^62 



SCIENCE-GOSSIP. 



ON COLOURING OF BIRDS' EGGS. 



By J. A. Wheldon. 



T AM glad to find my notes on this subject (ante 

 p. 200) have drawn such an interesting and 

 courteous reply from Mr. Hughes [ante p. 241), for 

 which I thank him. It is only by fully discussing 

 such difficulties as are presented that the truth can 

 be arrived at. 



In some respects the drift of my remarks has not 

 been fully understood. I certainly did not intend to 

 express, nor do I think ray remarks should be con- 

 strued as expressing, any doubt as to the results of 

 Mr. Hughes's interesting experiments on the chemical 

 composition of the various pigments. These decidedly 

 confirm, or afford additional proof if such is required, 

 that iron is an undoubted constituent of the egg- 

 colouring matters. This I am prepared to fully 

 admit. Indeed it was proved long ago, by the 

 researches of Mr. H. C. Sorby, who, chiefly by 

 means of spectroscopic analysis, distinguished no less 

 than seven different egg pigments, all nearly allied 

 to the colouring matter of the blood and bile ; indeed 

 two of the most important, which he termed oorhodein 

 and oocyan, closely approach haemoglobin and bile 

 pigment in composition. Now haemoglobin is 

 derived from the red corpuscles of the blood, and is 

 stated to contain 0-4 per cent, of iron, as well as a 

 little sulphur, which is another of the elements de- 

 tected by Mr. Hughes. 



The chemical composition of these organic colouring 

 matters is so complex, however, that the slightest 

 rearrangement of their elements causes an alteration 

 • of tint, which might easily be produced by physio- 

 logical action, such as some selective power of the 

 pigment glands acting on the haemoglobin of the 

 blood. I have seen it suggested somewhere that the 

 effect of the environment upon the mental or nervous 

 constitution of the mother bird during the laying 

 period has probably some influence on the pigmenta- 

 ,tion of the shell. 



The real point at issue between Mr. Hughes and 

 myself is whether the nature of the food bears any 

 relation to or produces any effect on the colouring of 

 the eggs. I was first led to doubt this from the 

 apparently insurmountable difficulty of arranging 

 birds into dietary classes, which would afford parallel 

 groups of eggs separable on account of colour. Nor 

 is this taxonomic difficulty the only one, for after 

 considering the chemical and physiological aspects of 

 the question I feel more strongly convinced than 

 before that we must look in other directions for an 

 explanation of this mystery, which appears to evade 

 solution as pertinaciously as does that affecting the 

 remarkable sculpturing of the eggs of insects and the 

 integuments of seeds and pollen grains. 



To arrive at an idea of how slight an influence 



food is likely to have on the colouring matter of 

 egg-shells, we must consider how the pigment supply 

 is obtained. As is the case with other secretions, I 

 presume it to be extracted by special glands from 

 the blood supply. All birds must obtain iron and 

 sulphur, no matter what the colour of their eggs may 

 be, in order to maintain the supply of red blood 

 corpuscles. It does not appear to me necessary that 

 these mineral essentials should be always entirely 

 drawn from the food supply, for the grit used by 

 many birds and the drinking water frequently con- 

 tain them. The vital point seems to be this, that no 

 matter how varied the raw material which enters the 

 stomach, the result is invariably the production of 

 blood of fairly uniform composition. I cannot con- 

 ceive that the particular diet from which the blood 

 is derived can have any influence on the subsequent 

 action of special glands on that fluid, except in ex- 

 tremely abnormal conditions produced by starvation, 

 poison, &c. , which do not enter into the argument. 

 Given plenty of food of any of the classes named by 

 Mr. Hughes, fish, seed, or insect, and the bird- 

 machine will evolve so much blood of probably 

 identical composition, from which each species will 

 extract a different shell-colouring peculiar to its kind. 

 I think it will be found the exception, rather than 

 the rule, that colours in the animal world are much 

 influenced by the chemical composition of the food. 

 I exclude, of course, diseased conditions produced 

 by poisons, under which I would class the action of 

 hemp seed on the plumage of bullfinches, and of 

 cayenne upon canaries. As a solitary example of 

 my meaning, it could not be established that the pig- 

 ment of the skin and^hair of a negro differed from 

 that of a European because of the nature of their 

 diet ; and the same would apply to the various shades 

 of skin, hair, and iris noticeable in many races of 

 man and animals. 



I frankly admit that I cannot explain why various 

 birds nesting in similar positions should have different 

 coloured eggs, nor can I call to mind any serious 

 attempt to account for thfs anomaly. It is one of the 

 afore-mentioned weak points in the argument for 

 protective coloration. I do think, notwithstanding 

 Mr. ?Iughes's doubts, that a spotted egg has some 

 claims to be considered more protectively coloured 

 than a pure white one amongst the flickering lights 

 and shades of a hedgerow or the foliage of a tree ; 

 the rusty spotted robin's egg might be readily over- 

 looked amongst the russet leaves it delights in strew- 

 ing about its nest. Spots and streaks on animals, 

 moreover, have been long looked upon as protective ; 

 and in my own experience mottled caterpillars and 

 pale green ones are often equally difficult of discovery 



