Colour in birds' eggs: the collections of the Natural History Museum, Tring

The paper by Lack (1958) has long been used as justification that egg colour has no taxonomic significance, and is probably purely adaptive and related to nest site. While there is no doubt that many eggs are cryptically coloured, crypsis cannot be the only factor involved, or one would expect much more uniformity of colour in eggs of birds which nest in similar situations. Lack was not an oologist, and in fact his conclusions reveal a lack of understanding of the way in which patterning in eggs (which is more important than actual colour) works. Most unfortunately, he also made a considerable number of errors, which raise doubts as to the validity of his conclusions. There are many such errors in his discussion of subspecific variations (Lack, 1958, 148) though it is unclear whether he actually looked at the eggs in question or relied entirely on the statements of Baker (1933). In fact significant subspecific variation in egg colour is extremely rare, and usually suggests that the forms involved may be specifically distinct Lack admitted this in one instance, that of Brachypteryx montana cruralis. He also claimed that eggs of Zoothera dauma imbricata are paler than those of other races. This is not sustainable. There are so few eggs of this form in collections (not all of which are pale) that it is impossible to say if this difference is consistent, and in any case eggs as pale as those of imbricata sometimes occur in other races. Lack also claimed that eggs of Z. mollissima simlaensis are greeny blue while those of the nominate are creamy in ground colour. In fact there are very few authentic eggs of mollissima, and the only known eggs of simlaensis (now merged in whiteheadi) do not differ from those of the nominate. The eggs that Lack believed to belong to this race, are now considered to be almost certainly eggs of Zoothera dixoni. Saxicola ferrea ferrea has blue eggs thickly specked with brown so as to obscure the ground. S.f. haringtoni differs in having almost immaculate eggs. However, Lack claimed that eggs of the latter are sometimes found which are “well speckled”. No such eggs occur in The Natural History Museum collection, the only one Lack claimed to have examined. A few eggs of haringtoni have very faint speckling, but never to the degree found in the nominate. Lack then pointed out that Brachypteryx montana cruralis has white eggs, whereas B.m. sinensis has brown eggs, “like various other species of Brachypteryx”. In fact, sinensis has cinnamon pink eggs, very different from the brown eggs of other species. Lack commented that Turdus t. torquatus and T.t. alpestris differ in nesting habits. It is hard to understand why he mentioned this, as the eggs of the two forms do not differ. These examples are cited to show that Lack’s claims cannot be taken as authoritative, and are in fact, flawed. Lack’s very crude attempt to categorise egg colour reveals his lack of understanding of the subject. His categories are: immaculate white, immaculate blue, speckled whitish, speckled blue, blotched whitish, blotched blue, obscured buff or pink, and dark brown. Some of these are simply not categories at all, others embrace a wide variety of different patterns of egg marking. Two basic colours occur in birds’ eggs, blue and reddish/yellowish. Blue, when present, occurs throughout the shell-surface (there is no known instance of a white egg with blue spots). Reddish pigment occurs throughout the shell surface, and/or in the form of spots, speckles, blotches, scrawls or other marks. The combination of these two produces all the variables involved. It is actually more complicated than here stated, but a technical discussion is outside the