may not be overheard. Mr. Wallace remarks (49. Westminster Review,
July 1867, p. 5.) that it is only in the tropics, among forests which
never lose their foliage, that we find whole groups of birds, whose
chief colour is green. It will be admitted by every one, who has ever
tried, how difficult it is to distinguish parrots in a leaf-covered
tree. Nevertheless, we must remember that many parrots are ornamented
with crimson, blue, and orange tints, which can hardly be protective.
Woodpeckers are eminently arboreal, but besides green species, there
are many black, and black-and-white kinds all the species being
apparently exposed to nearly the same dangers. It is therefore probable
that with tree-haunting birds, strongly-pronounced colours have been
acquired through sexual selection, but that a green tint has been
acquired oftener than any other, from the additional advantage of
protection.
In regard to birds which live on the ground, every one admits that they
are coloured so as to imitate the surrounding surface. How difficult it
is to see a partridge, snipe, woodcock, certain plovers, larks, and
night-jars when crouched on ground. Animals inhabiting deserts offer
the most striking cases, for the bare surface affords no concealment,
and nearly all the smaller quadrupeds, reptiles, and birds depend for
safety on their colours. Mr. Tristram has remarked in regard to the
inhabitants of the Sahara, that all are protected by their isabelline
or sand-colour. (50. Ibis, 1859, vol. i. p. 429, et seq. Dr. Rohlfs,
however, remarks to me in a letter that according to his experience of
the Sahara, this statement is too strong.) Calling to my recollection
the desert-birds of South America, as well as most of the ground-birds
of Great Britain, it appeared to me that both sexes in such cases are
generally coloured nearly alike. Accordingly, I applied to Mr. Tristram
with respect to the birds of the Sahara, and he has kindly given me the
following information. There are twenty-six species belonging to
fifteen genera, which manifestly have their plumage coloured in a
protective manner; and this colouring is all the more striking, as with
most of these birds it differs from that of their congeners. Both sexes
of thirteen out of the twenty-six species are coloured in the same
manner; but these belong to genera in which this rule commonly
prevails, so that they tell us nothing about the protective colours
being the same in both sexes of desert-birds. Of the other thirteen
species, three belong to genera in which the sexes usually differ from
each other, yet here they have the sexes alike. In the remaining ten
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