BIRDS—continued.
Discussion as to why the males alone of some species, and both sexes of
others, are brightly coloured—On sexually-limited inheritance, as
applied to various structures and to brightly-coloured
plumage—Nidification in relation to colour—Loss of nuptial plumage
during the winter.
We have in this chapter to consider why the females of many birds have
not acquired the same ornaments as the male; and why, on the other
hand, both sexes of many other birds are equally, or almost equally,
ornamented? In the following chapter we shall consider the few cases in
which the female is more conspicuously coloured than the male.
In my ‘Origin of Species’ (1. Fourth edition, 1866, p. 241.) I briefly
suggested that the long tail of the peacock would be inconvenient and
the conspicuous black colour of the male capercailzie dangerous, to the
female during the period of incubation: and consequently that the
transmission of these characters from the male to the female offspring
had been checked through natural selection. I still think that this may
have occurred in some few instances: but after mature reflection on all
the facts which I have been able to collect, I am now inclined to
believe that when the sexes differ, the successive variations have
generally been from the first limited in their transmission to the same
sex in which they first arose. Since my remarks appeared, the subject
of sexual coloration has been discussed in some very interesting papers
by Mr. Wallace (2. ‘Westminster Review,’ July 1867. ‘Journal of
Travel,’ vol. i. 1868, p. 73.), who believes that in almost all cases
the successive variations tended at first to be transmitted equally to
both sexes; but that the female was saved, through natural selection,
from acquiring the conspicuous colours of the male, owing to the danger
which she would thus have incurred during incubation.
This view necessitates a tedious discussion on a difficult point,
namely, whether the transmission of a character, which is at first
inherited by both sexes can be subsequently limited in its transmission
to one sex alone by means of natural selection. We must bear in mind,
as shewn in the preliminary chapter on sexual selection, that
characters which are limited in their development to one sex are always
latent in the other. An imaginary illustration will best aid us in
seeing the difficulty of the case; we may suppose that a fancier wished
to make a breed of pigeons, in which the males alone should be coloured
of a pale blue, whilst the females retained their former slaty tint. As
with pigeons characters of all kinds are usually transmitted to both
sexes equally, the fancier would have to try to convert this latter
form of inheritance into sexually-limited transmission. All that he
could do would be to persevere in selecting every male pigeon which was
in the least degree of a paler blue; and the natural result of this
process, if steadily carried on for a long time, and if the pale
variations were strongly inherited or often recurred, would be to make
his whole stock of a lighter blue. But our fancier would be compelled
to match, generation after generation, his pale blue males with slaty
females, for he wishes to keep the latter of this colour. The result
would generally be the production either of a mongrel piebald lot, or
more probably the speedy and complete loss of the pale-blue tint; for
the primordial slaty colour would be transmitted with prepotent force.
Supposing, however, that some pale-blue males and slaty females were
produced during each successive generation, and were always crossed
together, then the slaty females would have, if I may use the
expression, much blue blood in their veins, for their fathers,
grandfathers, etc., will all have been blue birds. Under these
circumstances it is conceivable (though I know of no distinct facts
rendering it probable) that the slaty females might acquire so strong a
latent tendency to pale-blueness, that they would not destroy this
colour in their male offspring, their female offspring still inheriting
the slaty tint. If so, the desired end of making a breed with the two
sexes permanently different in colour might be gained.
The extreme importance, or rather necessity in the above case of the
desired character, namely, pale-blueness, being present though in a
latent state in the female, so that the male offspring should not be
deteriorated, will be best appreciated as follows: the male of
Soemmerring’s pheasant has a tail thirty-seven inches in length, whilst
that of the female is only eight inches; the tail of the male common
pheasant is about twenty inches, and that of the female twelve inches
long. Now if the female Soemmerring pheasant with her SHORT tail were
crossed with the male common pheasant, there can be no doubt that the
male hybrid offspring would have a much LONGER tail than that of the
pure offspring of the common pheasant. On the other hand, if the female
common pheasant, with a tail much longer than that of the female
Soemmerring pheasant, were crossed with the male of the latter, the
male hybrid offspring would have a much SHORTER tail than that of the
pure offspring of Soemmerring’s pheasant. (3. Temminck says that the
tail of the female Phasianus Soemmerringii is only six inches long,
‘Planches coloriees,’ vol. v. 1838, pp. 487 and 488: the measurements
above given were made for me by Mr. Sclater. For the common pheasant,
see Macgillivray, ‘History of British Birds,’ vol. i. pp. 118-121.)
