WORMS 



187 



symptom is scarcely likely to be recorded for birds harbouring similar 

 parasites. Mechanical injuries, such as perforation of the intestinal 

 wall, severe bleeding, irritation and inflammation of various tissues, 

 blocking of ducts, thickening or maceration of various internal sur- 

 faces, the formation of ulcers or even cancerous growths are the types of 

 injury which attract attention. In nature, however, birds, unlike the 

 barnyard fowl, are not generally subjected to conditions which favour 

 infection with large numbers of nematodes simultaneously. It must also 

 be remembered that infections only last for a limited time, generally 

 less than a year. In due course the worms die and except in a few cases 

 they do not multiply inside the host. Therefore, if only one or two 

 specimens are present at one time, the bird probably recovers from the 

 injuries they inflict and symptoms due to their toxic secretions cease 

 when the parasites are eliminated. Nevertheless, when a bird-watcher 

 puts up his glasses to watch starlings or rooks feeding in the fields he 

 should pause and feel grateful that, unlike the birds, he can cook his 

 breakfast. 











Shore crab, Carcinus maenas, 

 the intermediate host of several worms infecting birds (x .5 



