Clellie Lynch: The 'hows' of bird migration
"O, Swallow, where do you go in winter?"
EAST CHATHAM, N.Y. — As the sun peeks through the still-verdant trees, I amble along the dusty lane toward the pond. Goldenrod, some with spikes of florets, others with yellow firework bursts, bloom along the roadside, dotted here and there with asters and loosestrife. Lush vines, both virgin's bower (wild clematis) and wild cucumber, climb up and over weeds and shrubs, their white blossoms like chalky inroads over the vegetation. A robin flits across the road in front of me. Chickadees, nuthatches and titmice call, hidden by the leafy curtain. Occasionally a thrush appears, a warbler sings and a young oriole lands on a low branch nearby.
I approach the pond; the overhead wires are lined with dozens of swallows. Scanning I find mostly tree swallows, more than a handful of barn swallows and pair of cliff swallows. One swallow may not a summer make, but these gathering multitudes like animated clothespins are a sure sign of autumn and the onset of fall migration.
Bird migration is a seasonal phenomenon noted from the earliest of times. In our hemisphere, springtime brings the birds north to their breeding ground and come autumn they are off to warmer climes where there's plenty of food. Some species migrate by day; others, by night. Some travel enormous distances; others, just shift a bit south. Some travel in huge flocks, others, in small family groups. Some fly at great heights; others, fly low winging their way through the woods and around cities. Some fly continually until they reach their destination; others, stop daily.
The "why" is obvious — the birds need to eat and the frozen north offers little in the way of seeds and insects. The "how" is more difficult to explain. How do they know when to leave, what route to take, where good stopping places are, where to settle? There have been multiple explanations throughout the centuries.
REDSTARTS TO ROBINS?
Five thousand years ago, when geese and cranes were returning from faraway lands, the farmers of Cyprus knew it was time to plant crops. Peasants often looked at the regular arrival and disappearance of birds as utterly mysterious and viewed them as either good or bad omens.
In the third century BC, the brilliant philosopher Aristotle, fascinated by the natural world, studied, recorded and analyzed animal movements. His observations led him to theorize about the regular appearance and disappearance of different species throughout the year. First, after noting summer redstarts disappeared in the fall (going to the sub-Sahara) and robins appeared (from northern Europe) in the winter, he decided that the redstarts became the robins. Transmutation seemed to be true for some of the smaller species. Other species like the swallow he believed hibernated in hollow tree trunks in the colder months. A majority of species he concluded went elsewhere for part of the year and came back to nest. Where they went to and why they came back was little understood. Aristotle called this migration.
Many years later, or so it says in the literature, in the 12th century, a German Cistercian prior, Caesarius von Heisterbach, took a swallow from its nest and attached a small parchment that said, "O swallow, where do you go in the winter?" to its foot. Lo and behold, the bird returned in the spring with the note, "In Asia, in the home of Petrus." Would have been good if the note also explained the how and why! Well, it is a good story the forerunner to bird banding.
In the 16th and 17th centuries, curious scientists set out to explore, observe, record and theorize about the natural world. Nearly every explorer ship carried a natural historian that collected specimens and drew illustrations of what he observed. On return, eager scientists compiled information from these different sources and soon an inkling about where various species went at different times of the year emerged.
Frances Willughby, the father of Ornithology, mentioned in his book, "Ornithology" (1676) that certain species were migratory. Once Linnaeus established his formal taxonomic system, pooling information from many places became easier. Still, even a careful observer like Gilbert White, in "The Natural History of Selbourne" (1789) perpetuated the myth that swallows, swifts and martins descended into the muck of ponds and lakes to hibernate for the winter. Curious, no one ever saw a swallow either disappear into or emerge from the water.
By this time many amateur ornithologists were observing and recording the arrival and dispersal dates for various species. Gunners too were useful in gathering information about where certain species were taken. Their motto: "What's hit is history; what's missed is mystery!" Scientists were beginning to identify patterns of bird migration.
Noting that islands and headlands were often rife with birds and bird nests, ornithologists requested lighthouse keepers throughout Europe to record what they saw daily. This massive project lasted from 1880 until 1914, ceasing with the onset of World War I. Now there are few working lighthouses and often the defunct sites have become bird observation areas, conservation lands or federal and state parks. Think: Cape May, New Jersey, Point Pelee, Ontario, Point Reyes, California.
At the beginning of the 20th century, when gunning slipped out of fashion, when the Christmas Count came into existence, when the Migratory Bird Treaty Act (1918) was passed, ornithologists, taking a lesson from that Cistercian prior, began banding birds and tossing them on their way, asking birders and others to send back any retrieved information. It wasn't until the 20th-century that specific flyways became known and mapped by Frederick C. Lincoln, author of The Wildfowl Flyways of North America (1935) and other books about migration.
By the 21st century, radio sets and modern antiaircraft equipment were modified to detect birds, enabling the tracking of individual birds by computer over a range of 6 + miles, signaling the bird's position, height, speed, wing and heart beats. Radar also reveals where the flocks of migrating avians go en masse. Many websites have illustrative interactive migration maps: allaboutbirds.org; nationalgeographic.com; journeynorth.org; birdcast.info.
At this time of year, billions of birds are on the move. In North America, about 80 percent of the species and 94 percent of the individuals breeding in northern conifer forests migrate to the Tropics. About 62 percent of the species and 75 percent of the individuals breeding in deciduous forests hasten to the Tropics too. Of the 650 species that nest in the US, only 130 are NOT migratory. Perhaps they are the lucky ones; the forests of the Amazon are on fire.
Clellie Lynch is a regular Eagle contributor.
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