124 NATURAL HISTORY. 
months come on. They migrate to the south, where 
the warm weather will give them the same worms or 
other food which they can no longer obtain at the north. 
Different kinds of birds have each their time for migra- 
tion, which can be calculated upon with considerable ac- 
curacy. It does not depend on the degree of tempera- 
ture, although it is somewhat influenced by it, as birds go 
north sooner when the season is early than when it is late. 
212. The most mysterious part of this migration is, 
that the same birds will often return in the following 
season to the same spot which they left, although they 
have traversed in their migration hundreds, and perhaps 
even thousands of miles. This fact has been proved in 
the case of swallows by tying silken threads to their feet, 
so that there should be no mistake as to their identity. 
Spallanzani, a celebrated Italian physiologist of the last 
century, saw the same couples return to their old nests 
\. for eighteen years in succession. 
Questions. —What are the two grand divisions of warm-blooded 
Vertebrates? In what respects do Birds differ from Mammals? 
What is said of feathers? What analogy does the office of a wing 
bear to that of a hand? Point out the resemblance between them in 
their frame-work. What is said of the breast-bone in birds? What 
is said of the muscular power required for flying? What of the in- 
ability of man to fly? What essentially contributes to the lightness 
of birds? Describe the digestive apparatus of birds. How is it 
varied in different kinds of birds? What is said of the crop? What 
is said of the different periods of incubation? What is the real agent 
of the process? What is said of the Ostrich? What of the Mound 
Birds? What is said of the nests of birds? What of the formation 
of the bird in the egg? How does it get out? What is said of the 
development of the senses of birds? What of the nictitating mem- 
brane? What of the ear? What is said of the legs of birds? De- 
scribe the arrangement of the perching apparatus. How is it used in 
taking prey? What is said of the plumage of birds? What of their 
powers of song? What are the two uses of the tail? What is said 
of the migration of birds? Is it only, or even the chief object of this, 
to find a suitable temperature? How much influence has the weather 
on migration? What is the most wonderful fact in regard to it? 
