102 THE NATURAL HISTORY 



LETTER XL 



Selborne, Sept. 2, 1774. 

 Dear Sir, 



Before your letter arrived, and of my own accord, I had 

 been remarking and comparing the tails of the male and 

 female swallow, and this ere any young broods appeared ; 

 so that there was no danger of confounding the dams 

 with their pulli : and besides, as they were then always 

 in pairs, and busied in the employ of nidification, there 

 could be no room for mistaking the sexes, nor the 

 individuals of different chimnies the one for the other. 

 From all my observations, it constantly appeared that 

 each sex has the long feathers in its tail that give it that 

 forked shape ; with this difference, that they are longer 

 in the tail of the male than in that of the female. 



Nightingales, when their young first come abroad, and 

 are helpless, make a plaintive and a jarring noise : and 

 also a snapping or cracking, pursuing people along the 

 hedges as they walk : these last sounds seem intended 

 for menace and defiance. 



The grasshopper-lark chirps all night in the height ol 

 summer. 



Swans turn white the second year, and breed the 

 third. 



Weasels prey on moles, as appears by their being 

 sometimes caught in mole-traps. 



Sparrow-hawks sometimes breed in old crows' nests, 

 and the kestril in churches and ruins. 



There are supposed to be two sorts of eels in the 

 island of Ely. The threads sometimes discovered in eels 

 are perhaps their young : the generation of eels is very 

 dark and mysterious. 



