SCIENCE GOSSIP. 



267 



not be disputed that most nocturnal birds have dark 

 plumage ; those comparatively light in colour are 

 species upon which the process of evolution has noc 

 yet conferred the power of forming a large amount 

 of pigment, but in the meantime what can be pro- 

 duced is not partly wasted upon the eggs. The 

 nightjar, having crepuscular habits, is neither strictly 

 nocturnal nor very deeply coloured ; and in its case 

 pigment is especially useful on the egg, as this bird 

 deposits them on the ground. It should be noticed 

 that the nightjar's egg is not white, spotted brown, 

 but practically completely covered with pigment. I 

 think this egg is a good example to show that the 

 white eggs with brown or red spots are not pro- 

 tectively coloured. Place a robin's and a nightjar's 

 egg on a brown background, which stands for the 

 robin's nest or the ground ; the robin's egg, although 

 smaller, can be seen much further off. The 

 Egyptian nightjar, which lays its eggs on the sand, 

 has a lighter and yellower tint. 



There are eighteen species of British birds that hunt 

 their prey at night. Of these, fourteen lay white eggs ; 

 the four exceptions being the nightjar and the three 

 petrels I mentioned previously, which lay eggs with 

 minute spots, probably different in nature from the 

 pigment of the plumage. The nightjar is the only 

 one of the eighteen species depositing its eggs on the 

 open surface of the ground. Petrels and shearwaters 

 lay their eggs, differently from most sea-birds, at the 

 end of burrows. 



Before proceeding I must state the result of another 

 experiment — viz. that blue or green eggs are bleached 

 by chlorine gas, if care is taken that the shell be kept 

 moist whilst exposed to its action. This certainly 

 points to the colour being caused by a hydrocarbon- 

 green. Sea-birds' eggs cannot be bleached in this 

 manner. 



Coming now to the fourth paragraph in Mr. 

 Wheldon's article, I admit it is difficult to fully 

 account for the difference between the eggs of the 

 hedge-sparrow and the robin. Yet the former does 

 eat some seeds ; quite enough to produce the small 

 quantity of pigment required. Its near ally, the 

 Alpine accentor, eats still more, owing to the scarcity 

 of insect life during a large portion of the year on the 

 mountains it inhabits. If the hedge-sparrow of the 

 plains has descended from its mountain equivalent, 

 as is not improbable, it might retain the power of 

 laying blue eggs. The young of the hedge-sparrow 

 are browner and more spotted than the adults, thus 

 approaching the coloration of the Alpine accentor. 

 I do not see anything against my theory in the case 

 of the tree-pipit, &c, next cited. They are all 

 insectivorous birds, and what colour exists on their 

 eggs is brown or brownish, and could be easily pro- 

 duced by carbonate of iron ; also the wagtails have 

 darker markings than the tree-pipit or grasshopper 

 warbler, whilst the black redstart, laying a white 

 egg, has the most intensely coloured plumage 

 of all. 



Regarding the swallow tribe, although the colours 



of the eggs vary, none have any trace of blue or 

 green, the various species simply having different 

 powers of depositing the same pigment. Many fowls 

 use all the pigment in their plumage ; when any is 

 deposited on the eggs it is carbonate of iron, derived 

 from either the seeds or insects which they eat. 

 Some varieties have the power of depositing the pig- 

 ment ; others cannot. But the darkest breeds of 

 domestic fowls lay white eggs, and the relatively pale 

 — Cochins and Brahmas, for instance — buff ones. 

 Pheasants have a mixed diet, including both seeds 

 and insects, so it is natural their eggs should vary 

 somewhat ; but as they eat chiefly insects, the pre- 

 vailing colour is brown. I believe the colours of 

 canaries and parrots are caused by hydrocarbons, and 

 are protective in the varied hues of their native 

 forests. Many parrots are green, the colour of 

 the wild canary being yellow-green. The marked 

 intensification when canaries are fed with cayenne 

 pepper also points to the pigment being in this case 

 a hydrocarbon. The pigment which would other- 

 wise be deposited on the egg is thus more advan- 

 tageously used up by the plumage. The goldfinch, 

 and I might add the siskin, are similar cases. The 

 former is very partial to evergreen shrubs, and the 

 latter to fir-trees. Doves and pigeons, especially 

 the original wild types, have also much green and 

 blue about their plumage. 



I agree that the gannets are exceptional in so far 

 as they have very little power of depositing pigment 

 either on eggs or plumage ; but such exceptions are 

 not contrary to my theory. The egg of a gannet is 

 not really white ; it has a blue undershell. The eggs 

 of several sea-birds, which are apparently white, have 

 a blue or green undershell or shell membrane. The 

 differences in the colour of guillemots' eggs are caused 

 by individual variations in the power of depositing 

 pigment. Of course all the food consumed by the 

 bird capable of conversion into colouring matter 

 need not be deposited. 



I now come to a rather important point. On sea- 

 birds' eggs the precipitate formed by an excess of 

 acid is blood-red instead of brown. To prove re- 

 statement, compare that produced on a grouse's egg 

 and on a common gull's. I have found that all sea- 

 birds' eggs give off an unmistakable smell of sulphur- 

 etted hydrogen when heated to produce Prussian 

 blue, as previously described, and all the observed 

 facts are most completely explained if sulphide of iron 

 exists in the pigment. For let us suppose we have 

 produced the green colour with a little acid and 

 some ferricyanide. On adding more acid the 

 chlorine would decompose the sulphide to form a 

 chloride, and the liberated sulphur would displace 

 the iron of the ferricyanide to form sulphocyanide of 

 potassium. We thus get a mixture of the latter salt 

 with a persalt of iron, and the mixture of these two 

 salts produces, as is well known, a blood-red colour. 

 When heated, the free hydrogen present that has 

 been liberated from the acid combines with the 

 sulphur and escapes as sulphuretted hydrogen, and 



K 4 



