25 
on the migration of birds. 
swallow incessantly skimming over the surface of ponds and 
brooks ; and their thus early hovering over water has 
strengthened the idea of their having lately emerged from 
their watery abode, where they are supposed to have lain 
dormant during the winter. But they are driven by necessity 
to feed on the gnat. Like the swift and martin, their more 
favourite food is a small beetle of the scarabasus kind, which, 
on dissection, I have found in far greater abundance in their 
stomachs than any other insects. 
The tumid state of the testes and ovaria sometimes comes 
on prematurely, and in the same manner sometimes subsides. 
When this happens, swallows and martins desert their nest- 
lings, and leave them to perish in the nest. The economy of 
the animal seems to be regulated by some external impulse, 
which leads to a train of consequences. When this change 
in the testes and ovaria takes place, the bird becomes im- 
pelled by a stronger principle, that is, the desire of self pre- 
servation. This sometimes happens when they produce a 
very late hatch. A pair of martins hatched four broods of 
young ones in the house of a tradesman in this place in the 
year 1786. The latter brood was hatched in the early part 
of October. About the middle of the month the old birds 
went off, and left their young ones, about half fledged, to 
perish. The pair returned to the nest the 17th of May, 1787, 
and threw the skeletons out. 
Thus scarcely a winter passes but we hear of a nest of 
robins, hedge-sparrows, and some others of the smaller birds. 
We have been informed by Pennant, and it has been noticed 
also by others, that the cuckoo has been heard to give his 
song so early as the middle of February, two months sooner 
MDCCCXXIV. E 
