Fall Bird Migration
A bird's real home is the place where it raises its family. Many birds have also a temporary home. In North America, as you know, there is great difference between winter and summer.
The butterflies and bees and grasshoppers and bugs that swarmed in the fields and woods during the summer, disappear when cold weather comes. The frost kills some of them, and the rest go to bed for the winter in the ground, or under a stone, or behind a piece of bark.
When insects are scarce and those that are left are well hidden, it is hard for birds to find food. About this time of year most of our birds fly south for the winter. They don't fly away because food is hard to find. Most of them go before their food supply becomes scarce.
Distances of Fall Bird Migration:
The distances that birds migrate vary greatly with the species. Some birds migrate southward:
See where birds go in winter.
- only a few miles and are permanent residents
- others travel farther, to the Southern States
- hundreds of species leave the United States
- Some winter in Central America
- some in the northern part of South America
- and still others in the southern part of South America
- Some warblers which nest in Alaska travel to Brazil, a distance of seven thousand miles
As one watches the birds in their flight, it is interesting to think of the countries to which they are headed, and of the varied scenery which their keen eyes have looked upon.
The hummingbird that visits our garden flowers leaves for the Panama Canal
- the Baltimore oriole that swings its nest from our elm trees heads off to the Andes in Colombia
- the rose-breasted grosbeak spends his winter just over the equator in Ecuador
- the kingbird flies above the waters of Lake Titicaca in Bolivia and Peru
- the bobolink travels to Paraguay
- the red-eyed vireo visits the coffee plantations of southwestern Brazil
- the barn swallow that builds his mud nest in our barns will return to the Pampas in Argentina for his winter sojourn
- while some of the nighthawks that nest in Alaska may travel to the southern part of South America, to Patagonia, a distance of about seven thousand miles and of about one hundred and fifteen degrees of latitude
The speed with which birds migrate varies with different species of birds and with the same species of bird in different parts of its journey.
In general, birds travel faster during the latter part of their journey than during the first part.
Usually birds migrate only a few hours during the night and then rest for a day or two, so that the average rate at which a species migrates is much less than for an individual bird.
Our common small birds probably travel at the rate of about thirty miles an hour while migrating; ducks and geese may travel at the rate of forty-five miles an hour. Thus during a single night birds may travel from two hundred to four hundred miles.
As birds travel between their summer and winter homes, it is found that they follow fairly well-defined routes. In the central United States the Mississippi Valley is the most common route and in the eastern United States, the coast-line.
The route by which a bird travels south is usually the same as the one by which it returns north, although there are some exceptions to this rule.
When birds which are en route for South America reach the Gulf Coast of the Southern States, several routes are possible. A few birds pass from Florida and follow the chain of islands extending southeast — the Bahamas, Haiti, Porto Rico, and the Lesser Antilles — and the to South America.
A few fly from southern Florida to Cuba, then to Jamaica, and then make the flight of five hundred miles from Jamaica to South America: the bobolink takes this route.
A few birds' fall migration, like the cliff swallows, follow along the coast of Mexico; but the great majority of species fly directly from the Gulf Coast of the Southern States across the Gulf of Mexico to the southern shore of the Gulf, a distance of from five hundred to seven hundred miles. From there the journey is continued through Central America to South America.
Another fall migration route much used by water-birds extends from Nova Scotia to the Lesser Antilles and the northern coast of South America. It was the birds which were migrating along a portion of this route that guided Columbus to land.
Many theories have been advanced to explain how birds find their way. Probably no one theory will satisfy all conditions. There are doubtless many factors needed to give a satisfactory explanation. One important factor is the bird's sight.
Birds have very keen eyesight, and it seems probable that birds flying at a great height may be guided by conspicuous landmarks, such as mountains ranges, coast-lines, and river-valleys which extend in the same direction as the routes of migration.
In North America, the coast-lines and mountain-ranges and the Mississippi Valley extend in the general direction in which most of the birds migrate. But this explanation alone is not sufficient, as birds may migrate at
right angles to these landmarks, and may find their way in a fog when landmarks are invisible, or over large bodies of water where no landmarks can be seen. And frequently birds fly so close to the ground or water that they cannot see any landmarks. And again birds may travel straight for long distances over routes which they have never seen before.
Sense of direction
Still another suggestion is that birds have a sense of direction which enables them to find their way. This is simply ascribing a power to birds without any real explanation, but experiments which have been made with birds seem to show quite conclusively that some birds do possess this sense of direction.
Several birds were captured on Bird Key south of Florida, and were placed in the hold of a steamship and taken north to Cape Hatteras, a distance of about one thousand miles from their nesting-sites, and released. Five days later, two of them were back on their nests. In this case no other explanation seems possible than that the birds found their way through a sense of direction, as the birds had never flown over this route before, and could not see the way over which they had come, and so could not make use of any landmarks.
Causes of migration
The most puzzling of all questions concerning migration is, why do birds migrate?
1. Food and temperature.
It is very commonly stated that lack of food and low temperatures cause birds to migrate. But even a very hasty examination of the facts shows that these do not explain migration. The fall migration begins during the late summer, when the temperature is still high, and at a time when insect life is abundant. Furthermore, during the spring migration, birds are traveling into regions where the temperature is lower and insect life is less abundant than in the regions which they are leaving. And again, some tropical sea-birds migrate from one section to another where the conditions of temperature and food-supply are practically the same.
2. Glacial theory. One theory relates the origin of bird migration closely with the glacial age. Fossils which have been found show that before this age North America had a warm climate, even in its northern portions. This climate must have been well adapted for bird life during all parts of the year. As the ice-sheet began to extend south, the birds were driven before it, and as it melted and receded north, the birds followed it back. In accordance with this theory, the habit which the birds thus acquired of moving back and forth, following the oscillations of the ice-sheet, was inherited eventually by the birds as an instinct and still exists to this day.
3. Physiological explanation. None of these theories are generally accepted by bird students as giving a satisfactory explanation of migration. It is probable that birds have a physiological instinct which prompts them to migrate in order to rear their young, just as their instinct leads to other actions, such as singing, mating, nest- building, egg-laying, and incubating. But this statement, of course, gives no explanation as to how and why this instinct originated.
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