xliv INTRODUCTION. 
or graze the surface of the deep. If the Wren, the Creeper, 
and the Titmouse rest for an instant on a tree to snatch a hasty 
morsel, in the next they are on the wing, to fulfil their destina- 
tion. However abundant may be the nourishment which 
presents itself to supply their wants, in general, birds of passage 
rarely remain more than two days together in a place. 
The cries of many birds, while engaged in their aérial voy- 
age, are such as are only heard on this important occasion, and 
appear necessary for the direction of those which fly in assem- 
bled ranks. 
During these migrations it has been observed that birds 
fly ordinarily in the higher regions of the air, except when 
fogs force them to seek a lower elevation. This habit is 
particularly prevalent with Wild Geese, Storks, Cranes, and 
Herons, which often pass at such a height as to be scarcely 
distinguishable. 
We shall not here enter into any detailed description of the 
manner in which each species conducts its migration, but 
shall content ourselves with citing the single remarkable exam- 
ple of the motions of the Cranes, Of all migrating birds, these 
appear to be endowed with the greatest share of foresight. 
They never undertake the journey alone; throughout a circle 
of several miles they appear to communicate the intention 
of commencing their route. Several days previous to their 
departure they call upon each other by a peculiar cry, as if 
giving warning to assemble at a central point; the favorable 
moment being.at length arrived, they betake themselves to 
flight, and, in military style, fall into two lines, which, uniting 
at the summit, form an extended angle with two equal sides. 
At the central point of the phalanx, the chief takes his station, 
to whom the whole troop, by their subordination, appear to 
have pledged their obedience. The commander has not only 
the painful task of breaking the path through the air, but he 
has also the charge of watching for the common safety; to 
avoid the attacks of birds of prey; to range the two lines in a 
circle at the approach of a tempest, in order to resist with 
more effect the squalls which menace the dispersion of the 
