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Chapter XVI
BIRDS--concluded.
The immature plumage in relation to the character of the plumage in both
sexes when adult--Six classes of cases--Sexual differences between the
males of closely-allied or representative species--The female assuming the
characters of the male--Plumage of the young in relation to the summer and
winter plumage of the adults--On the increase of beauty in the birds of the
world--Protective colouring--Conspicuously coloured birds--Novelty
appreciated--Summary of the four chapters on Birds.
We must now consider the transmission of characters, as limited by age, in
reference to sexual selection. The truth and importance of the principle
of inheritance at corresponding ages need not here be discussed, as enough
has already been said on the subject. Before giving the several rather
complex rules or classes of cases, under which the differences in plumage
between the young and the old, as far as known to me, may be included, it
will be well to make a few preliminary remarks.
With animals of all kinds when the adults differ in colour from the young,
and the colours of the latter are not, as far as we can see, of any special
service, they may generally be attributed, like various embryological
structures, to the retention of a former character. But this view can be
maintained with confidence, only when the young of several species resemble
each other closely, and likewise resemble other adult species belonging to
the same group; for the latter are the living proofs that such a state of
things was formerly possible. Young lions and pumas are marked with feeble
stripes or rows of spots, and as many allied species both young and old are
similarly marked, no believer in evolution will doubt that the progenitor
of the lion and puma was a striped animal, and that the young have retained
vestiges of the stripes, like the kittens of black cats, which are not in
the least striped when grown up. Many species of deer, which when mature
are not spotted, are whilst young covered with white spots, as are likewise
some few species in the adult state. So again the young in the whole
family of pigs (Suidae), and in certain rather distantly allied animals,
such as the tapir, are marked with dark longitudinal stripes; but here we
have a character apparently derived from an extinct progenitor, and now
preserved by the young alone. In all such cases the old have had their
colours changed in the course of time, whilst the young have remained but
little altered, and this has been effected through the principle of
inheritance at corresponding ages.
This same principle applies to many birds belonging to various groups, in
which the young closely resemble each other, and differ much from their
respective adult parents. The young of almost all the Gallinaceae, and of
some distantly allied birds such as ostriches, are covered with
longitudinally striped down; but this character points back to a state of
things so remote that it hardly concerns us. Young cross-bills (Loxia)
have at first straight beaks like those of other finches, and in their
immature striated plumage they resemble the mature red-pole and female
siskin, as well as the young of the goldfinch, greenfinch, and some other
allied species. The young of many kinds of buntings (Emberiza) resemble
one another, and likewise the adult state of the common bunting, E.
miliaria. In almost the whole large group of thrushes the young have their
breasts spotted--a character which is retained throughout life by many
species, but is quite lost by others, as by the Turdus migratorius. So
again with many thrushes, the feathers on the back are mottled before they
are moulted for the first time, and this character is retained for life by
certain eastern species. The young of many species of shrikes (Lanius), of
some woodpeckers, and of an Indian pigeon (Chalcophaps indicus), are
transversely striped on the under surface; and certain allied species or
whole genera are similarly marked when adult. In some closely-allied and
resplendent Indian cuckoos (Chrysococcyx), the mature species differ
considerably from one another in colour, but the young cannot be
distinguished. The young of an Indian goose (Sarkidiornis melanonotus)
closely resemble in plumage an allied genus, Dendrocygna, when mature. (1.
In regard to thrushes, shrikes, and woodpeckers, see Mr. Blyth, in
Charlesworth's 'Mag. of Nat. Hist.' vol. i. 1837, p. 304; also footnote to
his translation of Cuvier's 'Regne Animal,' p. 159. I give the case of
Loxia on Mr. Blyth's information. On thrushes, see also Audubon, 'Ornith.
