Of the assumption of the breeding plumage by certain birds without a moult, or with only a moult of such feathers as have to be replaced by decorative plumes and crests, I have written recently in the 'Avicultural Magazine,' vol. viii. pp. 132–5; therefore, I need not enter into that matter again here. Those interested in the dispute as to the possibility of a change in the colour of feathers can easily refer to this paper, and to one which I published in 'The Ibis' for 1897.
It has always seemed to me a strange thing that ornithology, probably the most advanced study in the biological series, should in one respect be at fault, namely, in the reliance which the systematic student places upon the sexing of his specimens by collectors, and his objections to any other method but dissection for ascertaining the sex. Where would the aviculturist be, if (apart from colour differences) he were unable to be sure whether a bird was male or female?
In collections one frequently sees skins which have been incorrectly sexed by collectors—the young of Cyanospiza ciris, showing the commencement of the scarlet under surface of the cock bird on the flanks, yet labelled female; or, perchance, such easily sexed birds as the common Linnet, with no indication as to whether they are males or females. If one points out differences in form of beak, width of skull, length of wing, width of wing-markings, or other (apparently trifling, but actually trenchant) characters, one is met by the assertion that these are all variable, and therefore unreliable. I discovered a reason for the supposed unreliability of the sexual characters some years since when comparing my male, female, and young of Sialia sialis (the American Blue-bird) with the beautiful series in the British Museum collection. The males of Thrushes are rather longer and more slenderly built birds than the females; they have narrower skulls, and longer and more slender bills; but, when I compared my young cock Blue-bird (which had acquired its adult plumage) with its parents, I discovered that it was shorter than its mother, had quite as broad a skull, and a distinctly broader and shorter bill. I found that this young bird was uniform in every respect with all males of the same length in the Museum series, and that this apparently variable character was therefore due to the fact that Sialia acquires its adult colouring
