1586 The Zoologist— March, J 869. 



together, will occasionally show certain well-marked differences: when 

 1 first became aware of this fact 1 thought they might probably be 

 referable to distinct species ; but after examining every parasite 1 could 

 detect on certain birds, have come to the conclusion that these dif- 

 ferences are due to age and sex. I have no doubt that totally distinct 

 parasites may be found on some birds, nor do I indeed well see how 

 this can be otherwise, although I have not yet, except the " tick," de- 

 tected more than one kind of parasite on each species. In the eagles, 

 hawks, &c., preying on other birds, and in the crowded breeding 

 colonies of sea-fowl, also in many other ways, an interchange of para- 

 sites might be effected. In handling or skinning a bird infested with 

 these small creatures I have frequently found them running over my 

 hands and fingers. Birds, again, of different species, packed together 

 in the same box or han)per, might in this way alone produce two or 

 three distinct forms. 



The size of these minute insects is not by any means proportioned 

 to the relative size of the particidar species of bird they inhabit. We 

 might naturally suppose the larger the bird the larger the parasite : 

 the very reverse is often, however, the case. The parasite of the golden 

 eagle is remarkably small and much below the average; that of the 

 ring ouzel is very large, the head alone being equal in bulk to the 

 entire parasite of the eagle. The parasite of the merganser is about 

 equal in size to the golden eagle's, while that infesting the ice 

 duck is fully ten times its bulk. Tiie smallest I have yet examined 

 is that of the little auk — one-third less than one taken from a 

 bearded tit. 



Closely-allied species of birds have very often very opposite 

 parasites : those of the common and velvet scoter are totally distinct, 

 and in no one point resemble each other. There is, on the other side, 

 often a strong resemblance between these insects when the species 

 they infest are widely distinct. The parasites of the peregrine falcon 

 and reeve are strikingly alike. 



Bird parasites may be divided into two classes, the long-headed and 

 the broad-headed : there are many gradations between these two types. 

 In the long-headed the head resembles the spade on a playing-card, 

 but narrower; that of the broad-headed kind may be likened to that 

 of a true British bull-dog, and has a most determined and ferocious 

 expression : the mouth in these creatures is under the head, the flat 

 eyes on the upper surface, and placed on each side in a slight cavity : 

 these eyes are very conspicuous in the parasite of the green plover, 