Biog.' vol. ii. p. 195. On Chrysococcyx and Chalcophaps, Blyth, as quoted
in Jerdon's 'Birds of India,' vol. iii. p. 485. On Sarkidiornis, Blyth, in
'Ibis,' 1867, p. 175.) Similar facts will hereafter be given in regard to
certain herons. Young black-grouse (Tetrao tetrix) resemble the young as
well as the old of certain other species, for instance the red-grouse or T.
scoticus. Finally, as Mr. Blyth, who has attended closely to this subject,
has well remarked, the natural affinities of many species are best
exhibited in their immature plumage; and as the true affinities of all
organic beings depend on their descent from a common progenitor, this
remark strongly confirms the belief that the immature plumage approximately
shews us the former or ancestral condition of the species.
Although many young birds, belonging to various families, thus give us a
glimpse of the plumage of their remote progenitors, yet there are many
other birds, both dull-coloured and bright-coloured, in which the young
closely resemble their parents. In such cases the young of the different
species cannot resemble each other more closely than do the parents; nor
can they strikingly resemble allied forms when adult. They give us but
little insight into the plumage of their progenitors, excepting in so far
that, when the young and the old are coloured in the same general manner
throughout a whole group of species, it is probable that their progenitors
were similarly coloured.
We may now consider the classes of cases, under which the differences and
resemblances between the plumage of the young and the old, in both sexes or
in one sex alone, may be grouped. Rules of this kind were first enounced
by Cuvier; but with the progress of knowledge they require some
modification and amplification. This I have attempted to do, as far as the
extreme complexity of the subject permits, from information derived from
various sources; but a full essay on this subject by some competent
ornithologist is much needed. In order to ascertain to what extent each
rule prevails, I have tabulated the facts given in four great works,
namely, by Macgillivray on the birds of Britain, Audubon on those of North
America, Jerdon on those of India, and Gould on those of Australia. I may
here premise, first, that the several cases or rules graduate into each
other; and secondly, that when the young are said to resemble their
parents, it is not meant that they are identically alike, for their colours
are almost always less vivid, and the feathers are softer and often of a
different shape.
RULES OR CLASSES OF CASES.
I. When the adult male is more beautiful or conspicuous than the adult
female, the young of both sexes in their first plumage closely resemble the
adult female, as with the common fowl and peacock; or, as occasionally
occurs, they resemble her much more closely than they do the adult male.
II. When the adult female is more conspicuous than the adult male, as
sometimes though rarely occurs, the young of both sexes in their first
plumage resemble the adult male.
III. When the adult male resembles the adult female, the young of both
sexes have a peculiar first plumage of their own, as with the robin.
IV. When the adult male resembles the adult female, the young of both
sexes in their first plumage resemble the adults, as with the kingfisher,
many parrots, crows, hedge-warblers.
V. When the adults of both sexes have a distinct winter and summer
plumage, whether or not the male differs from the female, the young
resemble the adults of both sexes in their winter dress, or much more
rarely in their summer dress, or they resemble the females alone. Or the
young may have an intermediate character; or again they may differ greatly
from the adults in both their seasonal plumages.
VI. In some few cases the young in their first plumage differ from each
other according to sex; the young males resembling more or less closely the
adult males, and the young females more or less closely the adult females.
CLASS I.
In this class, the young of both sexes more or less closely resemble the
adult female, whilst the adult male differs from the adult female, often in
the most conspicuous manner. Innumerable instances in all Orders could be
given; it will suffice to call to mind the common pheasant, duck, and
house-sparrow. The cases under this class graduate into others. Thus the
two sexes when adult may differ so slightly, and the young so slightly from
the adults, that it is doubtful whether such cases ought to come under the
present, or under the third or fourth classes. So again the young of the
two sexes, instead of being quite alike, may differ in a slight degree from
each other, as in our sixth class. These transitional cases, however, are
few, or at least are not strongly pronounced, in comparison with those
which come strictly under the present class.
