ART. 98 - The Soaring of Birds

Summary

The recent correspondence in Nature upon this subject ought not to close without some reference to a possible explanation of soaring which does not appear to have been yet suggested.

I premise that if we know anything about mechanics it is certain that a bird without working his wings cannot, either in still air or in a uniform horizontal wind, maintain his level indefinitely. For a short time such maintenance is possible at the expense of an initial relative velocity, but this must soon be exhausted. Whenever therefore a bird pursues his course for some time without working his wings we must conclude either (1) that the course is not horizontal, (2) that the wind is not horizontal, or (3) that the wind is not uniform. It is probable that the truth is usually represented by (1) or (2); but the question I wish to raise is whether the cause suggested by (3) may not sometimes come into operation.

In Nature, Vol. XXIII. p. 10, Mr S. E. Peal makes very distinct statements as to the soaring of pelicans and other large birds in Assam. The course is in large and nearly circular sweeps, and at each lap some 10 or 20 feet of elevation is gained. When there is a wind, the birds may in this way “without once flapping the wings” rise from a height of 200 to a height of 8000 feet.