102 FLEAS, FLUKES AND CUCKOOS 



Other's nests, which seem in every way equally suitable to fleas. All the 

 three common species referred to above have been found in the nest of 

 the pheasant but so far the only flea recorded from the partridge is 

 C. garei. It would appear that in some way the partridge is unattractive 

 to C. gallinae and D. gallinulae^ a fact which overrides the suitability of 

 the nesting site. 



The swan is a bird from which no fleas whatsoever, even stragglers, 

 have been recorded, despite the fact that its nest is both obvious and 

 accessible to collectors, and appears ideal for the requirements of C. 

 garei. We can do no more than guess at the reasons for its immunity. 

 Possibly the blood of the swan is unattractive to fleas, or its skin so 

 tough that the flea's mouth parts fail to pierce it, or the nest debris is in 

 some way unsuitable to the requirements of the larvae. 



One of the facts which strikes every student of bird fleas is the rela- 

 tively large variety of species which parasitise martins and swallows. 

 No less than seventeen fleas are specific to these hosts. Out of our own 

 fauna of fourteen bird fleas, five are martin fleas and of the rest, all but 

 four have been recorded from them as stragglers. Moreover, an un- 

 usually high proportion of martins' nests are infested, and the flea 

 population within individual nests is relatively very large. It has 

 already been pointed out that the habits of martins are helpful to 

 fleas. Thus they frequently build compact mud nests, or make nests in 

 holes in the ground or in caves, which suit the larvae and originally 

 favoured successful straggling. They also associate in colonies, so that 

 fleas off a dead host have not far to seek another suitable victim. What 

 is even more important, martins tend to return year after year to the 

 same nesting sites. These characteristics in themselves, however, are 

 not sufficient to ensure either a large or a varied fauna. For example 

 the swifts also construct mud nests, associate in colonies and return 

 year after year to the same site, but they have failed to acquire a single 

 flea peculiar to themselves. The only species recorded from their nests 

 are the hen flea and sparrow flea, both of which were probably im- 

 ported by sparrows which frequently usurp old nests and leave behind 

 a few parasites. The fact is we do not know why one whole group of 

 birds, such as the swifts, is unattractive and another, Hke the martins, 

 is apparently equally attractive to fleas. 