Our fancier, in order to make his new breed with the males of a
pale-blue tint, and the females unchanged, would have to continue
selecting the males during many generations; and each stage of paleness
would have to be fixed in the males, and rendered latent in the
females. The task would be an extremely difficult one, and has never
been tried, but might possibly be successfully carried out. The chief
obstacle would be the early and complete loss of the pale-blue tint,
from the necessity of reiterated crosses with the slaty female, the
latter not having at first any LATENT tendency to produce pale-blue
offspring.
On the other hand, if one or two males were to vary ever so slightly in
paleness, and the variations were from the first limited in their
transmission to the male sex, the task of making a new breed of the
desired kind would be easy, for such males would simply have to be
selected and matched with ordinary females. An analogous case has
actually occurred, for there are breeds of the pigeon in Belgium (4.
Dr. Chapuis, ‘Le Pigeon Voyageur Belge,’ 1865, p. 87.) in which the
males alone are marked with black striae. So again Mr. Tegetmeier has
recently shewn (5. The ‘Field,’ Sept. 1872.) that dragons not rarely
produce silver-coloured birds, which are almost always hens; and he
himself has bred ten such females. It is on the other hand a very
unusual event when a silver male is produced; so that nothing would be
easier, if desired, than to make a breed of dragons with blue males and
silver females. This tendency is indeed so strong that when Mr.
Tegetmeier at last got a silver male and matched him with one of the
silver females, he expected to get a breed with both sexes thus
coloured; he was however disappointed, for the young male reverted to
the blue colour of his grandfather, the young female alone being
silver. No doubt with patience this tendency to reversion in the males,
reared from an occasional silver male matched with a silver hen, might
be eliminated, and then both sexes would be coloured alike; and this
very process has been followed with success by Mr. Esquilant in the
case of silver turbits.
With fowls, variations of colour, limited in their transmission to the
male sex, habitually occur. When this form of inheritance prevails, it
might well happen that some of the successive variations would be
transferred to the female, who would then slightly resemble the male,
as actually occurs in some breeds. Or again, the greater number, but
not all, of the successive steps might be transferred to both sexes,
and the female would then closely resemble the male. There can hardly
be a doubt that this is the cause of the male pouter pigeon having a
somewhat larger crop, and of the male carrier pigeon having somewhat
larger wattles, than their respective females; for fanciers have not
selected one sex more than the other, and have had no wish that these
characters should be more strongly displayed in the male than in the
female, yet this is the case with both breeds.
The same process would have to be followed, and the same difficulties
encountered, if it were desired to make a breed with the females alone
of some new colour.
Lastly, our fancier might wish to make a breed with the two sexes
differing from each other, and both from the parent species. Here the
difficulty would be extreme, unless the successive variations were from
the first sexually limited on both sides, and then there would be no
difficulty. We see this with the fowl; thus the two sexes of the
pencilled Hamburghs differ greatly from each other, and from the two
sexes of the aboriginal Gallus bankiva; and both are now kept constant
to their standard of excellence by continued selection, which would be
impossible unless the distinctive characters of both were limited in
their transmission.
The Spanish fowl offers a more curious case; the male has an immense
comb, but some of the successive variations, by the accumulation of
which it was acquired, appear to have been transferred to the female;
for she has a comb many times larger than that of the females of the
parent species. But the comb of the female differs in one respect from
that of the male, for it is apt to lop over; and within a recent period
it has been ordered by the fancy that this should always be the case,
and success has quickly followed the order. Now the lopping of the comb
must be sexually limited in its transmission, otherwise it would
prevent the comb of the male from being perfectly upright, which would
be abhorrent to every fancier. On the other hand, the uprightness of
the comb in the male must likewise be a sexually-limited character,
otherwise it would prevent the comb of the female from lopping over.
From the foregoing illustrations, we see that even with almost
unlimited time at command, it would be an extremely difficult and
complex, perhaps an impossible process, to change one form of
transmission into the other through selection. Therefore, without
distinct evidence in each case, I am unwilling to admit that this has
been effected in natural species. On the other hand, by means of
successive variations, which were from the first
sexually limited in their transmission, there would not be the least
difficulty in rendering a male bird widely different in colour or in
any other character from the female; the latter being left unaltered,
or slightly altered, or specially modified for the sake of protection.
