BUILDING INSTINCT OF BEAVER. 537 
Sylvia sutoria, or Tailor-bird, a little eastern bird allied to 
our linnet ; which, by the aid of filaments of cotton drawn 
from the cotton-plant, sews leaves together with its beak and 
feet, in such a manner as to conceal the nest which they 
enclose from the observation of its enemies (fig. 279). 
_ 706. The association of a number of individuals of a certain 
_species, for the performance of labours in which they all unite 
to one common end, is another most remarkable example of 
the operation of instinct. Several Mammals exhibit this 
tendency in a greater or less degree; but the most interesting 
) of all, in this point of view, is the Beaver (fig. 280), which is 
ow chiefly found in Canada, though it formerly abounded 
m the Continent of Europe. During the summer it lives 
litarily in burrows, which it excavates for itself on the 
rders of lakes and streams; but as the cold season ap- 
proaches, it quits its retreat, and unites itself with its fellows, 
to construct, in common with them, a winter residence. It is 
mly in the most solitary places that their architectural in- 
inct fully developes itself. Having associated in troops of 
m two to three hundred each, they choose a lake or river 
hich is deep enough to prevent its being frozen to the 
ttom ; and they generally prefer running streams, for the 
e of the convenience which these afford in the transporta- 
tion of the materials of their erection. They begin by 
tructing a sloping dam, whereby the water is kept-up to 
