80 FLEAS, FLUKES AND CUCKOOS 



In the course of several experiments Buxton showed that out of 

 50 fleas placed on a captive mouse only approximately 14 survived on 

 the seventh day — the rest having presumably been eaten or killed by 

 the host. Undoubtedly birds destroy many fleas and their remains are 

 sometimes found in the host's crop. On the other hand there is no 

 evidence that they form part of the normal diet of any insectivorous 

 bird. Another point brought out by Buxton was the higher proportion 

 of fleas which survived on captive baby mice as opposed to those on 

 adult mice. Fledgelings are also relatively helpless in the face of attacks 

 by parasites and the various species which pullulate in their nests must 

 greatly reduce their strength and vitality. 



By far the most uncomfortable nests are those of the sand- 

 martin. Ceratophyllus styx, which teems in thousands in their burrows, 

 over-winters either as an adult or as a pupa which hatches in the spring. 

 It is sad to think that when the sand-martin reaches its breeding haunts 

 in April, having successfully endured the hardships and hazards of 

 migration, it is met by a reception committee in the form of thousands 

 of ravenous fleas which can be seen waiting round the entrance to thenests. 



It is perhaps obvious that one of the reasons why martins are 

 generally so heavily infested with fleas, both with regard to actual 

 numbers and variety of species, is because of this habit of returning 

 again and again to the same nesting site. Holes in mud banks or quarries 

 (Plate XXXV), and mud nests provide a favourable habitat for the 

 early stages in the life-cycle, but this reason alone is insufficient to 

 account for the numbers concerned. Compared with a house-martin 

 flea such as C.farreni, the species infecting warblers, finches and thrushes 

 have an extremely precarious existence. The temperature in a bird's 

 nest during the incubation period and the rearing of the young fledgelings 

 is sufficiently high to speed up metamorphosis of the flea to a maximum 

 degree. The number of blood feeds, temperature, copulation and fertile 

 egg laying are intimately linked and in the spring the flea population 

 must be seething with activity within the nest. These palmy days are 

 all too brief and at the end of 1-2 months the young birds are fledged 

 and leave the nest never to return. Maybe one or two fleas, busy 

 feeding on their host at the time, are carried away on each fledgeling. 

 The great majority, whatever stage they have reached in their develop- 

 ment, are left to perish miserably in the deserted nest. 



On several occasions fleas have been observed leaving birds' nests 

 in large swarms, and in Russia migration from abandoned mammals' 



