20 
The late Dr. Jenner 
having also endeavoured to controvert the notion of an hi- 
bernating system, I beg to draw their attention to what I 
conceive to be the true cause of migration. 
At the coming on of spring we observe our more domestic 
birds, those that approach our houses, and are most familiar 
to us, assuming new habits. The voice, gesticulation, and 
the attachment which the male begins to show to the female, 
plainly indicate some new agency acting upon the constitu- 
tion.* This newly excited influence, which so conspicuously 
alters the habits of our birds at home, is, at the same time, 
exerting itself abroad upon those which are destined to re- 
sort hither. It is the preparation which nature is making for the 
production of an offspring by a new arrangement in the structure 
of the sexual organs , (viz.) the enlargement of the testes in the 
male , and the ovaria in the female. 
No sooner is the impulse arising from this change suffi- 
ciently felt, than the birds are directed to seek a country 
where they can for a while be better accommodated with 
succours for their infant brood, than in that from which they 
depart. 
* The Rook, among many others, exhibits a familiar instance of the change of 
voice. 
f Birds of the same species that are commonly stationary in this island through- 
out the year (I say commonly , for all, I believe, occasionally migrate), are migrators 
in other countries. The adult bird might, perhaps, find a subsistence for itself in 
the country it quits during the incubating season, but the nestling is probably the 
object nature chiefly holds in view, both with respect to food, and to the tempera- 
ture of the air in which it is first to feel existence. The one may be unfit or too 
scanty, and the other too hot or too cold. It is wonderful to see with what peculiar 
care the parent birds select the food for their young until they are four or five days 
old. For the most part it is purely animal, but not an atom even of that is suffered 
to go into the nestling’s stomach, that is not perfectly adapted to the tender state of 
