FLEAS 99 



fleas can transmit the trypanosomes referred to above. Their harm- 

 lessness in this respect may be more apparent than real, due to the fact 

 that we are ignorant of the true role they play. However, fleas have 

 only been closely associated with birds for a relatively short period, and 

 therefore they may not have had time to become adapted as carriers or 

 intermediate hosts of other bird parasites. 



Occasionally plague-carrying mammal fleas straggle on to birds. 

 Thus, in the Rothschild collection there is a specimen of the tropical 

 rat flea taken off a bird. Certainly the hen stick- tight flea, which is also 

 frequently a parasite of rodents, can carry plague from wild to domestic 

 rats. Birds, when they nest or roost in rat-infested houses or chicken 

 coops must be regarded as potential reservoirs of plague. On the other 

 hand the Ceratophyllidae, which are the commonest and largest group 

 of bird fleas, are not very effective vectors of plague. 



In the case of the hen stick- tight flea, which is not a British species, the 

 direct effect of an infection is very serious. Poultry lose weight, egg laying 

 is reduced, and birds not infrequently succumb to heavy infestations. 



The effect of a bite on a human being, in the case of non-sedentary 

 species of fleas, varies considerably from one individual to another. The 

 local swelling which causes a certain amount of irritation is probably 

 due to the enzyme which entered the wound in the insect's saliva. As 

 time goes on, a certain immunity is usually developed and elderly 

 people hardly react at all to a flea's bite. Whether this is also the case 

 in birds is not known. 



In any case it is extremely difficult to gauge the direct effect of fleas 

 on nestlings. The few specimens living on the bodies of adult birds can 

 be of little or no importance but several thousand fleas in a nest full of 

 young birds must present a serious menace. The mortality from all 

 causes among nestlings is high, sometimes sixty-five per cent, of all 

 those hatched in the case of small passerines. It seems reasonable to 

 suppose that when a single population of fleas runs into four figures the 

 constant drain of blood must be a contributing cause of death among 

 the more delicate young birds. 



Bird Fleas and the Evolution of Birds 



In the Mallophaga as we have seen, these parasites have in many 

 cases evolved at a somewhat slower rate than their hosts. The bird's 



FFC H 



