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 are many 

 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 (CheUdon 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 



