136 THE NATURAL HISTORY OF SELBORNE. 
As they were grubbing a vast hollow pollard-ash that had been 
the mansion of owls for centuries, he discovered at the bottom 
a mass of matter that at first he could not account for. After 
some examination he found that it was a congeries of the 
bones of mice (and perhaps of birds and bats) that had been 
heaping together for ages, being cast up in pellets out of the 
crops of many generations of inhabitants. For owls cast up 
the bones, fur, and feathers of what they devour, after the 
manner of hawks. He believes, he told me, that there were 
bushels of this kind of substance. 
When brown owls hoot their throats swell as big as a hen’s 
egg. I have known an owl of this species live a full year 
without any water. Perhaps the case may be the same with 
all birds of prey. When owls fly they stretch out their legs 
behind them as a balance to their large heavy heads, for as 
most nocturnal birds have large eyes and ears they must have 
large heads to contain them. Large eyes, I presume, are neces- 
sary to collect every ray of light, and large concave ears to 
command the smallest degree of sound or noise. 
It will be proper to premise here that the sixteenth, 
eighteenth, twentieth, and twenty-first letters have been pub- 
lished already in the P%zlosophical Transactions ; but as nicer 
observation has furnished several corrections and additions, 
it is hoped that the republication of them will not give 
offence; especially as these sheets would be very imperfect 
without them, and as they will be new to many readers who 
had no opportunity of seeing them when they made their first 
appearance. | 
The Airundines are a most inoffensive, harmless, entertain- 
ing, social, and useful tribe of birds; they touch no fruit in 
our gardens; delight, all except one species, in attaching them- 
selves to our houses; amuse us with their migrations, songs, 
and marvellous agility; and clear our outlets from the annoy- 
ances of gnats and other troublesome insects. Some districts 
