246 FLEAS, FLUKES AND CUCKOOS 



are found in nests in holes are equally numerous in holes without nests. 

 There are also many common plant-eating mites and insects which are 

 passively introduced along with moss and lichen and other vegetable 

 nesting material. These flourish in a wide range of bird habitations, 

 and, ill cases such as the mite, Oribata geniculatus, can become dominant 

 species, but they are in no way peculiar to this type of habitat. Despite 

 the large proportion of wanderers, accidental visitors, occasional and 

 casual residents, and constant if independent inhabitants, there remains 

 quite a high proportion — say between 20 and 25 per cent. — of the 

 species present, which at one stage or another of their life-cycle are 

 obligate nidicoles and dependent on nests. Of these a few are host- 

 specific, and are found only in the nests of one species, or of a group of 

 related species of birds, but many are catholic in their tastes. 



Various factors influence and determine the fauna of birds' nests, 

 and our knowledge concerning them is ridiculously small, but one or 

 two generalisations can be made with confidence. Nests which are 

 built in holes, and which are returned to and re-occupied year after 

 year, contain on an average a larger number of individual nidicoles 

 and a greater variety of species than other nests. In this respect the host 

 itself seems to be less important. Thus the wood-pigeon has the smallest 

 nest fauna of any British bird so far examined, both as regards numbers 

 and variety, and the closely related stock-dove, which generally nests 

 in holes, has the largest. Needless to say there are many exceptions. 

 The crow family as a rule have revoltingly verminous nests, and the 

 carrion-crow with about 80 species recorded can boast a richer fauna 

 than most hole dwellers. The type of nest, whether it is domed or flat, 

 or just a scrape, massive or flimsy, constructed of mud or moss, stick or 

 stones, sea-weed or sand, naturally influences the nest fauna. The site 

 chosen, the age of the nest, whether it has contained young, its distance 

 from the ground, the proximity to water or human habitations or other 

 birds' nests is also important. The habits of the host, particularly its 

 choice of food, which to a great extent determines the nature of the 

 debris within the nest, have a considerable bearing on the species 

 of arthropods found there. Thus a beetle, Trox scaler, which chiefly 

 feeds on old bones and hides, is characteristic of owls' nests and 

 one would not look for it under a sitting firecrest or blackcap. 



The population of a bird's nest is not, of course, stable. The various 

 nidicoles have different requirements of food, temperature, humidity 

 or light; and this will influence which species occur in specific nests, in 



