130 ON THE INSTINCTS OF BIRDS. 
situated in an exposed district ; and as the corners 
of those windows are too shallow to protect the nests 
from injury, they are washed down every hard rain ; 
yet the birds drudge on to no purpose, from summer 
to summer, without changing their aspect or house. 
These actions, it cannot be denied, seem to indi- 
cate a more limited degree of sagacity in birds than 
might be inferred from those previously spoken 
of. This apparent contradiction, however, may be 
easily reconciled by admitting what, in all probability, 
will be thought sufficiently obvious, that the dictates 
of the understanding are frequently too feeble to 
resist the powerful influence of instinctive impulse. 
Several examples illustrative of this view of the sub- 
ject will be found interspersed through the remainder 
of the essay; there is not any necessity, therefore, 
for entering into a more detailed consideration of it 
here. 
After the business of nidification is completed, 
parturition commences, which is succeeded by incu- 
bation; and as birds will frequently continue to 
deposit their eggs in the same nest, though all, ex- 
cept one or two, should be removed as fast as they 
are laid, or exchanged for others of a different size 
and colour, and as they will sometimes, after having 
produced their appointed number, sit upon a single 
egg, on the eggs of other birds introduced for the 
purpose of experiment, on artificial ones of chalk, or 
even upon stones of any irregular figure, it is plain 
