LA VETA — “Have your vultures left yet?” Mary at the Ryus Avenue Bakery asked me recently. (There are vulture trees in my back yard and near hers.) “Pretty much,” I replied. “Well, most of ours are gone. They usually leave just after Oktoberfest, and we say that when they hear the “Om-Pa-Pa band, they know it’s time to go,” she said with a smile.
I had arrived at La Veta in mid-May, not long after the vultures. Now they were heading for Mexico. Other changes were taking place as well. The robins and hummingbirds in my yard had vanished. Taking their place were noisy grackles and chattering Brewer’s blackbirds, descending to replenish their fat supplies for the rest of their trip south. On the lakes, cormorants and loons appeared, feeding before continuing south.
The fall bird migration was underway. By definition, migration is the movement of animals from breeding grounds to non-breeding grounds and from areas of low or decreasing resources to ones of high or increasing resources, flying south in the fall and north in the spring.
Not all birds migrate. The crows, great-horned owls, and magpies will probably stay for the winter. Most species in North America do migrate, some moving short distances, such as up or down a mountain. The chickadees that have shown up in town recently may have spent the summer in the Spanish Peaks. Some birds are partial migrators, crossing one or several states, such as the western meadowlarks. Long-distance migrators, such as the tiny western flycatchers fly as far as Mexico, Central or South America. Even within species that migrate long-distances, such as robins, a few individuals may stay all winter, and some practice irruptive migration, moving as a result of unusual weather or food resources.
Bird migration has puzzled naturalists for centuries. Modern science has provided many answers, using leg tags and radar to track the birds, but there are still many unsolved questions.
How do the birds know when to leave? It is believed that they are triggered in part by changes in daylight and a decline in food. There may well be a genetic predisposition that sends a message, but there is enough variation from year to year and from flock to flock to indicate that the birds make some of the decisions themselves.
It is the long distance migrations that pose the biggest puzzle to scientists. Long distance migrators, such as the bobolinks that stop here on their journey from Canada to the southern end of South America, travel thousands of miles, years after year, usually following the same course. First year birds migrate to winter homes they’ve never seen, unattended by adults, and fly back in the spring. Some fly over vast expanses of water, some at night, at heights of up to 21,000 feet.
How do they know where to go and how to get there? Theories include following the stars, using magnetic fields, following an internal compass, tapping a genetic code, even using smell.
Why do they make such incredibly long journeys? One theory is that many migrant birds, including hummingbirds, orioles, and flycatchers, were originally tropical, forced to move north to find more food as glaciers retreated, drawn back each winter to their original home. Another theory is that such birds as sandpipers and plovers were from the far north, driven south by the glaciers, returning home to breed.
The more I learn, the more I find myself in awe of these fragile sojourners with their mysterious ways. La Veta’s vultures may not time their departure to Om-pa-pa music, but they still have us humans wondering.