The cause, no doubt, lies in the difference in the character of the vegetation.
One of the most difficult, as well as most interesting, questions in bird-migration has been, How are birds guided in their flight? By instinct, has been the usual answer; but, thanks to the labors of such men as Cooke, Allen, Brewster, and Scott, the question is now better understood. Recent observations made at lighthouses and astronomical observatories go to show that many, if not most, of our smaller birds fly at very great heights while migrating—heights even as great as one to two miles. And we now well know that "there are certain definite routes or paths along which birds pass in especially great numbers. These are usually coast-lines, river-valleys, or continuous mountain-ranges. Toward these converge innumerable less-frequented paths, each of which in turn has still smaller tributaries of its own. Thus bird-streams, like brooks, flow into common channels, and each particular region may be said to have its bird- as well as water-shed."
Perhaps the greater part of our birds migrate almost exclusively by night, and it seems true that clear, pleasant nights are selected during which to perform their migrations. That most species are unable to foretell the weather, even for a few hours, seems to be true; for during the migrations, if the early part of the night be clear, and a storm or severe rain come up later in the night, the birds will be stopped in their flight. On mornings after such nights I have always found more birds than at any other time. If the rain brought with it any considerable lowering of temperature, the woods and groves would be full of arrested migrants during the next day. These birds had evidently started when the sky was clear and the weather favorable.
Keeping these various facts in mind, it becomes quite easy to see how birds are guided in their course. From the great height at which they fly the whole country appears as a map upon which, in the light of moon and stars, the coast-lines, river systems, and mountain-ranges are outlined in every direction for many miles. "Guided by such landmarks as these, the older birds can have little difficulty in following paths they have repeatedly traversed before, and these direct and lead the flight of the younger birds." Mr. William Brewster, in his recent excellent paper on this subject,[1] and from which I have freely quoted, shows that, while the birds often migrate in waves or flocks, the different flocks do not move independently of each other. He believes that the flocks do not fly in close order, but scatter so as to approach or mingle with the stragglers or advance guard of other flocks, "thus in effect forming a continuous but straggling army, often hundreds of miles in length, and varying in breadth with the character of the country over which it is passing."
But when all is said, a great part of the details concerning the
- ↑ "Memoir of the Nuttall Ornithological Club," No. 1. Cambridge, 1886.
