NA TURAL HISTOR Y OF SELBORNE. 1 73 



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latebra being left unfinished arise from their meeting in those places 

 with strata too harsh, hard, and solid for their purpose, which they 

 relinquish, and go to a fresh spot that works more freely ? Or may 

 they not in other places fall in with a soil as much too loose and 

 mouldering, liable to flounder, and threatening to overwhelm them 

 and their labours ? 



One thing is remarkable that, after some years, the old holes are 

 forsaken and new ones bored ; perhaps because the old habitations 

 grow foul and fetid from long use, or because they may so abound 

 with fleas as to become untenantable. This species of swallow 

 moreover is strangely annoyed with fleas ; and we have seen fleas, 

 bed-fleas (pulex irritans), swarming at the mouths of these holes, 

 like bees on the stools of their hives. 



The following circumstance should by no means be omitted that 

 these birds do not make use of their caverns by way of hybernacula, 

 as might be expected ; since banks so perforated have been dug out 

 with care in the winter, when nothing was found but empty nests. 



The sand-martin arrives much about the same time with the 

 swallow, and lays, as she does, from four to six white eggs. But 

 as this species is cryptogame, carrying on the business of nidification, 

 incubation, and the support of its young in the dark, it would not be 

 so easy to ascertain the time of breeding, were it not for the coming 

 forth of the broods, which appear much about the time, or rather 

 somewhat earlier than those of the swallow. The nestlings are sup- 

 ported in common like those of their congeners, with gnats and 

 other small insects ; and sometimes they are fed with libelhilce 

 (dragon-flies) almost as long as themselves. In the last week in 

 June we have seen a row of these sitting on a rail near a great pool 

 as perchers, and so young and helpless, as easily to be taken by 

 hand ; but whether the dams ever feed them on the wing, as swallows 

 and house-martins do, we have never yet been able to determine ; 

 nor do we know whether they pursue and attack birds of prey. 



When they happen to breed near hedges and enclosures, they are 

 dispossessed of their breeding-holes by the house-sparrow, which 

 is on the same account a fell adversary to house-martins. 



These hirundines are no songsters, but rather mute, making only 

 a little harsh noise when a person approaches their nests. They 

 seem not to be of a sociable turn, never with us congregating with 

 their congeners in the autumn. Undoubtedly they breed a second 

 time, like the house-martin and swallow, and withdraw about 

 Michaelmas. 



