BUILDING INSTINCT OF BEAVER. 
5 37 
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 
Fig 280. —Beaver. 
now chiefly found in Canada, though it formerly abounded 
on the Continent of Europe. During the summer it lives 
solitarily in burrows, which it excavates for itself on the 
borders 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 
only in the most solitary places that their architectural in¬ 
stinct fully developes itself. Having associated in troops of 
from two to three hundred each, they choose a lake or river 
which is deep enough to prevent its being frozen to the 
bottom; and they generally prefer running streams, for the 
sake of the convenience which these afford in the transporta¬ 
tion of the materials of their erection. They begin by 
constructing a sloping dam, whereby the water is kept-up to 
