Migration
by Mike Stiles
Above our heads, largely unseen, a truly remarkable journey has been taking place - the migration of billions of birds to their wintering grounds. As Fall migration is winding down here on the central coast, let us take a closer look at the incredible journeys some of these tiny birds are making.
Blackpoll Warblers, for example, breed in the boreal forests from western Alaska, east across northern Canada, to Maine, Vermont, and New Hampshire. They migrate down the east coast to spend their winters in South America as far south as the Amazon basin in Brazil. They all follow the east coast southward, so the birds breeding in Alaska have a 3,000 mile journey just to get to the "start" of their migration. At Cape Hatteras in North Carolina, they head southeast, over the open ocean, at night. Prevailing winds will carry them out to sea until they are picked up by weather radar over Bermuda, where tropical trade winds blow them southwest, toward South America. When they make landfall in Venezuela, they will have flown 2,000 miles, with no rest, no water, and no food. Some continue another 1500 miles to Brazil, spend the winter, and then return in the spring - a total of 12,000 miles that year. The Blackpoll Warbler is 5 inches in length and weighs half an ounce; "you could mail two of them for a single first-class stamp".*
Stories of long distance migration stagger the mind. Four week old Bristle-thighed Curlews (a shorebird about 17 inches long), born this year in the Yukon Delta in far northwest Alaska, leave their breeding grounds about a week after the parents have left, and make a 5,000 mile, nonstop flight to Fiji and other islands in the south Pacific. Another shorebird, the Bar-tailed Godwit, flies from Siberia and western Alaska and will stop only after it makes it to New Zealand, a journey of 4 or 5 days and 6,800 miles. Hummingbirds that weigh about as much as a penny, fly nonstop from North to South America, over the Gulf of Mexico. (Contrary to old folklore, they do not hitch a ride on larger birds.) Brant, the small dark goose we see on Morro Bay every winter, leaves Izembek Lagoon in the Gulf of Alaska, and flies here in about 48 hours, losing one third of its body weight along the way.
Birds need fuel for these long journeys and can put on up to 55% extra body weight in fat stores. Waning day length will trigger these birds to become more active in feeding, doing nothing but gorging themselves to allow this fat build-up. Then they must migrate, for their food, especially in the northern latitudes, is about to disappear in the bitter cold of winter. It is said that birds could survive the Alaskan winters if they could find food, but they must travel south to a reliable food supply. Interestingly, it is said the Bar-tailed Godwit’s kidneys, liver, and other organs will atrophy and shrink just before departure, and some birds will consume muscle and organ mass, if the body fat is not enough for the journey.
Scientists have long pondered how birds, some straight out of the nest, are able to find their way to the same wintering grounds that their ancestors have used for millennia. For years it was thought that birds just used visual landmark cues and the sun for orientation, and that is certainly part of it. But most birds migrate at night, where the visual cues are limited. It has been found, through experiments in planetariums, that birds can recognize constellation formations to orient themselves. Research has shown that birds also detect the earth’s magnetic field, and even use olfactory senses in migration. They smell their way home.
Finding your direction is one thing, but it’s like a boy scout with a compass and no map. How do the birds know when they have arrived? Of course genetics and instinct must be involved, along with the bird’s cognitive ability to recognize certain habitats. Because many birds migrate between northern and southern hemispheres, it is thought that birds can interpret the varying strengths of the magnetic field at different latitudes by using magnetic minerals present in their cells and nervous system. One researcher has a found a connection between the eye and these magnetic portions of the nerves, and postulates that birds can actually see the earth’s magnetic fields. Migration truly is a wonder of nature.
November is the time to watch for the build-up of our wintering birds. Watch for Brant to arrive and build up to numbers into the thousands. Ducks and other water bird numbers will rapidly increase on Morro Bay. Some vagrant birds (birds that don’t have a good sense of direction) may still be found, but fall migration is essentially over.
*This is one of my favorite quotes from a book I pulled many of these anecdotes from, and well worth reading for more information about migration: "Living On The Wind: Across the Hemisphere With Migrating Birds" by Scott Weidensaul. |
