How do Birds find their Way?
“How do the birds find their way over the hundreds or thousands of miles between the winter and summer homes? Sight is probably the chief guide of those who fly by day, and it is known that these day travellers seldom make the long single flights that are so common with the birds that journey at night. Sight, undoubtedly, also guides them, to a large extent, in the night journeys, when the moon is bright. Migrating birds fly high, so that one can hardly hear their faint twittering. But if the sky is obscured and the clouds hang low, the flocks keep nearer to the earth, and their calls are more distinctly heard; while on very dark nights, the vibration of their wings can be heard close overhead.
TERNS AND SKIMMERS ON THE WING
(Summer Bird-Life, Cobbs Island, Va. Am. Museum Nat. Hist., N.Y.)
“Notwithstanding this, something besides sight guides these travellers in the upper air. (Here is a route for you to trace on the map.) In Alaska, a few years ago, members of the Biological Survey on the Harriman expedition went by steamer from the island of Unalaska to Bogoslof Island, a distance of about sixty miles. A dense fog had shut out every object beyond a hundred yards. When the steamer was halfway across, flocks of Murres, returning to Bogoslof after long quests for food, began to break through the fog wall astern, fly side by side with the vessels, and disappear in the mists ahead. By chart and compass, the ship was heading straight for the island; but its course was no more exact than that taken by the birds. The power which carried them unerringly home over the ocean wastes, whatever its nature, may be called ‘a sense of direction.’ We recognize in ourselves the possession of some such sense, though imperfect and easily at fault. Doubtless a similar, but vastly more acute, sense enabled the Murres, flying from home and circling wide over the water, to keep in mind the direction of their nests and return to them without the aid of sight. It is probable that this faculty is exercised during migration.
“Reports from lighthouses in southern Florida show that birds leave Cuba on cloudy nights when they cannot possibly see the Florida shores, and safely reach their destination, provided no change occurs in the weather. But if meantime the wind changes or a storm arises to throw them out of their reckoning, they become bewildered, lose their way, and fly toward the lighthouse beacon. Unless killed by striking the lantern, they hover near or alight on the balcony, to continue their flight when morning breaks, or, the storm ceasing, a clear sky allows them once more to determine the proper course.
“Birds flying over the Gulf of Mexico to Louisiana, even if they ascended to the height of five miles, would still be unable to see a third of the way across. Nevertheless this trip is successfully made twice each year by countless thousands of the warblers of the Mississippi Valley.
“Probably there are many short zigzags from one favoured feeding-spot to another, but the general course between the summer and winter homes is as straight as the birds can find without missing the usual stopping-places.