254 NATURAL HISTORY 



the second broods ; till at last they swarm in myriads 

 upon myriads round the villages on the Thames, dark- 

 ening the face of the sky as they frequent the aits of 

 that river, where they roost. They retire, the bulk of 

 them I mean, in vast flocks together, about the begin- 

 ning of October : but have appeared of late years in a 

 considerable flight in this neighbourhood, for one day 

 or two, as late as November the 3d and 6th, after they 

 were supposed to have been gone for more than a fort- 

 night. They therefore withdraw with us the latest of 

 any species. Unless these birds are very short-lived 

 indeed, or unless they do not return to the district 

 where they are bred, they must undergo vast devasta- 

 tions somehow, and somewhere ; for the birds that 

 return yearly bear no manner of proportion to the birds 

 that retire. 



House martins are distinguished from their congeners 

 by having their legs covered with soft downy feathers 

 down to their toes. They are no songsters ; but twitter 

 in a pretty inward soft manner in their nests. During 

 the time of breeding, they are often greatly molested 

 with fleas. 



I am, &c. 



LETTER XVII. 



TO THE SAME. 

 DEAR SIR, RINGMER, near LEWES, Dec. 9, 1773. 



I RECEIVED your last favour just as I was setting out 

 for this place; and am pleased to find that my mono- 

 graphy met with your approbation. My remarks are 

 the result of many years' observation ; and are, I trust, 

 true in the whole : though I do not pretend to say that 

 they are perfectly void of mistake, or that a more nice 

 observer might not make many additions, since subjects 

 of this kind are inexhaustible. 



