XII THE COLOURS OF BIRDS 259 



Eagle as an exampl^ of this. Wickmann further 

 explains the differences in the pigments of the eggs 

 of different birds as the result of differences in the 

 composition of the blood. It is well known that in 

 mammals the blood varies in different species, as is 

 shown by the differences in the shape of the crystals 

 of haemoglobin, the colour of the plasma, and so on ; 

 similar differences may express themselves in birds 

 as differences in the products of decomposition. 



For some criticisms of and additions to these 

 statements of Wickmann, reference may be made to 

 papers by Taschenberg and Von Nathusius. 



If the pigments of the shell are iron-free deriva- 

 tives of haemoglobin, then the question of the fate of 

 the iron' thus set free becomes interesting. Kruken- 

 berg is of opinion that it may be used to colour the 

 feathers in some cases ; he speaks of finding a large 

 amount of iron oxide in the feathers of the lammer- 

 geier, the feathers losing their dark brown colour 

 after the removal of the iron. 



Markings of Feathers 



We have already touched upon the interesting 

 questions connected with the markings of birds' 

 feathers, but a general survey of the colour phenomena 

 of birds would be incomplete without some further 

 reference to them. It is unfortunate that there is 

 so little certainty on the subject. 



First, as to the origin of markings, and the 

 simplest form of marking. On this point there are 

 many suggestions, unfortunately, however, in most 

 cases only suggestions. Hacker, in an interesting 



