•382 HIRUNDINID^E. 



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 Selbome an instance is mentioned, of a nest bnilt on the wings and 

 body of an owl, which hung from the rafter of a bam. 



