BIRDS IN LONDON 341 



of escaped house- pigeons, and thus ultimately of the wild 

 blue rock-dove, which still haunts some of the wildest cliffs 

 and caves on the coast and inland. They nest and roost, 

 accordingly, in the streets which are like deep ravines, and 

 on the tall buildings which recall the lofty cliffs. The roar 

 and flow of traffic far beneath them is curiously like the 

 movement and murmur of the sea, when heard and seen from 

 one of their lofty watch-towers. Wood-pigeons, on the other 

 hand, are seldom or never to be seen out of sight of a park 

 or garden, or at least of one of the trees which break the 

 line of so many London streets. House-pigeons feed more 

 freely in the parks and squares than wood-pigeons in the 

 streets ; but they never nest in trees, whereas wood-pigeons 

 have already so far modified their ancestral habits as to nest 

 now and then in a window-box on an upper floor, which is a 

 site more recalling a ledge on one of the rock-dove's cliffs. 

 Every group of London's half-wild house-pigeons recalls 

 Darwin's famous experiments with the many varieties of 

 their one species, by the diverse gradations of plumage 

 between the standard pattern of the fanciers and the original 

 wild stock with its characteristic dark wing-bars. The 

 perpetual tendency is to revert to the original type ; and if 

 it were not for perpetual new recruits of strange hues and 

 shapes from the pigeon-cotes, in a very few years the whole race 

 of London house-pigeons would become pure blue rocks again. 

 Grey wagtails are far less numerous than black-headed 

 gulls, but equally regular as winter visitors. Wagtails 

 are often badly named ; the name of yellow wagtail is 

 reserved for a summer migrant, and the name of grey 

 wagtail is given to a bird in which yellow is even more 

 conspicuous, while the common grey member of the family 

 is called the pied wagtail. Pied wagtails may be seen now 

 and then in the parks or along the river at most times of 