The force of the present law is well shewn in those groups, in which, as a
general rule, the two sexes and the young are all alike; for when in these
groups the male does differ from the female, as with certain parrots,
kingfishers, pigeons, etc., the young of both sexes resemble the adult
female. (2. See, for instance, Mr. Gould's account ('Handbook to the
Birds of Australia,' vol. i. p. 133) of Cyanalcyon (one of the
Kingfishers), in which, however, the young male, though resembling the
adult female, is less brilliantly coloured. In some species of Dacelo the
males have blue tails, and the females brown ones; and Mr. R.B. Sharpe
informs me that the tail of the young male of D. gaudichaudi is at first
brown. Mr. Gould has described (ibid. vol. ii. pp. 14, 20, 37) the sexes
and the young of certain black Cockatoos and of the King Lory, with which
the same rule prevails. Also Jerdon ('Birds of India,' vol. i. p. 260) on
the Palaeornis rosa, in which the young are more like the female than the
male. See Audubon ('Ornithological Biography,' vol. ii. p. 475) on the two
sexes and the young of Columba passerina.) We see the same fact exhibited
still more clearly in certain anomalous cases; thus the male of Heliothrix
auriculata (one of the humming-birds) differs conspicuously from the female
in having a splendid gorget and fine ear-tufts, but the female is
remarkable from having a much longer tail than that of the male; now the
young of both sexes resemble (with the exception of the breast being
spotted with bronze) the adult female in all other respects, including the
length of her tail, so that the tail of the male actually becomes shorter
as he reaches maturity, which is a most unusual circumstance. (3. I owe
this information to Mr. Gould, who shewed me the specimens; see also his
'Introduction to the Trochilidae,' 1861, p. 120.) Again, the plumage of
the male goosander (Mergus merganser) is more conspicuously coloured than
that of the female, with the scapular and secondary wing-feathers much
longer; but differently from what occurs, as far as I know, in any other
bird, the crest of the adult male, though broader than that of the female,
is considerably shorter, being only a little above an inch in length; the
crest of the female being two and a half inches long. Now the young of
both sexes entirely resemble the adult female, so that their crests are
actually of greater length, though narrower, than in the adult male. (4.
Macgillivray, 'Hist. Brit. Birds,' vol. v. pp. 207-214.)
When the young and the females closely resemble each other and both differ
from the males, the most obvious conclusion is that the males alone have
been modified. Even in the anomalous cases of the Heliothrix and Mergus,
it is probable that originally both adult sexes were furnished--the one
species with a much elongated tail, and the other with a much elongated
crest--these characters having since been partially lost by the adult males
from some unexplained cause, and transmitted in their diminished state to
their male offspring alone, when arrived at the corresponding age of
maturity. The belief that in the present class the male alone has been
modified, as far as the differences between the male and the female
together with her young are concerned, is strongly supported by some
remarkable facts recorded by Mr. Blyth (5. See his admirable paper in the
'Journal of the Asiatic Soc. of Bengal,' vol. xix. 1850, p. 223; see also
Jerdon, 'Birds of India,' vol. i. introduction, p. xxix. In regard to
Tanysiptera, Prof. Schlegel told Mr. Blyth that he could distinguish
several distinct races, solely by comparing the adult males.), with respect
to closely-allied species which represent each other in distinct countries.
For with several of these representative species the adult males have
undergone a certain amount of change and can be distinguished; the females
and the young from the distinct countries being indistinguishable, and
therefore absolutely unchanged. This is the case with certain Indian chats
(Thamnobia), with certain honey-suckers (Nectarinia), shrikes
(Tephrodornis), certain kingfishers (Tanysiptera), Kalij pheasants
(Gallophasis), and tree-partridges (Arboricola).
In some analogous cases, namely with birds having a different summer and
winter plumage, but with the two sexes nearly alike, certain closely-allied
species can easily be distinguished in their summer or nuptial plumage, yet
are indistinguishable in their winter as well as in their immature plumage.
This is the case with some of the closely-allied Indian wagtails or
Motacillae. Mr. Swinhoe (6. See also Mr. Swinhoe, in 'Ibis,' July 1863,
p. 131; and a previous paper, with an extract from a note by Mr. Blyth, in
'Ibis,' January, 1861, p. 25.) informs me that three species of Ardeola, a
genus of herons, which represent one another on separate continents, are
"most strikingly different" when ornamented with their summer plumes, but
are hardly, if at all, distinguishable during the winter. The young also
of these three species in their immature plumage closely resemble the
adults in their winter dress. This case is all the more interesting,
because with two other species of Ardeola both sexes retain, during the
winter and summer, nearly the same plumage as that possessed by the three
first species during the winter and in their immature state; and this
plumage, which is common to several distinct species at different ages and
seasons, probably shews us how the progenitors of the genus were coloured.