As bright colours are of service to the males in their rivalry with
other males, such colours would be selected whether or not they were
transmitted exclusively to the same sex. Consequently the females might
be expected often to partake of the brightness of the males to a
greater or less degree; and this occurs with a host of species. If all
the successive variations were transmitted equally to both sexes, the
females would be indistinguishable from the males; and this likewise
occurs with many birds. If, however, dull colours were of high
importance for the safety of the female during incubation, as with many
ground birds, the females which varied in brightness, or which received
through inheritance from the males any marked accession of brightness,
would sooner or later be destroyed. But the tendency in the males to
continue for an indefinite period transmitting to their female
offspring their own brightness, would have to be eliminated by a change
in the form of inheritance; and this, as shewn by our previous
illustration, would be extremely difficult. The more probable result of
the long-continued destruction of the more brightly-coloured females,
supposing the equal form of transmission to prevail, would be the
lessening or annihilation of the bright colours of the males, owing to
their continual crossing with the duller females. It would be tedious
to follow out all the other possible results; but I may remind the
reader that if sexually-limited variations in brightness occurred in
the females, even if they were not in the least injurious to them and
consequently were not eliminated, yet they would not be favoured or
selected, for the male usually accepts any female, and does not select
the more attractive individuals; consequently these variations would be
liable to be lost, and would have little influence on the character of
the race; and this will aid in accounting for the females being
commonly duller-coloured than the males.
In the eighth chapter instances were given, to which many might here be
added, of variations occurring at various ages, and inherited at the
corresponding age. It was also shewn that variations which occur late
in life are commonly transmitted to the same sex in which they first
appear; whilst variations occurring early in life are apt to be
transmitted to both sexes; not that all the cases of sexually-limited
transmission can thus be accounted for. It was further shewn that if a
male bird varied by becoming brighter whilst young, such variations
would be of no service until the age for reproduction had arrived, and
there was competition between rival males. But in the case of birds
living on the ground and commonly in need of the protection of dull
colours, bright tints would be far more dangerous to the young and
inexperienced than to the adult males. Consequently the males which
varied in brightness whilst young would suffer much destruction and be
eliminated through natural selection; on the other hand, the males
which varied in this manner when nearly mature, notwithstanding that
they were exposed to some additional danger, might survive, and from
being favoured through sexual selection, would procreate their kind. As
a relation often exists between the period of variation and the form of
transmission, if the bright-coloured young males were destroyed and the
mature ones were successful in their courtship, the males alone would
acquire brilliant colours and would transmit them exclusively to their
male offspring. But I by no means wish to maintain that the influence
of age on the form of transmission, is the sole cause of the great
difference in brilliancy between the sexes of many birds.
When the sexes of birds differ in colour, it is interesting to
determine whether the males alone have been modified by sexual
selection, the females having been left unchanged, or only partially
and indirectly thus changed; or whether the females have been specially
modified through natural selection for the sake of protection. I will
therefore discuss this question at some length, even more fully than
its intrinsic importance deserves; for various curious collateral
points may thus be conveniently considered.
Before we enter on the subject of colour, more especially in reference
to Mr. Wallace’s conclusions, it may be useful to discuss some other
sexual differences under a similar point of view. A breed of fowls
formerly existed in Germany (6. Bechstein, ‘Naturgeschichte
Deutschlands,’ 1793, B. iii. 339.) in which the hens were furnished
with spurs; they were good layers, but they so greatly disturbed their
nests with their spurs that they could not be allowed to sit on their
own eggs. Hence at one time it appeared to me probable that with the
females of the wild Gallinaceae the development of spurs had been
checked through natural selection, from the injury thus caused to their
nests. This seemed all the more probable, as wing-spurs, which would
not be injurious during incubation, are often as well-developed in the
female as in the male; though in not a few cases they are rather larger
in the male. When the male is furnished with leg-spurs the female
almost always exhibits rudiments of them,—the rudiment sometimes
consisting of a mere scale, as in Gallus. Hence it might be argued that
the females had aboriginally been furnished with well-developed spurs,
but that these had subsequently been lost through disuse or natural
selection. But if this view be admitted, it would have to be extended
to innumerable other cases; and it implies that the female progenitors
of the existing spur-bearing species were once encumbered with an
injurious appendage.
In some few genera and species, as in Galloperdix, Acomus, and the
Javan peacock (Pavo muticus), the females, as well as the males,
possess well-developed leg-spurs. Are we to infer from this fact that
they construct a different sort of nest from that made by their nearest
allies, and not liable to be injured by their spurs; so that the spurs
have not been removed? Or are we to suppose that the females of these
several species especially require spurs for their defence? It is a
more probable conclusion that both the presence and absence of spurs in
the females result from different laws of inheritance having prevailed,
independently of natural selection. With the many females in which
spurs appear as rudiments, we may conclude that some few of the
successive variations, through which they were developed in the males,
occurred very early in life, and were consequently transferred to the
females. In the other and much rarer cases, in which the females
possess fully developed spurs, we may conclude that all the successive
variations were transferred to them; and that they gradually acquired
and inherited the habit of not disturbing their nests.
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. (7.
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.) 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.
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 (8.
Mr. Ramsay, in ‘Proc. Zoolog. Soc.’ 1868, p. 50.) 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.
