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Bullseye
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The vocal organs and the feathers variously modified for producing
sound, as well as the proper instincts for using them, often differ in
the two sexes, but are sometimes the same in both. Can such
differences be accounted for by the males having acquired these organs
and instincts, whilst the females have been saved from inheriting
them, on account of the danger to which they would have been exposed
by attracting the attention of birds or beasts of prey? This does
not seem to me probable, when we think of the multitude of birds which
with impunity gladden the country with their voices during the
spring.* It is a safer conclusion that, as vocal and instrumental
organs are of special service only to the males during their
courtship, these organs were developed through sexual selection and
their constant use in that sex alone- the successive variations and
the effects of use having been from the first more or less limited
in transmission to the male offspring.
* Daines Barrington, however, thought it probable (Philosophical
Transactions, 1773, p. 164) that few female birds sing, because the
talent would have been dangerous to them during incubation. He adds,
that a similar view may possibly account for the inferiority of the
female to the male in plumage.
Many analogous cases could be adduced; those for instance of the
plumes on the head being generally longer in the male than in the
female, sometimes of equal length in both sexes, and occasionally
absent in the female,- these several cases occurring in the same group
of birds. It would be difficult to account for such a difference
between the sexes by the female having been benefited by possessing
a slightly shorter crest than the male, and its consequent
diminution or complete suppression through natural selection. But I
will take a more favourable case, namely the length of the tail. The
long train of the peacock would have been not only inconvenient but
dangerous to the peahen during the period of incubation and whilst
accompanying her young. Hence there is not the least a priori
improbability in the development of her tail having been checked
through natural selection. But the females of various pheasants, which
apparently are exposed on their open nests to as much danger as the
peahen, have tails of considerable length. The females as well as
the males of the Menura superba have long tails, and they build a
domed nest, which is a great anomaly in so large a bird. Naturalists
have wondered how the female Menura could manage her tail during
incubation; but it is now known* that she "enters the nest head first,
and then turns round with her tail sometimes over her back, but more
often bent round by her side. Thus in time the tail becomes quite
askew, and is a tolerable guide to the length of time the bird has
been sitting." Both sexes of an Australian kingfisher (Tanysiptera
sylvia) have the middle tail-feathers greatly lengthened, and the
female makes her nest in a hole; and as I am informed by Mr. R. B.
Sharpe these feathers become much crumpled during incubation.
* Mr. Ramsay, in Proc. Zoolog. Soc., 1868, p. 50.
In these two latter cases the great length of the tail-feathers must
be in some degree inconvenient to the female; and as in both species
the tail-feathers of the female are somewhat shorter than those of the
male, it might be argued that their full development had been
prevented through natural selection. But if the development of the
tail of the peahen had been checked only when it became inconveniently
or dangerously great, she would have retained a much longer tail
than she actually possesses; for her tail is not nearly so long,
relatively to the size of her body, as that of many female
pheasants, nor longer than that of the female turkey. It must also
be borne in mind that, in accordance with this view, as soon as the
tail of the peahen became dangerously long, and its development was
consequently checked, she would have continually reacted on her male
progeny, and thus have prevented the peacock from acquiring his
present magnificent train. We may therefore infer that the length of
the tail in the peacock and its shortness in the peahen are the result
of the requisite variations in the male having been from the first
transmitted to the male offspring alone.
We are led to a nearly similar conclusion with respect to the length
of the tail in the various species of pheasants. In the Eared pheasant
(Crossoptilon auritum) the tail is of equal length in both sexes,
namely sixteen or seventeen inches; in the common pheasant it is about
twenty inches long in the male and twelve in the female; in
Soemmerring's pheasant, thirty-seven inches in the male and only eight
in the female; and lastly in Reeve's pheasant it is sometimes actually
seventy-two inches long in the male and sixteen in the female. Thus in
the several species, the tail of the female differs much in length,
irrespectively of that of the male; and this can be accounted for,
as it seems to me, with much more probability, by the laws of
inheritance,- that is by the successive variations having been from
the first more or less closely limited in their transmission to the
male sex than by the agency of natural selection, resulting from the
length of tail being more or less injurious to the females of these
several allied species.
We may now consider Mr. Wallace's arguments in regard to the
sexual colouration of birds. He believes that the bright tints
originally acquired through sexual selection by the males would in
all, or almost all cases, have been transmitted to the females, unless
the transference had been checked through natural selection. I may
here remind the reader that various facts opposed to this view have
already been given under reptiles, amphibians, fishes and lepidoptera.
Mr. Wallace rests his belief chiefly, but not exclusively, as we shall
see in the next chapter, on the following statement,* that when both
sexes are coloured in a very conspicuous manner, the nest is of such a
nature as to conceal the sitting bird; but when there is a marked
contrast of colour between the sexes, the male being gay and the
female dull-coloured, the nest is open and exposes the sitting bird to
view. This coincidence, as far as it goes, certainly seems to favour
the belief that the females which sit on open nests have been
specially modified for the sake of protection; but we shall
presently see that there is another and more probable explanation,
namely, that conspicuous females have acquired the instinct of
building domed nests oftener than dull-coloured birds. Mr. Wallace
admits that there are, as might have been expected, some exceptions to
his two rules, but it is a question whether the exceptions are not
so numerous as seriously to invalidate them.
* Journal of Travel, edited by A. Murray, vol. i., 1868, p. 78.
There is in the first place much truth in the Duke of Argyll's
remark* that a large domed nest is more conspicuous to an enemy,
especially to all tree-haunting carnivorous animals, than a smaller
open nest. Nor must we forget that with many birds which build open
nests, the male sits on the eggs and aids the female in feeding the
young: this is the case, for instance, with Pyranga aestiva,*(2) one
of the most splendid birds in the United States, the male being
vermilion, and the female light brownish-green. Now if brilliant
colours had been extremely dangerous to birds whilst sitting on
their open nests, the males in these cases would have suffered
greatly. It might, however, be of such paramount importance to the
male to be brilliantly coloured, in order to beat his rivals, that
this may have more than compensated some additional danger.
* Journal of Travel, edited by A. Murray, vol. i., 1868, p. 281.
*(2) Audubon, Ornithological Biography, vol. i., p. 233.
Mr. Wallace admits that with the king-crows (Dicrurus), orioles, and
Pittidae, the females are conspicuously coloured, yet build open
nests; but he urges that the birds of the first group are highly
pugnacious and could defend themselves; that those of the second group
take extreme care in concealing their open nests, but this does not
invariably hold good;* and that with the birds of the third group
the females are brightly coloured chiefly on the under surface.
Besides these cases, pigeons which are sometimes brightly, and
almost always conspicuously coloured, and which are notoriously liable
to the attacks of birds of prey, offer a serious exception to the
rule, for they almost always build open and exposed nests. In
another large family, that of the humming-birds, all the species build
open nests, yet with some of the most gorgeous species the sexes are
alike; and in the majority, the females, though less brilliant than
the males, are brightly coloured. Nor can it be maintained that all
female humming-birds, which are brightly coloured, escape detection by
their tints being green, for some display on their upper surfaces red,
blue, and other colours.*(2)
* Jerdon, Birds of India, vol. ii., p. 108. Gould's Handbook of
the Birds of Australia, vol. i., p. 463.
*(2) For instance, the female Eupetomena macroura has the head and
tail dark blue with reddish loins; the female Lampornis porphyrurus is
blackish-green on the upper surface, with the lores and sides of the
throat crimson; the female Eulampis jugularis has the top of the
head and back green, but the loins and the tail are crimson. Many
other instances of highly conspicuous females could be given. See
Mr. Gould's magnificent work on this family.
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