382 
HIRUNDINIDyE. 
satisfaction of seeing it tenanted by young birds. This nest had 
for its support the wing of one of the departed falcons nailed 
against the wall, and on the centre of which it rested.* The 
entire height of the shed, which was erected solely for the pro- 
tection of the hawks, is not above seven feet. The nests are 
about six feet from the ground, and built against a beam of timber 
placed on the top of the low wall supporting the roof. The 
height of the roof from the ground is four feet two inches, which 
leaves only two and a half feet clear for the swallow's flight 
between it and the heads of the hawks, as they perch upon their 
blocks. One of the nests is only six feet from the block occu- 
pied by a hawk, and from which this bird has liberty to move to 
half that distance. The swallows, however, flew closely past these 
rapacious birds without being in any way molested by them. 
Leaving their young to perish. Mr. Blackwall in his Re- 
searches in Zoology, mentions the remarkable fact from personal 
investigation, that swallows, house-martins, and sand-martins not 
unfrequently leave their last brood of young to perish, and occa- 
sionally leave their eggs before they are incubated. He speculates 
on the causes of this “ voluntary act of desertion," and combats 
the opinion of Dr. Jenner, that it is prompted by “the desire to 
migrate, produced by a change in the reproductive system” 
Having given less attention to the subject than either author, 
I should perhaps be silent, but a few remarks on so apparently 
singular a proceeding may possibly not be considered presumptu- 
ous. In the instances alluded to, the young broods and eggs 
were deserted late in the season, and I should suppose at the 
migratory period. The paramount object would then seem to be 
migration, and when favourable weather and wind prevail, the 
love of offspring yields to the stronger impulse, and the parents 
take their departure. Had this favourable time been long enough 
protracted, they would have continued to tend their offspring and 
bring them to maturity. It is quite different at the season when 
the first brood is being produced. The primary principle which 
* In White’s Selborne an instance is mentioned, of a nest built on the wings and 
body of an owl, which hung from the rafter of a barn. 
