OF SELBORNE 175 



colour from Ihat which lay loose and bleached in the 

 sun. 



In what space of time these little artists are able to 

 mine and finish these cavities I have never been able to 

 discover, for reasons given above ; but it would be a 

 matter worthy of observation, where it falls in the way of 

 any naturalist to make his remarks. This I have often 

 taken notice of, that several holes of different depths are 

 left unfinished at the end of summer. To imagine that 

 these beginnings were intentionally made in order to be in 

 the greater forwardness for next spring, is allowing per- 

 haps too much foresight and rerum pnidentia to a simple 

 bird. May not the cause of these lalebrx 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 untenable. This species of swallow moreover is 

 strangely annoyed with fleas : and we have seen 

 fleas, bed-fleas (pulex initans), swarming at the 

 mouths of these holes, like bees upon 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 hgbernacula, 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 

 ^th the swallow, and lays, as she does, froni four to six 



