RSSChange of address: For millions of animals, this season is one of migration
Elk are opportunistic grazers and are able to digest a wide range of vegetation. Forage on winter ranges is less nutritous than the fresh green growth of spring and summer and as snows retreat, elk start moving toward the high country.
This family group of sandhill cranes is among the hundreds of cranes seen passing through western Colorado this month during the fall migration. While many of these cranes will winter along the Rio Grande in southern New Mexico, some of the birds may over-winter near Delta if food is available.
Snow geese numbers are estimated to exceed 5 million birds and this flock at the Bosque del Apache National Wildlife Refuge in southern New Mexico is just a fraction of the thousands of snows that winter along the Rio Grande.
The holiday guests finally are gone, just when you feared they’d never leave.
Just the thought of someone arriving unannounced for Christmas vacation leaves you sleepless.
Yet throughout the world, tens of millions of animals are making their twice-yearly change of address, a process we know as migration.
As author, educator and renowned birder Scott Weidensaul notes in his fascinating book, “Living on The Wind: Across the Hemisphere with Migratory Birds,” migration is far more complicated than it seems.
Many creatures may be said to migrate. Anglers tie their vacations to the movement of stoneflies on the Gunnison River or cicadas on the Green River.
Each fall, a new generation of monarch butterflies travels from breeding areas in the United States and Canada to the mountains of central Mexico.
Elk and deer follow seasonal movement patterns, humans leave one country and head to another, and even ocean plankton show movements that may indicate migration.
Entomologist Hugh Dingle, quoted in Weidensaul’s book, said insect/plankton movements may not be true migrations because they are “single-direction movements.”
Still, Dingle said, the movements “allow exploitation of different habitats as life history requirements alter or environments change seasonally or successionally.”
To Weidensaul and to countless ornithologists, scientists and researchers, migration entails a “seasonal to-and-from” movement.
It is “what we often call a ‘return migration,’ because the animal eventually winds up where it began,” Weidensaul writes.
That movement may be mind-boggling, with some birds moving thousands of miles each year, or hardly noticeable, with other birds moving but a few hundred feet in elevation.
Migration can be soul-stirring: Seeing kettling flocks of Swainson’s hawks numbering in the tens of thousands or long Vs of sandhill cranes arrowing through the cold November skies.
Or it may be as abrupt as a red-tailed hawk coming down from Grand Mesa to chase mice and rabbits at Fruitgrower’s Reservoir near Eckert.
Many birds, including most of the colorful songbirds flitting around your home all summer, go prodigious distances.
Some lesser sandhill cranes breed in Siberia and winter in southern New Mexico; blackpoll warblers — weighing half an ounce — wing it from breeding grounds in western Alaska to New England to winter in western Bolivia and northern Brazil, a round-trip flight of nearly 12,000 miles.
And the bar-tailed godwit, normally a slim shorebird weighing a feather or two more than 10 ounces, fattens up for its five-day, 6,800-mile ride on the north wind from breeding grounds in western Alaska to its winter retreat in New Zealand.
“They put on so much fat, they have a sort of boxy appearance before they leave Alaska,” said Bob Gill, a shorebird researcher at the USGS Science Center in Anchorage, in a 2005 article by science writer Nick Rozelle of the Geophysical Institute, University of Alaska Fairbanks. “The probably use all of that fat and then burn protein (muscle) for added energy.”
Why do animals migrate? The Cornell University Laboratory of Ornithology says a combination of factors — changes in day length and food supplies, lower temperatures and genetic predisposition — are key to migration.
Genetics may explain why some birds of the same species follow different migratory patterns, or why some long-distance travelers bypass food sources nearer to their starting point.
Elk movements are keyed by day length, weather and food supplies, according to the Rocky Mountain Elk Foundation.
Summer forage is more nutritious than what’s on the winter range and elk will stay on their summer ranges as long as adequate forage is available.
The Elk Foundation says some elk in Yellowstone National Park don’t migrate, instead staying year-round near thermal areas where underground heat keeps snow melted and plants growing during the coldest winters.
Spring migration is spurred by longer days, warming temperatures and the growth of new vegetation in the high country. Cow elk seek familiar calving grounds.
And the millennia-spanning story of human migration, which Smithsonian Magazine (July 2008) said, “brought our species to a position of world dominance that it has never relinquished and signaled the extinction of whatever competitors remained,” is a fascinating story in itself.
COMMENTS
Commenting is not available in this channel entry.