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
coloration 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 (9. ‘Journal of
Travel,’ edited by A. Murray, vol. i. 1868, p. 78.), 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.
There is in the first place much truth in the Duke of Argyll’s remark
(10. ‘Journal of Travel,’ edited by A. Murray, vol. i. 1868, p. 281.)
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 (11. Audubon, ‘Ornithological
Biography,’ vol. i. p. 233.), 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.
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 (12. Jerdon, ‘Birds of India,’ vol. ii. p. 108.
Gould’s ‘Handbook of the Birds of Australia,’ vol. i. p. 463.); 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. (13. 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.)
In regard to birds which build in holes or construct domed nests, other
advantages, as Mr. Wallace remarks, besides concealment are gained,
such as shelter from the rain, greater warmth, and in hot countries
protection from the sun (14. Mr. Salvin noticed in Guatemala (‘Ibis,’
1864, p. 375) that humming-birds were much more unwilling to leave
their nests during very hot weather, when the sun was shining brightly,
as if their eggs would be thus injured, than during cool, cloudy, or
rainy weather.); so that it is no valid objection to his view that many
birds having both sexes obscurely coloured build concealed nests. (15.
I may specify, as instances of dull-coloured birds building concealed
nests, the species belonging to eight Australian genera described in
Gould’s ‘Handbook of the Birds of Australia,’ vol. i. pp. 340, 362,
365, 383, 387, 389, 391, 414.) The female Horn-bill (Buceros), for
instance, of India and Africa is protected during incubation with
extraordinary care, for she plasters up with her own excrement the
orifice of the hole in which she sits on her eggs, leaving only a small
orifice through which the male feeds her; she is thus kept a close
prisoner during the whole period of incubation (16. Mr. C. Horne,
‘Proc. Zoolog. Soc.’ 1869. p. 243.); yet female horn-bills are not more
conspicuously coloured than many other birds of equal size which build
open nests. It is a more serious objection to Mr. Wallace’s view, as is
admitted by him, that in some few groups the males are brilliantly
coloured and the females obscure, and yet the latter hatch their eggs
in domed nests. This is the case with the Grallinae of Australia, the
Superb Warblers (Maluridae) of the same country, the Sun-birds
(Nectariniae), and with several of the Australian Honey-suckers or
Meliphagidae. (17. On the nidification and colours of these latter
species, see Gould’s ‘Handbook to the Birds of Australia,’ vol. i. pp.
504, 527.)
If we look to the birds of England we shall see that there is no close
and general relation between the colours of the female and the nature
of the nest which is constructed. About forty of our British birds
(excluding those of large size which could defend themselves) build in
holes in banks, rocks, or trees, or construct domed nests. If we take
the colours of the female goldfinch, bullfinch, or blackbird, as a
standard of the degree of conspicuousness, which is not highly
dangerous to the sitting female, then out of the above forty birds the
females of only twelve can be considered as conspicuous to a dangerous
degree, the remaining twenty-eight being inconspicuous. (18. I have
consulted, on this subject, Macgillivray’s ‘British Birds,’ and though
doubts may be entertained in some cases in regard to the degree of
concealment of the nest, and to the degree of conspicuousness of the
female, yet the following birds, which all lay their eggs in holes or
in domed nests, can hardly be considered, by the above standard, as
conspicuous: Passer, 2 species; Sturnus, of which the female is
considerably less brilliant than the male; Cinclus; Motallica boarula
(?); Erithacus (?); Fruticola, 2 sp.; Saxicola; Ruticilla, 2 sp.;
Sylvia, 3 sp.; Parus, 3 sp.; Mecistura; Anorthura; Certhia; Sitta;
Yunx; Muscicapa, 2 sp.; Hirundo, 3 sp.; and Cypselus. The females of
the following 12 birds may be considered as conspicuous according to
the same standard, viz., Pastor, Motacilla alba, Parus major and P.
caeruleus, Upupa, Picus, 4 sp., Coracias, Alcedo, and Merops.) Nor is
there any close relation within the same genus between a
well-pronounced difference in colour between the sexes, and the nature
of the nest constructed. Thus the male house sparrow (Passer
domesticus) differs much from the female, the male tree-sparrow (P.
montanus) hardly at all, and yet both build well-concealed nests. The
two sexes of the common fly-catcher (Muscicapa grisola) can hardly be
distinguished, whilst the sexes of the pied fly-catcher (M. luctuosa)
differ considerably, and both species build in holes or conceal their
nests. The female blackbird (Turdus merula) differs much, the female
ring-ouzel (T. torquatus) differs less, and the female common thrush
(T. musicus) hardly at all from their respective males; yet all build
open nests. On the other hand, the not very distantly-allied
water-ouzel (Cinclus aquaticus) builds a domed nest, and the sexes
differ about as much as in the ring-ouzel. The black and red grouse
(Tetrao tetrix and T. scoticus) build open nests in equally
well-concealed spots, but in the one species the sexes differ greatly,
and in the other very little.
