Some Possible Reasons for Bird Migration

Wild geese in flight.

        Sounding in the distance like the far-off yapping of very small dogs, twice a year come the calls of the wild geese flying overhead. Their V-shaped formations are a reminder in autumn that long lines of birds are moving south, in spring that they are on their way north once more. Some birds, like the cardinals, English sparrows, or crows, stay out the year wherever winter finds them, but many others, seen in early spring and summer, start their trip south long before cold weather comes. Some of these may travel only a few hundred miles; some may go thousands. The meadowlark, whose clear notes are heard throughout the fields and pastures of Illinois from April to September, spends the rest of the year in Texas, Louisiana, and the neighboring states. The bobolink, from the same fields, heads in September for the tip of Florida. From there it flies the 150 miles to Cuba, then on to Jamaica, and next makes the long 500-mile stretch across the Caribbean Sea to the South American coast. It continues to move steadily southward and finally settles for the winter months in the central part of South America. Although the dates on which birds start their migration and the places to which many of them go are known, yet the reasons why they go are not known.

       There are many explanations; most of them suggest that once upon a time birds spent all their time in whatever place they happened to be, but as the years went on something in their home became so unfavorable to them that they moved away at certain periods of the year. Perhaps some of those living in the warm southern parts of the world were crowded north at the breeding season when the scramble started for nest-space, worms, and seeds for the young. Perhaps others farther north, either natives or visitors, were pushed down to camping grounds in the south each year by the sleets and snows of winter. It eventually came about then that the birds which yearly went south, began to do so without waiting for the first snow. Many were young ones, entirely unacquainted with winter, that would go alone or along with their elders while still there was the food and warmth of late summer around the nest. Apparently, in certain kinds of birds, the urge to fly south and later return north, had become an instinct—something that the bird could do without any experience or training, something that benefited it without planning on its part.

       If the bird knows nothing of winter and yet has the instinct to leave the spot where winter presently may come, something must make this instinct go to work at the proper season, but what this something is no one knows—it may well be different for various kinds of birds. Many influences differ from year to year—for example, the temperature and the speed of the wind are perhaps high one day and then low on the same day of the month in another year, so that their influence on the very regular take-off time of some migrants can not be great. It is thought by many people that it may be the amount of light each day, changing like clockwork with the time of the year, that determines when some birds start south. Most birds settle down to sleep when the sun sinks, so that as summer turns to autumn the hour to roost each night comes earlier, and chances for hopping and flying are fewer. This lack of exercise seems to cause certain parts of the bird’s body—those that produce the eggs and lead the bird to build nests—to become smaller, and as they do, the bird loses all inclination to busy itself with family affairs. Apparently it is only then that the instinct to leave the nesting grounds and fly south becomes strong. In the spring the migratory instinct seems to start operating in a different way, for after the bird has spent long days in feeding and flying and the parts important for egg-making are again grown large, it makes the return trip north. Perhaps more fortunate than other birds, those that migrate can usually have, all the year round, food, water, and warmth in abundance, advantages that evidently outweigh the dangers of the long round trip made each year. Elizabeth Best

By Learning Junction: Why do birds and animals migrate?