Page:The Migration of Birds - Thomas A Coward - 1912.pdf/69

Rh hour, the bird will cover a distance of 30 miles in one hour, though the force exerted by the bird is the force necessary for 20 miles in a calm. Conversely, if a 10-mile air current meets it, it will unconsciously he carried only 10 miles. If the speed of the bird is the same as the opposing force of the wind, it will remain stationary; I have seen ducks in a blizzard rise head to wind and fly rapidly, making no progress but maintaining their position over the water, to which they dropped again when the storm passed. Some black-headed gulls on the same water did not attempt this manoeuvre, and in a few seconds had vanished down wind. The swimmer, in a swiftly-flowing river, may hold himself in position so long as he can swim at the same rate as the stream ho is contending with, but he cannot make headway if the speed of the water exceeds his. He may, however swift1y the stream moves, swim in any direction, but his actual progress will be down stream; if he aims to swim directly across, his real course will be diagonal.

The fact that birds fly in any direction in a wind, and when at low elevation pay little heed to the direction of the wind, when the breeze is light, simply means that they can fly faster than the medium they are in; if the medium travels faster than they do, they will be carried in it to their advantage or disadvantage.