Wilts.  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 necessary to collect every ray of light, and large concave ears to command the smallest degree of sound or noise.

I am, etc.

* * * * *

[It will be proper to premise here that the sixteenth, eighteenth, twentieth, and twenty-first letters have been published already in the “Philosophical 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 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 marvellous agility; and clear our outlets from the annoyances of gnats and other troublesome insects.  Some districts in the south

seas, near Guiaquil, are desolated, it seems, by the infinite 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 was it not for the friendly interposition of the swallow tribe.

“Many species of birds have their peculiar lice; but the hirundines alone seem to be annoyed with dipterous insects, which infest every species, and are so large, in proportion to themselves, that they must be extremely irksome and injurious to them.  These are the hippoboscœ hirundinis, with narrow subulated wings, abounding in every nest; and are hatched by the warmth of the bird’s own body during incubation, and crawl about under its feathers.

“A species of them is familiar to horsemen in the south of England under the name of forest-fly; and to some of side-fly, from its running sideways like a crab.  It creeps under the tails, and about the groins, of horses, which, at their first coming out of the north, are rendered half frantic by the tickling sensation; while our own breed little regards them.

“The curious Reaumur discovered the large eggs,

or rather pupœ, of these flies as big as the flies themselves, which he hatched in his own bosom.  Any person that will take the trouble to examine the old nests of either species of swallows may find in them the black shining cases or skins of the pupœ of these insects; but for other particulars, too long for this place, we refer the reader to ‘L’Histoire d’Insectes’ of that admirable entomologist.  Tom. iv., pl. ii.”

LETTER XVI.

Selborne, Nov. 20th, 1773.

Dear Sir,—In obedience to your injunctions I sit down to give you some account of the house-martin, or martlet; and if my monography of this little domestic and familiar bird should happen to meet with your approbation, I may probably soon extend my inquiries to the rest of the British hirundines—the swallow, the swift, and the bank-martin.

A few house-martins begin to appear about the 16th April; usually some few days later than the swallow.  For some time after they appear the hirundines in general pay no attention to the business of nidification, but play and sport about, either to recruit from the fatigue of their journey, if they do migrate at all, or

else that their blood may recover its true tone and texture after it has been so long benumbed by the severities of winter.  About the middle of May, if the weather be fine, the martin begins to think in earnest of providing a mansion for its family.  The crust or shell of this nest seems to be formed of such dirt or loam as comes most readily to hand, and is tempered and wrought together with little bits of broken straws to render it tough and tenacious.  As this bird often builds against a perpendicular wall without any projecting ledge under, it requires its utmost efforts to get the first foundation firmly fixed, so that it may safely carry the superstructure.  On this occasion the bird not only clings with its claws, but partly supports itself by strongly inclining its tail against the wall, making that a fulcrum; and thus steadied, it works and plasters the materials into the face of the brick or stone.  But then, that this work may not, while it is soft and green, pull itself down by its own weight, the provident architect has prudence and forbearance enough not to advance her work too fast; but by building only in the morning, and by dedicating the rest of the day to food and amusement, gives it sufficient time to dry and harden.  About half an inch seems to be a sufficient layer for a day.  Thus careful workmen, when they build mud-walls (informed at first, perhaps, by this little bird), raise but a moderate

layer at a time, and then desist, lest the work should become top-heavy, and so be ruined by its own weight.  By this method in about ten or twelve days is formed a hemispheric nest with a small aperture towards the top, strong, compact, and warm; and perfectly fitted for all the purposes for which it was intended.  But then nothing is more common than for the house-sparrow, as soon as the shell is finished, to seize on it as its own, to eject the owner, and to line it after its own manner.

After so much labour is bestowed in erecting a mansion, as Nature seldom works in vain, martins will breed on for several years together in the same nest, where it happens to be well sheltered and secure from the injuries of weather.  The shell or crust of the nest is a sort of rustic work, full of knobs and protuberances on the outside; nor is the inside of those that I have examined smoothed with any exactness at all; but is rendered soft and warm, and fit for incubation, by a lining of small straws, grasses, and feathers, and sometimes by a bed of moss interwoven with wool.  In this nest they tread, or engender, frequently during the time of building; and the hen lays from three to five white eggs.

At first when the young are hatched, and are in a naked and helpless condition, the parent birds, with tender assiduity, carry out what comes away from

their young.  Was it not for this