For centuries, people who have kept cage birds have noticed that the migratory species go through a period of restlessness each spring and fall, repeatedly fluttering toward one side of their cage. Migration can be triggered by a combination of changes in day length, lower temperatures, changes in food supplies, and genetic predisposition. The mechanisms initiating migratory behavior vary and are not always completely understood. Supporting this theory is the fact that most North American vireos, flycatchers, tanagers, warblers, orioles, and swallows have evolved from forms that originated in the tropics. As their breeding zones moved north during periods of glacial retreat, the birds continued to return to their tropical homes as winter weather and declining food supplies made life more difficult. The seasonal abundance of insect food and greater day length allowed them to raise more young (4–6 on average) than their stay-at-home tropical relatives (2–3 on average). Why make such an arduous trip north in spring? One idea is that through many generations the tropical ancestors of these birds dispersed from their tropical breeding sites northward. They also incorporate responses to weather, geography, food sources, day length, and other factors.įor birds that winter in the tropics, it seems strange to imagine leaving home and embarking on a migration north. They’ve evolved over thousands of years and are controlled at least partially by the genetic makeup of the birds. While short-distance migration probably developed from a fairly simple need for food, the origins of long-distant migration patterns are much more complex. The pattern of migration can vary within each category, but is most variable in short and medium distance migrants. Despite the arduous journeys involved, long-distance migration is a feature of some 350 species of North American birds. Long-distance migrants typically move from breeding ranges in the United States and Canada to wintering grounds in Central and South America.Medium-distance migrants cover distances that span a few hundred miles.Short-distance migrants make relatively small movements, as from higher to lower elevations on a mountainside.They are able to find adequate supplies of food year-round. One way to look at migration is to consider the distances traveled. The term migration describes periodic, large-scale movements of populations of animals. As winter approaches and the availability of insects and other food drops, the birds move south again. Escaping the cold is a motivating factor but many species, including hummingbirds, can withstand freezing temperatures as long as an adequate supply of food is available. Here’s more about how migration evolved.īirds that nest in the Northern Hemisphere tend to migrate northward in the spring to take advantage of burgeoning insect populations, budding plants and an abundance of nesting locations. The two primary resources being sought are food and nesting locations. Why do birds migrate?īirds migrate to move from areas of low or decreasing resources to areas of high or increasing resources. Of the more than 650 species of North American breeding birds, more than half are migratory. But geese are far from our only migratory birds. Barrère and Leland's "Dictionary of Slang, Jargon & Cant" describes the noun as an American term of contempt for a person, "a sham 'swell ' a simpleton," and suspect it might be from jayhawker.Geese winging their way south in wrinkled V-shaped flocks is perhaps the classic picture of migration-the annual, large-scale movement of birds between their breeding (summer) homes and their nonbreeding (winter) grounds. They were said to be disliked by hunters because their cries aroused deer. The sacredness of an obligation is a thing which you can't cram into no blue-jay's head." A jay will lie, a jay will steal, a jay will deceive, a jay will betray and, four times out of five, a jay will go back on his solemnest promise. "A jay hasn't got any more principle than a Congressman. Perhaps via a decaying or ironical use of jay in the old slang sense "flashy dresser." Century Dictionary (1890s) notes it as actors' slang for "an amateur or poor actor" and as an adjective a general term of contempt for audiences. "fourth-rate, worthless" (as in a jay town), 1888, American English, earlier as a noun, "hick, rube, dupe" (1884) apparently from some disparaging sense of jay (n.).
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |