Dear M^r. Darwin,

With respect to your first question, asking for more illustrations of the fact of one sex only differing in certain kindred races (or species) of birds, I cannot do better than refer you to the paper indicated in my last; for I remember that there are one or two cases mentioned there that I do not think that I have since mentioned: e.g. Tephrodornis and Thamnobia.¹

Next, about cuckoos. It is the barred upper rather than the “under surfaces” which you must mean. Well, then, there is the hepaticus plumage of C. canorus, rare in this species and probably confined to the female sex; it being a repetition of the barred nestling dress.² In C. striatus (apud Schlegel, vide “Ibis,” 1866, p. 359), this hepatic plumage is more common, and still more so in C. poliocephalus (which is figured in this dress erroneously as C. himalayanus in Gould’s “Century of Himalayan Birds”).³

In C. Sonneratii this nestling-like hepatic colouring is permanent, there being no ashy phase as in the others.⁴ Thirdly, the first plumage of the crossbills is striated, very like that of a redpole, or of a hen siskin,—and the same remark applies to the young goldfinch and greenfinch, which species are streakless when adult. Does not Yarrell figure the young of the crossbill in his ‘British Birds’?⁵

The turacos constitute the exclusively African family Musophagidæ, comprising Musophaga, Turacus, (see Corythaix), Schizorhis, and (in my opinion, but as a distinct subfamily,) Colius. Musophaga and Turacus are united by Schlegel, and these are the birds to which I especially referred as “turacos”.⁶ However long ago I ventured upon the remark that the sexes are alike in these three families (Musophagidæ, Capitonidæ, and Rhamphastidæ, the two latter being very nearly allied), it still holds to the best of my information, unless there may be an exception or two in the S. American genus Capito, about which consult Sclater.⁷ In fact you had better ask him in the following words to prevent any confusion

—“Do you know of any Musophagidæ, Rhamphastidæ, or Capitonidæ, (inclusive of Mægalæmidæ), in which the sexes differ in plumage”?⁸ The name Bucco formerly included the Capitonidæ, but is now transferred to the very distinct S. American group formerly styled Tamatia with its kindred genera—⁹

Yours very truly, | E Blyth