Notwithstanding the foregoing objections, I cannot doubt, after reading
Mr. Wallace’s excellent essay, that looking to the birds of the world,
a large majority of the species in which the females are conspicuously
coloured (and in this case the males with rare exceptions are equally
conspicuous), build concealed nests for the sake of protection. Mr.
Wallace enumerates (19. ‘Journal of Travel,’ edited by A. Murray, vol.
i. p. 78.) a long series of groups in which this rule holds good; but
it will suffice here to give, as instances, the more familiar groups of
kingfishers, toucans, trogons, puff-birds (Capitonidae),
plantain-eaters (Musophagae, woodpeckers, and parrots. Mr. Wallace
believes that in these groups, as the males gradually acquired through
sexual selection their brilliant colours, these were transferred to the
females and were not eliminated by natural selection, owing to the
protection which they already enjoyed from their manner of
nidification. According to this view, their present manner of nesting
was acquired before their present colours. But it seems to me much more
probable that in most cases, as the females were gradually rendered
more and more brilliant from partaking of the colours of the male, they
were gradually led to change their instincts (supposing that they
originally built open nests), and to seek protection by building domed
or concealed nests. No one who studies, for instance, Audubon’s account
of the differences in the nests of the same species in the Northern and
Southern United States (20. See many statements in the ‘Ornithological
Biography.’ See also some curious observations on the nests of Italian
birds by Eugenio Bettoni, in the ‘Atti della Società Italiana,’ vol.
xi. 1869, p. 487.), will feel any great difficulty in admitting that
birds, either by a change (in the strict sense of the word) of their
habits, or through the natural selection of so-called spontaneous
variations of instinct, might readily be led to modify their manner of
nesting.
This way of viewing the relation, as far as it holds good, between the
bright colours of female birds and their manner of nesting, receives
some support from certain cases occurring in the Sahara Desert. Here,
as in most other deserts, various birds, and many other animals, have
had their colours adapted in a wonderful manner to the tints of the
surrounding surface. Nevertheless there are, as I am informed by the
Rev. Mr. Tristram, some curious exceptions to the rule; thus the male
of the Monticola cyanea is conspicuous from his bright blue colour, and
the female almost equally conspicuous from her mottled brown and white
plumage; both sexes of two species of Dromolaea are of a lustrous
black; so that these three species are far from receiving protection
from their colours, yet they are able to survive, for they have
acquired the habit of taking refuge from danger in holes or crevices in
the rocks.
With respect to the above groups in which the females are conspicuously
coloured and build concealed nests, it is not necessary to suppose that
each separate species had its nidifying instinct specially modified;
but only that the early progenitors of each group were gradually led to
build domed or concealed nests, and afterwards transmitted this
instinct, together with their bright colours, to their modified
descendants. As far as it can be trusted, the conclusion is
interesting, that sexual selection together with equal or nearly equal
inheritance by both sexes, have indirectly determined the manner of
nidification of whole groups of birds.
According to Mr. Wallace, even in the groups in which the females, from
being protected in domed nests during incubation, have not had their
bright colours eliminated through natural selection, the males often
differ in a slight, and occasionally in a considerable degree from the
females. This is a significant fact, for such differences in colour
must be accounted for by some of the variations in the males having
been from the first limited in transmission to the same sex; as it can
hardly be maintained that these differences, especially when very
slight, serve as a protection to the female. Thus all the species in
the splendid group of the Trogons build in holes; and Mr. Gould gives
figures (21. See his Monograph of the Trogonidae, 1st edition.) of both
sexes of twenty-five species, in all of which, with one partial
exception, the sexes differ sometimes slightly, sometimes
conspicuously, in colour,—the males being always finer than the
females, though the latter are likewise beautiful. All the species of
kingfishers build in holes, and with most of the species the sexes are
equally brilliant, and thus far Mr. Wallace’s rule holds good; but in
some of the Australian species the colours of the females are rather
less vivid than those of the male; and in one splendidly-coloured
species, the sexes differ so much that they were at first thought to be
specifically distinct. (22. Namely, Cyanalcyon, Gould’s ‘Handbook to
the Birds of Australia,’ vol. i. p. 133; see, also, pp. 130, 136.) Mr.
