118 HABITS OF BIRDS. 
accidentally precipitated, as we have seen happen, 
from the rocks above. On the departure of their 
enemy, the sheep began to feed again, but warily at 
first, raising their heads at intervals to assure them- 
selves that he was really gone; while, to render as- 
surance doubly sure, their leader again took his sta- 
tion as sentinel for the common weal. 
Now we are disposed to conclude, that these two 
cases of the sheep and the raven may, so far as food 
is concerned, be taken as the general conditions of 
the solitary and gregarious habits of birds. One 
whose food is confined to living prey will prowl 
alone, because along with associates he might fare 
scantily ; while those who feed on seeds and other 
vegetable substances, easily obtained in abundance, 
congregate that they may feed more safely, by ap- 
pointing, as the mountain sheep do, a sentinel to 
warn them of danger. ‘The rgven, indeed, can hard- 
ly be looked upon as subsisting wholly on living 
prey. It is not furnished by nature with suffi- 
ciently formidable weapons for this purpose ; and 
almost uniformly, when carrion cannot be obtain- 
ed, it attacks lambs, sickly sheep, or such as have 
fallen into a ditch or bog, perching on the head 
and pecking out the eyes. In temperate climates, 
birds that prey on carrion are less necessary as 
scavengers than in tropical countries, where flocks 
of vultures collect together from distances that 
have astonished all observers by whom the circum- 
stance is recorded. The gregariousness of these 
birds, however, may be plausibly referred to the 
wise care of Providence to have offensive car- 
casses speedily removed; and it is manifestly 
with this design that such birds are endowed with 
extremely acute senses, either of vision or of smell, 
probably both, so as to enable them to discover car- 
rion afar off. 
This is well exemplified in two species, which 
have been frequently confounded, the Turkey buz- 
