142 
THE NATURAL HISTORY 
[LETT. 
heaping together for ages, being cast np in pellets out of the 
crops of many generations of inhahitants. 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. 
AVhen 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 tliey stretch out tlieir 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 necessary to collect 
every ray of light, and large concave ears to command the 
smallest degree of sound or noise.^ 
The hirundiries are a most inoffensive, harmless, entertaining, 
social, and useful tribe of birds ; they touch no fruit in our 
gardens ; delight, all except one species, in attaching themselves 
to our houses ; amuse us with their migrations, songs, and mar- 
vellous agility ; and clear our outlets from the annoyances of 
gnats and other troublesome insects. Some distiicts in the 
South Seas, near Guiaquil,^ are desolated, it seems, by the in- 
finite swarms of venomous mosquitoes, which fill the air, and 
render those coasts insupportable. It would be worth inquiring 
whether any species of hirundines is found in those regions. 
Whoever contemplates the myriads of insects that sport in the 
sunbeams of a summer evening in this country, will soon be 
convinced to what a degree our atmosphere would be choked 
with them were it not for the friendly interposition of the 
swallows. 
Many species of birds have their peculiar lice ; but the hirun- 
dines alone seem to be annoyed with dipterous insects, which 
infest every species, and are so large, in proportion to them- 
selves, that they must be extremely irksome and injurious 
to them. These are the Hippoboscce hirundines, with narrow 
^ It will be proper to premise here that the Letters LIIL, LV., LVII,, 
and LX., have been published already in the "Philosophical Transactions," 
but nicer observation has furnished several corrections and additions. 
2 See Ulloa's Travels." 