R.B. Sharpe, who has especially studied this group, has shewn me some
American species (Ceryle) in which the breast of the male is belted
with black. Again, in Carcineutes, the difference between the sexes is
conspicuous: in the male the upper surface is dull-blue banded with
black, the lower surface being partly fawn-coloured, and there is much
red about the head; in the female the upper surface is reddish-brown
banded with black, and the lower surface white with black markings. It
is an interesting fact, as shewing how the same peculiar style of
sexual colouring often characterises allied forms, that in three
species of Dacelo the male differs from the female only in the tail
being dull-blue banded with black, whilst that of the female is brown
with blackish bars; so that here the tail differs in colour in the two
sexes in exactly the same manner as the whole upper surface in the two
sexes of Carcineutes.
With parrots, which likewise build in holes, we find analogous cases:
in most of the species, both sexes are brilliantly coloured and
indistinguishable, but in not a few species the males are coloured
rather more vividly than the females, or even very differently from
them. Thus, besides other strongly-marked differences, the whole under
surface of the male King Lory (Aprosmictus scapulatus) is scarlet,
whilst the throat and chest of the female is green tinged with red: in
the Euphema splendida there is a similar difference, the face and wing
coverts moreover of the female being of a paler blue than in the male.
(23. Every gradation of difference between the sexes may be followed in
the parrots of Australia. See Gould’s ‘Handbook,’ etc., vol. ii. pp.
14-102.) In the family of the tits (Parinae), which build concealed
nests, the female of our common blue tomtit (Parus caeruleus), is “much
less brightly coloured” than the male: and in the magnificent Sultan
yellow tit of India the difference is greater. (24. Macgillivray’s
‘British Birds,’ vol. ii. p. 433. Jerdon, ‘Birds of India,’ vol. ii. p.
282.)
Again, in the great group of the woodpeckers (25. All the following
facts are taken from M. Malherbe’s magnificent ‘Monographie des
Picidees,’ 1861.), the sexes are generally nearly alike, but in the
Megapicus validus all those parts of the head, neck, and breast, which
are crimson in the male are pale brown in the female. As in several
woodpeckers the head of the male is bright crimson, whilst that of the
female is plain, it occurred to me that this colour might possibly make
the female dangerously conspicuous, whenever she put her head out of
the hole containing her nest, and consequently that this colour, in
accordance with Mr. Wallace’s belief, had been eliminated. This view is
strengthened by what Malherbe states with respect to Indopicus
carlotta; namely, that the young females, like the young males, have
some crimson about their heads, but that this colour disappears in the
adult female, whilst it is intensified in the adult male. Nevertheless
the following considerations render this view extremely doubtful: the
male takes a fair share in incubation (26. Audubon’s ‘Ornithological
Biography,’ vol. ii. p. 75; see also the ‘Ibis,’ vol. i. p. 268.), and
would be thus almost equally exposed to danger; both sexes of many
species have their heads of an equally bright crimson; in other species
the difference between the sexes in the amount of scarlet is so slight
that it can hardly make any appreciable difference in the danger
incurred; and lastly, the colouring of the head in the two sexes often
differs slightly in other ways.
The cases, as yet given, of slight and graduated differences in colour
between the males and females in the groups, in which as a general rule
the sexes resemble each other, all relate to species which build domed
or concealed nests. But similar gradations may likewise be observed in
groups in which the sexes as a general rule resemble each other, but
which build open nests.
As I have before instanced the Australian parrots, so I may here
instance, without giving any details, the Australian pigeons. (27.
Gould’s ‘Handbook to the Birds of Australia,’ vol. ii. pp. 109-149.) It
deserves especial notice that in all these cases the slight differences
in plumage between the sexes are of the same general nature as the
occasionally greater differences. A good illustration of this fact has
already been afforded by those kingfishers in which either the tail
alone or the whole upper surface of the plumage differs in the same
manner in the two sexes. Similar cases may be observed with parrots and
pigeons. The differences in colour between the sexes of the same
species are, also, of the same general nature as the differences in
colour between the distinct species of the same group. For when in a
group in which the sexes are usually alike, the male differs
considerably from the female, he is not coloured in a quite new style.
Hence we may infer that within the same group the special colours of
both sexes when they are alike, and the colours of the male, when he
differs slightly or even considerably from the female, have been in
most cases determined by the same general cause; this being sexual
selection.
It is not probable, as has already been remarked, that differences in
colour between the sexes, when very slight, can be of service to the
female as a protection. Assuming, however, that they are of service,
they might be thought to be cases of transition; but we have no reason
to believe that many species at any one time are undergoing change.
