CHANGE OF HABITS IN ANIMALS 403 



also to how great an extent the building of man's tenements 

 must have increased the opportunities for nesting and the 

 numbers of the house-nesting birds. And this has led to 

 another circumstance of some economic interest, for let us 

 suppose that on an average each nest at Baddingsgill shel- 

 tered but four chicks and that the average number of insects 

 caught daily to feed each chick was only 500 (a low estimate, 

 for Mr O. H. Wild tells me that he has counted 400 to 500 

 insects in the gullet of a Swift a single meal), then the 

 daily catch for this colony of 280 youngsters (the 140 adults 

 being omitted) during the nesting-season would average 

 1 40,000 insects, Gnats, Midges, Weevils and Greenflies, a 

 work of no small service to man, the tiller of the soil. 



House -sparrows, too, though a proportion of their num- 

 ber still retains the primitive tree-nesting habit, have to a 

 very large extent developed an inordinate love for the chim- 

 neys and water-rhones of houses an apparent affection for 

 man's habitations commemorated in their Linnaean name of 

 Passer domesticus. 



Shelter and warmth and easy supply of food have all 

 contributed to induce many creatures, which must originally 

 have dwelt in the open, to accept the unwilling hospitality 

 of man's abode. Houses have, indeed, a fauna of their own, 

 the members of which have to some extent altered their 

 habits in placing themselves under man's roof. The simplest 

 case is that of the creatures which take temporary shelter 

 in the warmth and security of houses in order to tide over 

 periods of severe conditions out of doors. Take the swarms 

 of Flies which appear year after year in some houses, 

 covering the windows until, as Mr Hugh Scott described, 



every pane of glass was densely covered with countless myriads of small 

 flies; on the upper sides of the projecting cross-pieces of wood between the 

 panes the flies rested in masses, literally crawling over each other, while all 

 the part of the ceiling near the window was almost as thickly covered as 

 the window itself. 



Such swarms, which have recently been recorded from 

 houses in Edinburgh and in Fifeshire, as well as from Cam- 

 bridge, consist mainly of Two- winged \\zsLimnop/iora 

 septem-notata being most common, although in one case 

 Chloropisca ornata was abundant. Careful investigation by 

 Professor J. H. Ash worth has shown that these thousands 



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