Listening Script Vocabulary
(Section 4: You will hear a talk on the topic of bird migration. First, you will have some time to look at questions 33 to 40 [20 seconds]. Listen carefully and answer questions 33 to 40.)
L: Good afternoon everyone, and welcome to the last lecture in this series, which, as you already know, is about the migration patterns of birds.
Birds migrate, or travel, to move from areas of low or decreasing resources to areas of high or increasing resources. The two primary resources being sought are food and nesting locations.
Birds that nest in the Northern Hemisphere tend to migrate northward in the spring to take advantage of growing insect populations, budding plants and an abundance of nesting locations. 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.
The term 'migration', describes periodic, large-scale movements of populations of animals but there are, of course, different types of migration. One way to look at migration is to consider the distances travelled. Permanent animal residents do not migrate. They are able to find adequate supplies of food year-round. Short-distance migrants, as you might expect, move only a short distance, as from higher to lower elevations on a mountainside. Medium-distance migrants cover distances that span from one to several states. And finally, long-distance migrants typically move from breeding ranges in the United States and Canada to wintering grounds in Central and South America. Despite the long and difficult journeys involved, long-distance migration is a feature of some 350 species of North American birds. The pattern of migration can vary within each category, but is most variable in short and medium distance migrants.
While short-distance migration probably developed from a fairly simple need for food, the origins of long-distance migration patterns are much more complex. They've evolved over thousands of years and are controlled at least partially by the genetic makeup of the birds. They also incorporate responses to weather, geography, food sources, day length, and other factors. For birds that winter in the tropics, it seems strange to imagine leaving home and embarking on a migration north. Why make such a difficult trip north in the spring? One idea is that through many generations, the tropical ancestors of these birds dispersed from their tropical breeding sites northward. 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). 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 mechanisms initiating migratory behaviour vary and are not always completely understood. Migration can be triggered by a combination of changes in day length, lower temperatures, changes in food supplies, and genetic predisposition. 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. German behavioural scientists gave this behaviour the name 'zugunruhe', meaning migratory restlessness. Different species of birds and even segments of the population within the same species, may follow different migratory patterns.
Migrating birds can cover thousands of miles in their annual travels, often travelling the same course year after year with little deviation. First-year birds often make their very first migration on their own. Somehow, they can find their winter home despite never having seen it before, and return the following spring to where they were born.
The secrets of their amazing navigational skills aren't fully understood, partly because birds combine several different types of senses when they navigate. Birds can get compass information from the sun, the stars, and by sensing the earth's magnetic field. They also get information from the position of the setting sun and from landmarks seen during the day. There's even evidence that sense of smell plays a role, at least for homing pigeons.
Some species, particularly waterfowl and cranes, follow preferred pathways on their annual migrations. These pathways are often related to important stopover locations that provide food supplies critical to the birds' survival…