XII 
THE COLOURS OF BIRDS 
259 
Eagle as an example 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 