Therefore we can hardly admit that the numerous females which differ
very slightly in colour from their males are now all commencing to
become obscure for the sake of protection. Even if we consider somewhat
more marked sexual differences, is it probable, for instance, that the
head of the female chaffinch,—the crimson on the breast of the female
bullfinch,—the green of the female greenfinch,—the crest of the female
golden-crested wren, have all been rendered less bright by the slow
process of selection for the sake of protection? I cannot think so; and
still less with the slight differences between the sexes of those birds
which build concealed nests. On the other hand, the differences in
colour between the sexes, whether great or small, may to a large extent
be explained on the principle of the successive variations, acquired by
the males through sexual selection, having been from the first more or
less limited in their transmission to the females. That the degree of
limitation should differ in different species of the same group will
not surprise any one who has studied the laws of inheritance, for they
are so complex that they appear to us in our ignorance to be capricious
in their action. (28. See remarks to this effect in ‘Variation of
Animals and Plants under Domestication,’ vol. ii. chap. xii.)
As far as I can discover there are few large groups of birds in which
all the species have both sexes alike and brilliantly coloured, but I
hear from Mr. Sclater, that this appears to be the case with the
Musophagae or plantain-eaters. Nor do I believe that any large group
exists in which the sexes of all the species are widely dissimilar in
colour: Mr. Wallace informs me that the chatterers of S. America
(Cotingidae) offer one of the best instances; but with some of the
species, in which the male has a splendid red breast, the female
exhibits some red on her breast; and the females of other species shew
traces of the green and other colours of the males. Nevertheless we
have a near approach to close sexual similarity or dissimilarity
throughout several groups: and this, from what has just been said of
the fluctuating nature of inheritance, is a somewhat surprising
circumstance. But that the same laws should largely prevail with allied
animals is not surprising. The domestic fowl has produced a great
number of breeds and sub-breeds, and in these the sexes generally
differ in plumage; so that it has been noticed as an unusual
circumstance when in certain sub-breeds they resemble each other. On
the other hand, the domestic pigeon has likewise produced a vast number
of distinct breeds and sub-breeds, and in these, with rare exceptions,
the two sexes are identically alike.
Therefore if other species of Gallus and Columba were domesticated and
varied, it would not be rash to predict that similar rules of sexual
similarity and dissimilarity, depending on the form of transmission,
would hold good in both cases. In like manner the same form of
transmission has generally prevailed under nature throughout the same
groups, although marked exceptions to this rule occur. Thus within the
same family or even genus, the sexes may be identically alike, or very
different in colour. Instances have already been given in the same
genus, as with sparrows, fly-catchers, thrushes and grouse. In the
family of pheasants the sexes of almost all the species are wonderfully
dissimilar, but are quite alike in the eared pheasant or Crossoptilon
auritum. In two species of Chloephaga, a genus of geese, the male
cannot be distinguished from the females, except by size; whilst in two
others, the sexes are so unlike that they might easily be mistaken for
distinct species. (29. The ‘Ibis,’ vol. vi. 1864, p. 122.)
The laws of inheritance can alone account for the following cases, in
which the female acquires, late in life, certain characters proper to
the male, and ultimately comes to resemble him more or less completely.
Here protection can hardly have come into play. Mr. Blyth informs me
that the females of Oriolus melanocephalus and of some allied species,
when sufficiently mature to breed, differ considerably in plumage from
the adult males; but after the second or third moults they differ only
in their beaks having a slight greenish tinge. In the dwarf bitterns
(Ardetta), according to the same authority, “the male acquires his
final livery at the first moult, the female not before the third or
fourth moult; in the meanwhile she presents an intermediate garb, which
is ultimately exchanged for the same livery as that of the male.” So
again the female Falco peregrinus acquires her blue plumage more slowly
than the male. Mr. Swinhoe states that with one of the Drongo shrikes
(Dicrurus macrocercus) the male, whilst almost a nestling, moults his
soft brown plumage and becomes of a uniform glossy greenish-black; but
the female retains for a long time the white striae and spots on the
axillary feathers; and does not completely assume the uniform black
colour of the male for three years. The same excellent observer remarks
that in the spring of the second year the female spoon-bill (Platalea)
of China resembles the male of the first year, and that apparently it
is not until the third spring that she acquires the same adult plumage
as that possessed by the male at a much earlier age. The female
Bombycilla carolinensis differs very little from the male, but the
appendages, which like beads of red sealing-wax ornament the
wing-feathers (30. When the male courts the female, these ornaments are
vibrated, and “are shewn off to great advantage,” on the outstretched
wings: A. Leith Adams, ‘Field and Forest Rambles,’ 1873, p. 153.), are
not developed in her so early in life as in the male. In the male of an
Indian parrakeet (Palaeornis javanicus) the upper mandible is coral-red
from his earliest youth, but in the female, as Mr. Blyth has observed
with caged and wild birds, it is at first black and does not become red
until the bird is at least a year old, at which age the sexes resemble
each other in all respects. Both sexes of the wild turkey are
ultimately furnished with a tuft of bristles on the breast, but in
two-year-old birds the tuft is about four inches long in the male and
hardly apparent in the female; when, however, the latter has reached
her fourth year, it is from four to five inches in length. (31. On
Ardetta, Translation of Cuvier’s ‘Regne Animal,’ by Mr. Blyth,
footnote, p. 159. On the Peregrine Falcon, Mr. Blyth, in Charlesworth’s
‘Mag. of Nat. Hist.’ vol. i. 1837, p. 304. On Dicrurus, ‘Ibis,’ 1863,
p. 44. On the Platalea, ‘Ibis,’ vol. vi. 1864, p. 366. On the
Bombycilla, Audubon’s ‘Ornitholog. Biography,’ vol. i. p. 229. On the
Palaeornis, see, also, Jerdon, ‘Birds of India,’ vol. i. p. 263. On the
wild turkey, Audubon, ibid. vol. i. p. 15; but I hear from Judge Caton
that in Illinois the female very rarely acquires a tuft. Analogous
cases with the females of Petrocossyphus are given by Mr. R. Sharpe,
‘Proceedings of the Zoological Society,’ 1872, p. 496.)
