24 
The late Dr. Jenner 
ation, and took several of them on the same night, the latter 
end of May. On dissection, the cause of their gradual and 
successive migrations appeared obvious, the testes and ovaria 
being in very different states of progressive forwardness. 
While one bird presented embryo eggs in the ovarium as 
large as peas, in another they were found no larger than 
hemp-seed. These were the extremes ; for in the other 
birds there appeared all the intermediate stages, from the 
enlargement of the ovaria, sufficient to give the stimulus 
for migration, to the degree of forwardness just described. 
The same gradations in the state of the testes of the male 
corresponded with that of the ovaria in the females. This 
progressive arrival is not confined to the swallow tribe : all 
the birds that come early in the spring appear in the same 
gradual manner. I cannot help observing, that here the wise 
design of Providence is very conspicuous. Their appearance 
keeps pace with that of the insects which are to afford them 
food. If the numbers which flock in upon us in May, were 
to arrive in April, when only part of them appear, all must 
be insufficiently supplied, and many of course perish from a 
want of the needful succours ; but by the middle of May, 
myriads of insects have produced eggs, and great numbers 
have either brought forth, or matured their progeny ; and it 
may be remarked, there is still a greater increase of insect 
food by the time the young birds begin to require it. Swal- 
lows, on their first coming, feed principally upon gnats. 
These insects are called forth from their wintry retreats 
when the air is but moderately heated, 48 degrees of Fahren- 
heit’s thermometer being sufficient to put them on the wing. 
It is in pursuit of them that we see, in cool weather, the 
