Even the stork in the heavens knows its seasons, and the turtle dove, the swift and the crane keep the time of their arrival.
Bird migration has been a subject of popular interest, particularly to scientists, for at least three millennia, yet many of its secrets remain only partially understood.
Today, it is recognized that birds, like some of our neighbors, retreat to the south when the winds and cold of winter begin to threaten. This was not always the case. Aristotle believed that some birds hibernated, an idea that continued to be supported as late as 1898. That year, ornithologist Elliott Coues cited almost two hundred references claiming that swallows hibernated, some even suggesting that they spent the winter underwater.
Even after migration was widely accepted, many wondered if any birds returned to the same locale after their southern sojourn. In 1803, one ornithologist checked this by tying silver bands around the legs of a brood of phoebes. Their return the following spring not only answered his question but initiated the now widespread practice of bird banding in North America. That ornithologist is better known as a wildlife artist. He was John James Audubon.
It is widely believed that bird migration developed in response to the Ice Ages, when miles-deep glaciers covered Canada and parts of the United States as recently as 13,000 years ago. As those glaciers retreated and the botany of the newly opened regions flourished, birds were encouraged to visit for at least part of each year. (Today, a similar attraction is provided by global warming; southern species like Carolina wrens and black vultures are moving north into our region.)
That glacier-retreat rationale for migration is difficult to test, but other aspects of migration have been explored, and those studies continue today.
When we head south, we carry road maps or use GPS, but birds have no such assistance. How, then, do they aim their flights? This has been studied extensively, beginning with the German researcher, Friedrich Merkel, who demonstrated that birds respond to the earth’s magnetism in orienting. He showed this by capturing a number of European robins and enclosing them in darkened cages surrounded by iron that blocked out the earth’s magnetism. He then used Helmholtz coils to control the magnetism of the cages. The restless birds oriented in the direction of the introduced magnetism. More recent studies have shown that birds use other clues as well, some passed on genetically, others related to their identification of constellations and—possibly—clues from quantum physics.
Until recently, most information about the migration of individual species has been accumulated from the local observations of bird watchers and the recapture records of banded birds. I cannot resist inserting here a widely quoted, but almost certainly apocryphal, story about the early banding operation. The bands were normally imprinted with the instructions, "Notify Fish and Wildlife Div. Wash. Biol. Surv.," those abbreviations representing Division Washington Biological Survey. Unfortunately, one group of bands was printed with Boil instead of Biol, which drew the following message from an Alberta farmer: "Gents: I shot one of your crows last week and followed instructions attached to it. I washed it, boiled it, and served it. It was awful. You should stop trying to fool the public with things like this."
Observers’ and banders’ records provide much information about migration, including when birds arrive in spring, when they leave in fall, where they spend the winter, and where they go in summer. Counts like the Audubon Christmas Counts provided data about numbers as well. We knew, for example, that robins showed up on the Niagara Frontier on about March 15 and most left by about November 10, and that the European starling was the commonest land bird recorded on the Buffalo Christmas Count. That is where things stood at the turn of the century.
Since then, things have changed rapidly.
In 2002, the Cornell Laboratory of Ornithology introduced eBird, an Internet opportunity for bird watchers across the continent to record their observations. To an old-time birder like me, the organized results are truly amazing. On the website ebird.org, you can find an encyclopedic collection of information about the occurrence of individual bird species. Anyone interested in birds can spend hours, as I did, exploring the information on this site.
There you can find, for example, not only when a tree swallow first appears in Erie county (mid-March) but when it becomes common (early April), when its numbers decrease in fall (mid-August), and when the last individuals are seen (early October). You can explore a detailed map to see where and when snow buntings have been reported. For example, you can learn that Diana Zubrzycki reported three just north of Clarence Center on February 14, 2015. And you can even watch occurrence maps for a few individual species. These show the United States divided into counties that color as the year progresses and birds arrive. For example, you can watch the map for the indigo bunting remain blank until late April when its range expands rapidly, reaching our area in May, then retreating from here in mid-August, and leaving the region entirely bare again by late October.
But today’s most interesting migration studies are being carried out through use of tracking devices attached to individual birds. These devices have been used for many years to study the movements of mammals, but their weight has been a problem for all but the largest birds.
Miniaturization has now led to GPS devices that weigh less than four grams (one-seventh ounce), which can be carried by midsized songbirds; it is expected that even lighter devices will soon be available. To give some context, while a robin weighs about seventy-five grams, a wood thrush weighs only about forty-five grams.
A few of the other records established by contemporary ornithologists, most through use of such tracking devices: highest migration flight, 24,000 feet over the Himalayas by bar-tailed geese; longest songbird migration, an 18,000-mile round trip by the Northern wheatear; fastest land bird migrant, the great snipe of Europe at sixty miles per hour; longest nonstop land bird migration, a bar-tailed godwit’s 7,258-mile flight from Alaska to New Zealand. That shorebird flew almost nine days without rest or sustenance.
Naturalist Gerry Rising is the author of Birds and Bird Watching.