PARASITISM 9 



blackflies are only parasitic in the adult stage and they too only visit the 

 host at irregular intervals, some by day and some by night. Very little is 

 known about the biology of these species and it is not quite certain 

 whether all are true obligate parasites. Some may be more correctly in- 

 cluded in the group of animals classed 2iS facultative parasites. These can 

 live and complete their life-cycles as free animals, but resort to para- 

 sitism if circumstances are favourable. For example, certain fly larvae 

 which normally hve on dead and decaying matter in nests are attracted 

 to any sores which may be present on the bodies of the young birds. They 

 may invade these areas and from then onwards f.ed upon the exudations 

 and the putrefying flesh in the cavity of the wounds. 



There are also very large numbers of organisms which can be 

 called accidental parasites. Certain Protozoa, maggots and worms are 

 over and over again accidently ingested by birds and other animals 

 and can survive for varying periods. They feed upon secretion or even 

 hving body tissues although this is in no way their normal mode of life. 



Among these groups we recognise many degrees of dependence, 

 diflferent and contrasting life-cycles and great variation with regard to 

 the mutual reactions of host and parasite. The feather lice and mange 

 mites, for example, which are so-called permanent obligate parasites, 

 pass generation after generation on the same individual animal, even 

 their eggs being attached to the outside or buried within the tissues of 

 the host. On the other hand, certain worms and many of the Protozoa 

 pass through tw^o or more hosts and, certain phases of their lives 

 present a great contrast, part being spent in warm- and part in cold- 

 blooded animals. Again, the tgg stage, or the spore and cyst stage, 

 of obligate parasites often passes from the host into the outside 

 world, and is the means by which another animal is reached and 

 the life-cycle recommenced. 



Perhaps the most fascinating form of dependence among bird 

 parasites is what is now defined as brood parasitism. The feeding and 

 rearing of the young is entrusted to a diflferent species of bird. This 

 type of behaviour is not uncommon in the animal kingdom as a whole 

 but it is very rare among vertebrates. Established brood parasitism in 

 mammals is unknown, although in isolated instances and under dom- 

 estic conditions certain species will voluntarily rear the young of others. 

 The European cuckoo is the best known type of brood parasite among 

 birds and the only one found in Britain. It is an obligate parasite, 

 incapable of rearing its own young. There are, however, numbers of 



