BIRD-MIGRATION. 8c 9 



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 astronomi- 

 cal 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 num- 

 bers. These are usually coast-lines, river-valleys, or continuous mount- 

 ain-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 par- 

 ticular 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 un- 

 able 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 al- 

 ways 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 follow- 

 ing 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,* 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, 1S86. 



