10 



FLEAS, FLUKES AND CUCKOOS 



birds, apart from the cuckoos, which practise brood parasitism, to a 

 greater or lesser degree. The cow-birds (Icteridae) of America, the 

 honey-guides (Indicatoridae), and certain weavers (Ploceidae) especial- 

 ly the widow-birds of Africa, adopt a similar mode of life. 



Not all these birds are as harmful to their hosts as the European 

 cuckoo, for in several species the foster parents rear the intruder as well 

 as their own young, which is left unmolested in the nest along with 

 the rightful owners. 



Some birds, of which the skuas (Plate XXXVIIIa) are good examples, 

 live by a curious form of food robbing known as clepto-parasitism. They 

 are large and powerful birds, capable of killing their prey in the usual 

 predatory manner, but instead they prefer to chase other sea birds and 

 by their relentless attacks force them to drop the prey they have 

 captured or to disgorge their last meal. 



Obligate parasites, facultative parasites, temporary or permanent 

 parasites, brood parasites, clepto-parasites, endo- or ecto-parasites — 

 all these categories have been invented by us for our own convenience 

 in order to simplify the task of description and exposition. They are 

 arbitrary classifications which do not bear critical analysis, and in 

 nature these types are found to merge gradually into one another. 

 Moreover, it is true to say even when closely related species of both 

 host and parasite are involved, no two parasitic relationships are 

 exactly alike. 



Fungus, Aspergillus fumigatus {x 293), 

 a facultative parasite in the lungs of birds 



