THE FAUNA OF BIRDS* NESTS 247 



which part of the structure and at what period of their history. Thus 

 within the nest itself there may be different levels at which various 

 species are more abundant than at others. For example, in a great tit's 

 nest, out of a total of 3,469 arthropod inhabitants, 490 were found in 

 the lining, 2,277 in the middle layers and 702 in the outer structure. 

 In a flycatcher's nest the position was reversed and out of a total of 

 1,568 specimens no less than 840 — over half — were found in the lining, 

 and the smallest numbers were present in the middle layer, where, in 

 the tit's nest, they reached a maximum. An observer once saw fly 

 larvae — up to that moment completely hidden — seethe to the surface 

 of the lining when droppings fell, and eagerly devour them. The nest is 

 occupied however, for only a very brief period and most populations — 

 especially if the larvae and adults of each group are counted together — 

 reach a maximum density during two or three days when the young are 

 about to leave or have just flown. Quite a large assortment of arthro- 

 pods overwinter in immature or adult stages in the nest. 



The habitations of the Hirundinidae (martins and swallows) probably 

 harbour the most interesting nidicoles of any group of British birds, 

 although several others, such as the jackdaw, starling and stock-dove 

 have a larger assortment. The crows as a family, have a richer though 

 less distinctive nest fauna. 



A conspicuous inhabitant of house-martins' and occasionally other 

 birds' nests is the swallow bug {Oeciacus hiriindinis, Plate XXXVIb). 

 We have already remarked (p. 16) that martins and men probably 

 shared cave dwellings in prehistoric times and they may both have 

 acquired this group of parasites in their former habitat. Very few birds 

 are preyed on by bugs. One of their essential requirements is a perma- 

 nent dwelling house, for during the day they hide in cracks and crevices 

 — in which they also lay their eggs — and only creep out at night for a 

 blood meal. There is one other bird bug in Britain, Cimex columbarius^ 

 which is a parasite of the domestic pigeon. In all probability it is a 

 sub-species of the human bed-bug (C lectularius^ see Plate XXXVIa) 

 which has passed accidentally on to pigeons and chickens since their 

 domestication by man, and has now become morphologically distinct. 

 Fertile hybrids can be obtained by crossing the two forms. The bugs 

 which infest wild birds can only survive if the host is the type which 

 returns to its old nest. Moreover the nest itself must remain fairly dry 

 during the host's absence. In the United States the barn-swallow [Hirundo 

 rustica erythrogaster) and purple martin {Progne subis) , and the oven-bird 



