504 REPTILES AND BIRDS. 
tail, and excessively short tarsi. The air is the true element of these 
birds; they fly with a facility, lightness, and rapidity quite incon- 
ceivable ; indeed, their existence when not reposing is one eternal 
flight. So expert are they that they can even feed their young on the 
wing when the latter begin to fly. Watch them in the air, and they 
will be seen to rise and fall, tracing the shortest curves, crossing and 
interlacing each other’s course, moderating their pace suddenly when 
at their utmost speed, in order to follow the eccentric course of some 
winged insect which they have doomed for their food. Such, indeed, 
is the rapidity of their progress that some of the species have been 
known to travel at the rate of thirty leagues an hour. 
This wonderful power, however, is only developed at the sacrifice 
of another locomotive faculty, for they are bad walkers. With their 
short limbs, activity on their feet is impossible; and if by chance 
they are placed on level ground, with difficulty they rise again on 
the wing. On the other hand, their sight is excellent, equal to even 
that of the Eagle or Falcon. According to Spallanzani, who made 
numerous experiments on Swallows, the Martin perceives the 
winged fly passing through the air at the distance of more than 
120 yards. 
Swallows are celebrated for their migratory journeys. In the 
early days of spring they reach Europe, not in flocks, but as isolated 
individuals or in pairs. ‘They devote themselves almost immediately 
afterwards either to repairing their last year’s nests, or, if these have 
been destroyed, to constructing new ones. Among the arrivals aremany 
young birds of the previous year which have not had nests, and yet it 
is not a little extraordinary that these, after six months’ absence, re- 
turn with unerring certainty to the old dwelling where hatched. ‘This 
fact has been too often recorded to admit of any doubt on the subject. 
The form, structure, and locality of their nests vary with the 
species. The House Martins (Chelidon urbica, Fig. 220) build in the 
upper angles of the windows of some country house, under the eaves 
of a roof, or on the interior wall of a chimney. A chimney seems 
an odd place to select for such a purpose; and White of Selborne 
relates, not without some expressions of wonder at such a choice, 
that near the middle of May one of these little birds began to 
form her nest about five or six feet down a chimney adjoining the 
kitchen fire. Their nests consist of a crust or shell of mud mixed 
with straw, and lined with fine grass and feathers. Other species, 
sometimes in vast numbers, establish themselves in the hollows 
of dead trees. Audubon estimated at the incredible number of 
11,000 the quantity of swallows which had taken up their dwelling 
