Page:The Migration of Birds - Thomas A Coward - 1912.pdf/78

62 were guided by the beam-winds; always keeping the wind on their flanks led them aright (14). He says that if they fly at 100 miles per hour, with a beam-wind of 30 miles per hour, they will reach a spot 100 miles from whence they started, but 30 miles to leeward of a line drawn at right angles to the wind. Thus, if they rely upon the wind, their course is more or less diagonally across it according to its strength. He maps out the supposed route according to prevailing winds, but fails to notice that the very route he maps may be caused simply by the leeward drift when flying on winds which are not with them. One portion of the journey is enough to illustrate what I mean. From Labrador to the east of Bermuda, the birds fly south-east, so, he argues, as to cross the south-west wind at right angles. But supposing the birds headed due south, meeting the south-west wind on their right front, they would of necessity, if the wind was strong, drift away to the east. It is improbable that they actually aim to strike the Bermudas, for it is only during certain weather conditions that they Visit these islands. In favourable weather the birds do not touch the Bermudas, but continue their flight direct to South America.

The leeward drift of birds in a strong beam-wind may be noticed during ordinary flight; it has occasioned one of the most remarkable of Gätke's