexico and Central America much to their liking as a winter home. Come spring, though, these medium-distance, Neotropical migrants will need an abundance of insects to feed their young. Riding the prevailing winds, they join the successive waves of insectivorous birds heading north around the Gulf. They start to appear in significant numbers along Texas shores in mid-April, later than many other Neotropical migrants that must travel much further north. The painted bunting nests in the southern portion of the country. Sometimes called the “nonpareil” for its incomparable good looks, the painted bunting is a small, sparrow-like bird that looks nothing like a sparrow. The multi-colored red, blue, chartreuse and yellow male is unmistakable, and his glistening lime-green consort is equally stunning. Though shy and sometimes difficult to see, birds forage on the ground, in weedy fields or in grassy marshes during migration and on their breeding grounds.
(Podiceps nigricollis) — medium-distance, seasonal Nearctic migrant
Early springtime brings lines of white pelicans, ibis and anhingas snaking high over marsh and beach, weaving and undulating, as they make their way from Texas coastal wintering grounds to breeding areas farther north. Just offshore or on coastal bays we often spot enormous rafts of eared grebes, by the thousands, bobbing buoyantly on choppy waters. These high-riding, compact, petite-featured birds with red eyes and upturned bills join the ranks of other medium-distance seasonal Nearctic migrants as they move northward. Nearctic migrants move north in the summer to breed in the northern portion of the Nearctic region (northern Canada, Alaska, northern U.S.) and return south in the fall to winter in the southernmost points of that same region. Biogeographers usually use the Isthmus of Tehuantepec in Mexico as the cut-off between the Neotropic and Nearctic regions. Those that winter here in Texas begin to don their nuptial plumage just before departure, going from a somber gun-metal gray above and buff below to snappy black and tan set off by gold-feathered ear fans. Sometimes called the “crazed grebe” for its high-pitched piping bugle and splashy courtship display, the eared grebe loves company, even on its breeding grounds. Birds forage mainly by diving and swimming underwater, propelled by their powerful lobed feet. They feed mostly on small fish, crustaceans and aquatic insects. Eared grebes leave their Texas coastal wintering grounds from late February to late May. The less conspicuous horned grebes and common loons, also medium distance Nearctic migrants, disappear quietly under cover of darkness as well.
(Coccothraustes vespertinus) — irruptive, opportunistic wanderer
Few of us in Texas welcome the icy blasts of winter weather that, on rare occasion, sweep across the state between December and February. But with these infrequent bouts of howling winds and swirling snow, a golden winter surprise may appear. The dapper evening grosbeak of northern forests is the occasional bright mustard-colored “irruptive,” whose invading flocks take our breath away. By definition, irruptive species are opportunistic wanderers that vacate their traditional home range only rarely and then under extreme duress. When food resources crash, nomads forced to flee their usual haunts strike out in waves in search of more hospitable grounds. The saffron yellow males are unmistakable, with their blackish hood, wings and tail and a white saddle on the lower back and yellow under-tail coverts. Though to-tally unpredictable, the thick-billed finches take refuge from adversity more often in the northwestern portion of the state than elsewhere. On four occasions, though, the East Texas Pineywoods was regaled with large nomadic flocks of evening grosbeaks that spent the winter gobbling seeds of box elders and other maples. Descending in colorful, noisy flocks, they thrilled “feeder-watchers” who watched them consume prodigious amounts of black sunflower seeds.
(Junco hyemalis) — short-distance latitudinal migrant
Known in many parts of the south as “snowbirds,” the dark-eyed junco deploys several different migration strategies. They are common denizens of the coniferous forests and forest edges, clearings, as far north as the muskegs. As cool-weather birds, they barely reach extreme northern Mexico during the non-breeding season. While the birds prefer coniferous forests in which to nest, they breed both in northern Canada and on mountaintops in the U.S. Most juncos are migratory, but the degree, distance and type vary greatly from year to year and from population to population. Some populations move long distances south in winter, others move only a short distance down the mountain to the valleys below, still others hardly at all. In mild winters, most remain well north. Unquestionably, the urge to migrate is much greater when food is in short supply. Dark-eyed juncos show up around woodland edges, suburban yards and winter bird feeders across Texas — some winters in profusion. Exhibiting various color morphs depending on the population, they all are easily recognized by their white outer-tail feathers, which show conspicuously in flight. The young of montane juncos move down-slope at the end of breeding season while the adults follow later on, exemplifying age-biased migration. Juncos, like red-winged blackbirds, likewise migrate in sexually segregated flocks. In fact, most of the dark-eyed juncos that winter in Texas are largely females and birds of the year, males tending to remain further north – perhaps staying closer to the breeding grounds.
(Aythya valisineria) — molt migrant
One of our largest diving ducks, the gregarious canvasback is wary and swift in flight, earning the respect of seasoned sportsmen everywhere. A handsome diving duck characteristic of prairie marshes in summer and saltwater bays in winter, the canvasback occurs almost exclusively in North America. Canvasbacks display a type of migration pattern called the molt migration that turns the usual linear north-south trip into a three-legged affair. Most waterfowl molt all of their flight feathers almost at once after breeding as they go into eclipse. Eclipse plumage renders the males as drab and camouflaged as the females. During the time they are unable to fly, they must hide away in secluded marshes to feed and rest until their feathers have grown in. The males of many species make special migratory legs to reach these traditional molting safe havens, before flying south for the winter. While post-breeding canvasbacks across North America use multiple routes to reach their wintering grounds, individuals of the prairie breeding population fly south on a broad front to the Texas Gulf Coast. These are the birds we see best at the Laguna Madre, typically in the company of redheads as they forage together on the bays. When migrating or traveling short distances, canvasbacks fly in a well-regimented V-formation. They prefer marshy habitats bordered by dense vegetation, where they dive for their food in shallow water. Birds forage largely on the roots and bases of underwater plants, with wild celery a particular favorite.
(Porphyrula martinica) — migrant in spite of itself
Despite their seemingly floppy flight, purple gallinules are consummate migrants and travel long distances to reach breeding and wintering grounds, sometimes in the wrong direction. Strays have been known to cross the Atlantic, showing up in Europe and southern Africa on several occasions. Vagrants may reach as far north as Canada at any season. Nicknamed the “swamp hen,” reflecting its watery habitat preferences, this gaudily plumaged bird with iridescent green back and royal purple head and neck, yellow-tipped red bill and ice-blue frontal shield is an unlikely-looking flying machine. But despite the dubious flight profile, purple gallinules totally withdraw from the northern parts of their breeding range and fly to the Neotropics for the winter. With their lanky and somewhat “wilted” silhouette, long yellow legs and dangly toes, they complete long-haul migratory treks worthy of distinction. Their flight may seem weak, but it is steady, and wing-beats are quick and regular. On long flights, they fly with their legs raised instead of letting them dangle. During migration, individuals occasionally show up in the oddest of places — they may even plop down in the middle of a busy city intersection. Clambering over marshy vegetation, or trotting spryly over lily pads, the birds eat a wide variety of plant and animal matter, including seeds, fruits and leaves of aquatic and terrestrial plants. Jerking their tail nervously while walking, they climb easily to the tops of marsh plants and even into trees, where they are typically seen in the evening.
For further information on patterns of bird migration, see the chapter on migration in Frank Gill’s textbook Ornithology; Living on the Wind by Scott Weidensaul; Bird Migration: A General Survey by Peter Berthold; and Bird Migration by Robert Burton.