44 
The late Dr. Jenner on the migration of birds. 
homes (this country) in spring, in quest of a country better 
suited to their intended purpose than their own ; that they 
are actuated by the same impulse in quitting this country, that 
causes the spring birds to come to it, and that want of food 
cannot be the inducement ; that the emigration of the winter 
birds is less complete than that of the others (the spring mi- 
grators ) ; that some species breed here, especially the wild- 
duck and wood-pigeon ; that the redwings and fieldfares are 
the most regular and uniform in their appearance and disap- 
pearance, and most probably never risk the trial of incubation 
here ;* that they quit the country temporarily in severe and 
long continued frost through want of food, and return to it 
again at the approach of more temperate weather ; that the 
arrival of water-birds forebodes the approach of intense frost, 
the usual return of the winter-birds, a thaw ; that examina- 
tions of the latter prove them to have taken long flights be- 
fore their return, and sets the fact of temporary migration 
beyond the reach of doubt. 
I have then made a digression, and introduced some obser- 
vations on the singing of birds ; and in a third part, given 
some additional particulars respecting the different sizes of 
the generative organs of birds, as they appear at different 
seasons of the year. 
* I must be understood by the word “ here,” to mean that part of Gloucester- 
shire under my own observation. 
