172 



SCIENCE-GOSSIP. 



ON COLOURING OF BIRDS' liGGS. 



By Reginald J- Hughes. 



'T~ HE cause of the various colours of birds' eggs has 

 not yet been satisfactorily explained ; in fact, 

 the only attempt to do so, deserving serious notice, is 

 that which considers their tints due to protective 

 colouration. This idea, however, hardly bears 

 examination. What is there protective, for instance, 

 in the colouring of the bright blue eggs of the hedge 

 sparrow or the thrush ? Even the white eggs spotted 

 with red, of which there are so many, such as those 

 of the roljin, can scarcely be said to be difficult to see 

 in the nest. Again, the eggs of sea-birds do not 

 always harmonise with the colour of the cliff where 

 they are placed. It is, indeed, much more important 

 that the colour of the hen bird should be protective 

 than that of the eggs which she usually covers during 

 incubation. A good example of protective colouring 

 of the bird is seen in the case of one of the few British 

 birds with green plumage, viz., the golden-crested 

 wren. This bird usually lives in Scotch fir and spruce 

 fir trees, where, as the leaves do not fall off in winter, 

 a green colouring is more useful than the brown of 

 most English birds that live in deciduous trees. Yet, 

 its eggs are not at all inconspicuous, being white, 

 spotted red. Of course I do not deny that there are 

 some cases of protective colouring: for instance, the 

 cuckoo, and some birds that lay ground-coloured eggs 

 in an open nest on the ground. 



In the present article I intend to consider the 

 colouring on birds' eggs as a convenient way of 

 getting rid of some superfluous elements in their 

 blood, and I think I shall be able to show a connec- 

 tion between the colours of birds and their food taken 

 during winter and early spring. We will generally 

 take British birds as examples, because I, and pro- 

 bably most of your readers, have a better practical 

 acquaintance with them, than with those birds not 

 occurring as breeders in these islands. 



I shall, for my purpose, divide our birds into five 

 classes, according to their food, viz. : — 



1. Birds whose food largely consists of insects that 

 feed on vegetable matter. 



2. Those birds feeding chiefly on fruit, berries and 

 seeds. 



3. Sea-birds subsisting on fishes and other marine 

 animals. 



4. Birds living on miscellaneous food, such as 

 freshwater fish, carrion and offal, amphibians, etc. 



5. Birds of prey. 



I propose to show that each of the above classes 

 produces eggs of a characteristic colour. As examples 

 of the first class, I will enumerate the robin, black- 

 cap, golden-crested wren, willow wren, grasshopper 

 warbler, common wren, tree-creeper, the familiar 

 I'aridae, Motacillidae, and Anthidae. It will be 

 ound that the eggs of all these have a white, or in 

 some cases reddish, ground colour, spotted or 



blotched with red or brown. Now I am of the: 

 opinion that carbonate of iron is the principal cause 

 of this red-brown colour. This forms the pigment in 

 the feathers and on the eggs of the buff breeds of" 

 domestic poultry, which lay brown eggs. The 

 administration of small doses of carl^onate of iron has- 

 been found to darken both eggs and plumage of fowls. 

 In order to test the truth of this suggestion, I 

 extracted the pigment from some eggs of this class by 

 rubbing them in warm water with a piece of white 

 satin. This is the best material for the purpose,, 

 because it is the easiest substance from which to 

 squeeze the colour. On evaporating the water I 

 obtained a small quantity of colouring matter, 

 tolerably free from the lime of the shell. When dried 

 and heated, it burnt with the usual flame of carbon,, 

 although it often disappeared without any flame. The 

 residue seemed to be composed of lime, soot, and. 

 sonie small reddish specks that were attracted by a 

 magnet. I have never found this colouring matter to 

 be capable of attraction before being burnt. Now 

 this is exactly how desiccated carbonate of iron acts- 

 when burnt. Before combustion it cannot be affected 

 by a magnet, but afterwards is easily attracted. 

 Vegetable^feeding insects are obviously a likely 

 source from w hence birds might derive carbon. 



My Class 2, birds feeding largely on seeds and 

 berries during winter, contains the song thrush, field- 

 fare, blackbird, linnet, brarabling, hawfinch, green- 

 finch, bullfinch, and many others that have the 

 ground colour of the eggs varying from green to blue, 

 generally blue-green. I consider the pigment to be- 

 probably some kind of hydro-carbon. The fact that 

 these eggs lose their colour if left long in a strong; 

 light, and also if heated, agrees with this conclusion. 

 The colour is verv much akin to that which causes 

 the green colour of caterpillars, and the necessary 

 elements are the most common constituents of fruit 

 and seeds. The colour is very difficult to extract 

 from eggs of this class, the shell invariably breaking, 

 during the process. It may be argued that fruit is as. 

 likely a source of carbonate of iron as insects. To 

 this I agree, and that is the reason why most of the 

 eggs of birds in this class are spotted red or brown. 

 It cannot at present be explained why some should' 

 be spotted and others not, such as the eggs of the 

 hedge sparrow, which are spotless. Possibly the 

 colouring matter is sometimes all taken up by the 

 plumage, where it is more useful for protective pur- 

 poses, when used in that manner. Some of the sea- 

 birds are good examples of this point, as will be seen 

 later. It might be thought domestic fowls' eggs, 

 belong to this class, as fowls feed largely on seeds, but 

 probably the green colour is used up by the feathers. 

 Many birds with the bluest eggs eat a considerable 

 number of worms, snails and slugs, which food may 



