112 THE NATURAL HISTORY [LETT 



heaping together for ages, being cast up in pellets otit of the 

 crops of many generations of inhabitants. Fur 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 

 e-gg. 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 necessary to collect 

 every ray of light, and large concave ears to command the 

 smallest degree of sound or noise. 1 



The hirundines 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 districts in the 

 South Seas, near Guiaquil, 2 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 hi r un- 

 it ines 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 Hippolosca; hirundines, with narrow 



1 It will be proper to premise here that tiie Letters LIII., LV., LVII., 

 and LX., have been published already in the " Philosophical Transactions," 

 Imt nicer observation has furnished several corrections and additions. 



See Ulloa's " Travels " 