These cases must not be confounded with those where diseased or old
females abnormally assume masculine characters, nor with those where
fertile females, whilst young, acquire the characters of the male,
through variation or some unknown cause. (32. Of these latter cases Mr.
Blyth has recorded (Translation of Cuvier’s ‘Regne Animal,’ p. 158)
various instances with Lanius, Ruticilla, Linaria, and Anas. Audubon
has also recorded a similar case (‘Ornitholog. Biography,’ vol. v. p.
519) with Pyranga aestiva.) But all these cases have so much in common
that they depend, according to the hypothesis of pangenesis, on
gemmules derived from each part of the male being present, though
latent, in the female; their development following on some slight
change in the elective affinities of her constituent tissues.
A few words must be added on changes of plumage in relation to the
season of the year. From reasons formerly assigned there can be little
doubt that the elegant plumes, long pendant feathers, crests, etc., of
egrets, herons, and many other birds, which are developed and retained
only during the summer, serve for ornamental and nuptial purposes,
though common to both sexes. The female is thus rendered more
conspicuous during the period of incubation than during the winter; but
such birds as herons and egrets would be able to defend themselves. As,
however, plumes would probably be inconvenient and certainly of no use
during the winter, it is possible that the habit of moulting twice in
the year may have been gradually acquired through natural selection for
the sake of casting off inconvenient ornaments during the winter. But
this view cannot be extended to the many waders, whose summer and
winter plumages differ very little in colour. With defenceless species,
in which both sexes, or the males alone, become extremely conspicuous
during the breeding-season,—or when the males acquire at this season
such long wing or tail-feathers as to impede their flight, as with
Cosmetornis and Vidua,—it certainly at first appears highly probable
that the second moult has been gained for the special purpose of
throwing off these ornaments. We must, however, remember that many
birds, such as some of the Birds of Paradise, the Argus pheasant and
peacock, do not cast their plumes during the winter; and it can hardly
be maintained that the constitution of these birds, at least of the
Gallinaceae, renders a double moult impossible, for the ptarmigan
moults thrice in the year. (33. See Gould’s ‘Birds of Great Britain.’)
Hence it must be considered as doubtful whether the many species which
moult their ornamental plumes or lose their bright colours during the
winter, have acquired this habit on account of the inconvenience or
danger which they would otherwise have suffered.
I conclude, therefore, that the habit of moulting twice in the year was
in most or all cases first acquired for some distinct purpose, perhaps
for gaining a warmer winter covering; and that variations in the
plumage occurring during the summer were accumulated through sexual
selection, and transmitted to the offspring at the same season of the
year; that such variations were inherited either by both sexes or by
the males alone, according to the form of inheritance which prevailed.
This appears more probable than that the species in all cases
originally tended to retain their ornamental plumage during the winter,
but were saved from this through natural selection, resulting from the
inconvenience or danger thus caused.
I have endeavoured in this chapter to shew that the arguments are not
trustworthy in favour of the view that weapons, bright colours, and
various ornaments, are now confined to the males owing to the
conversion, by natural selection, of the equal transmission of
characters to both sexes, into transmission to the male sex alone. It
is also doubtful whether the colours of many female birds are due to
the preservation, for the sake of protection, of variations which were
from the first limited in their transmission to the female sex. But it
will be convenient to defer any further discussion on this subject
until I treat, in the following chapter, of the differences in plumage
between the young and old.