THE EFFECT OF PARASITES ON THE HOST 37 



breeding season. One species of bird is generally parasitised in a 

 particular district and the cuckoo's attentions can result, over a period 

 of years, in a serious reduction in the numbers of the host. Some very 

 careful observations have been made on the reed-warbler in a circum- 

 scribed area in Germany. The first year in which counts were made 

 fourteen nests were present of which four contained cuckoos' eggs. Six 

 years later, in the same area, only eight were found and of these no less 

 than seven contained cuckoos' eggs. It seems quite possible that a 

 favoured host can be exterminated in certain districts by over-parasiti- 

 sation by the cuckoo. 



It cannot be too strongly emphasised that the effect of all types of 

 parasites on the host is detrimental. If we find that a bird seems little, 

 if at all, inconvenienced by the presence of Protozoa or worms or lice, 

 or a cuckoo in the nest, we can nevertheless assume that it would be 

 better off without them. There are, for example, certain worms which 

 live in the oviduct of birds and are known to inhibit egg-laying. There 

 are others which are likewise located in the oviduct, yet apparently 

 produce no symptoms of any kind. Over a long period, however, they 

 may well reduce the total numbers of eggs laid. Small effects such as 

 lack of vitality, loss of voice, excessive blinking, or perverted habits 

 like dirt eating are extremely difficult to gauge. Nevertheless, it is 

 only a question of degree. Potentially all parasites are harmful. 







Mussel, Mytilus eduLis, with a pearl 



