Dear Sir

I must apologize for my delay in replying to your enquiries, caused by my absence from home; as your question required the examination of my collections & notes. I have now great pleasure in replying to your kind letter.¹

My views generally were expressed in Ibis. 1859. Vol. 1— pp. 429. seqq written before the publication of your work on the ‘Origin of Species’.²

I find that 26 species of Saharan birds are clothed in a protective colouring, which differs from the ordinary colouring of their respective congeners, & closely resembles the colour of the surrounding desert—a buff fawn colour. 13 of these belong to genera where the sexes usually resemble each other in plumage, which similarity is continued in the desert forms.

Of the 13 others, where the sexes ordinarily differ in plumage, 10 desert species shew a like distinction, but generally speaking the distinction is confined to the under surface of the bird, as the breast or belly, while the back & head have the same sand coloured hue.

Thus the black in the ♂ desert sparrow, Corospiza simplex (which is a true Passer) & in the Pterocles is confined to the throat or belly, & is not visible when the bird crouches for concealment.³

In 3 species only of the 13 are the sexes similar, though ordinarily differently coloured in the same genus—

I should add that in a group of desert birds which depend for their safety not on escaping observation, but on refuge in holes or crevices of rocks, the plumage is remarkably bright & conspicuous. Thus the bright blue of Monticola cyanea, & the lustrous black of Dromolæa leucopygia & Dro. leucocephala have been noticed by me—⁴

In all these cases the young plumage follows the ♀.

I enclose a list of the desert birds of North Africa marked by protective plumage—& remain | Dear Sir | Yrs very faithfully & obliged | H. B. Tristram