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The migration of the swallows

Bird migration is a very curious thing. It takes a tremendous effort to wing it for thousands of miles just to nest in a specific territory. There are two conflicting theories about bird migration. The first postulates that half  a million years ago, during the Pleistocene ice age when most of North America was ice covered, birds dependent upon warm weather for food were forced south, but retained a memory of their northern breeding and hunting grounds. When the glaciers melted, the birds returned to North America during the warm months. A half a million years—that is a long time to remember, some strong instinct. 

Here is another theory: as the Northern Hemisphere warmed up, tropical birds expanded their territories into new hunting grounds, but are compelled each year to return south to escape a northern winter.

Whichever reason or combination of reasons, bird migration remains a great mystery. It is possible that birds use magnetism (magnetite has been discovered in the tiny avian brain of pigeons) to navigate. Biologists have performed experiments that indicate some birds can navigate by sun or celestial positions. The use of landmarks—topographical clues—plays an important part in bird orientation.

Since 1983, I have been jotting the arrival date of birds on the calendar, with the information later going into a yearly journal of bird information. Violet green, then tree, followed by cliff and barn swallows start appearing in early spring. The violet green and tree swallows probably come from Mexico or Central America. The cliff and barn could be coming from as far away as Argentina. The earliest arrival was March 3 in '93; the latest was May 1 in '97, but most swallow arrivals have been clustered in the last half of March.

Every August a large flock of swallows starts dropping from the sky on a clear morning. Since 'our' (bird box users, under bridge nest builders) swallows moved to meatier pastures in July, this flock really catches our attention. The birds are noisy and hyperactive. When perched on the 1/8 mile of power lines that cross our hayfields, they can be counted. I get as high as four hundred or so when suddenly, in concert, like a zipper opening, the birds leave the double wires. Released of the weight, the lines bounce. The swallows circle, dip, weave, dive, and drop again to the wires. The lines sag.

The mixed flock will hang around all morning, then completely disappear. We figured that the resident swallows and their offspring returned to circle around our fields and imprint the spring destination. The converging valleys may be important topographical elements to recognize.

August is our month for float trips so I don't have complete records of swallow departure, but the little mosquito eaters are always gone by mid month.

Until this year. We were floating British Columbia Rivers in early August. We didn't see any swallows the rest of the month so figured the big flock excitement had occurred when we were gone. We were sorry to have missed them. I always like to yell, “Good bye, have a safe trip, see ya' next year.”

There were plenty of opportunities to yell good bye in September. The first ten days of the month brought us a nearly continual contingent of excited tree swallows. There is no way to know if one large flock was returning every couple of days or if there were separate flocks, each heading south with the convergent valleys and convenient power lines over an open field on their map.

Tree swallows are one of the species that biologists are studying for the effects of global warming. These little bug eaters are among the most numerous of the migrating species and are relatively easy to study. One study showed the egg-laying date of North American tree swallows advanced by nine days during the period from 1959 to 1991. Reported in 1999 by Peter O. Dunn, University of Wisconsin and David W. Winkler, Cornell University, the research correlated 3,450 nest records. "We conclude," wrote the biologist, "that tree swallows across North America are breeding earlier and that the most likely cause is a long term increase in spring temperature."

Not only are these little opportunists making babies earlier, I think they are departing later. We live in interesting times.

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Lou Springer Lou Springer lives in Heron when not out on a river somewhere.

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birds, outdoors, swallows

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