In all these cases, the nuptial plumage which we may assume was originally
acquired by the adult males during the breeding-season, and transmitted to
the adults of both sexes at the corresponding season, has been modified,
whilst the winter and immature plumages have been left unchanged.
The question naturally arises, how is it that in these latter cases the
winter plumage of both sexes, and in the former cases the plumage of the
adult females, as well as the immature plumage of the young, have not been
at all affected? The species which represent each other in distinct
countries will almost always have been exposed to somewhat different
conditions, but we can hardly attribute to this action the modification of
the plumage in the males alone, seeing that the females and the young,
though similarly exposed, have not been affected. Hardly any fact shews us
more clearly how subordinate in importance is the direct action of the
conditions of life, in comparison with the accumulation through selection
of indefinite variations, than the surprising difference between the sexes
of many birds; for both will have consumed the same food, and have been
exposed to the same climate. Nevertheless we are not precluded from
believing that in the course of time new conditions may produce some direct
effect either on both sexes, or from their constitutional differences
chiefly on one sex. We see only that this is subordinate in importance to
the accumulated results of selection. Judging, however, from a wide-spread
analogy, when a species migrates into a new country (and this must precede
the formation of representative species), the changed conditions to which
they will almost always have been exposed will cause them to undergo a
certain amount of fluctuating variability. In this case sexual selection,
which depends on an element liable to change--the taste or admiration of
the female--will have had new shades of colour or other differences to act
on and accumulate; and as sexual selection is always at work, it would
(from what we know of the results on domestic animals of man's
unintentional selection), be surprising if animals inhabiting separate
districts, which can never cross and thus blend their newly-acquired
characters, were not, after a sufficient lapse of time, differently
modified. These remarks likewise apply to the nuptial or summer plumage,
whether confined to the males, or common to both sexes.
Although the females of the above closely-allied or representative species,
together with their young, differ hardly at all from one another, so that
the males alone can be distinguished, yet the females of most species
within the same genus obviously differ from each other. The differences,
however, are rarely as great as between the males. We see this clearly in
the whole family of the Gallinaceae: the females, for instance, of the
common and Japan pheasant, and especially of the gold and Amherst pheasant
--of the silver pheasant and the wild fowl--resemble one another very
closely in colour, whilst the males differ to an extraordinary degree. So
it is with the females of most of the Cotingidae, Fringillidae, and many
other families. There can indeed be no doubt that, as a general rule, the
females have been less modified than the males. Some few birds, however,
offer a singular and inexplicable exception; thus the females of Paradisea
apoda and P. papuana differ from each other more than do their respective
males (7. Wallace, 'The Malay Archipelago,' vol. ii. 1869, p. 394.); the
female of the latter species having the under surface pure white, whilst
the female P. apoda is deep brown beneath. So, again, as I hear from
Professor Newton, the males of two species of Oxynotus (shrikes), which
represent each other in the islands of Mauritius and Bourbon (8. These
species are described with coloured figures, by M. F. Pollen, in 'Ibis,'
1866, p. 275.), differ but little in colour, whilst the females differ
much. In the Bourbon species the female appears to have partially retained
an immature condition of plumage, for at first sight she "might be taken
for the young of the Mauritian species." These differences may be compared
with those inexplicable ones, which occur independently of man's selection
in certain sub-breeds of the game-fowl, in which the females are very
different, whilst the males can hardly be distinguished. (9. 'Variation
of Animals,' etc., vol. i. p. 251.)
As I account so largely by sexual selection for the differences between the
males of allied species, how can the differences between the females be
accounted for in all ordinary cases? We need not here consider the species
which belong to distinct genera; for with these, adaptation to different
habits of life, and other agencies, will have come into play. In regard to
the differences between the females within the same genus, it appears to me
almost certain, after looking through various large groups, that the chief
agent has been the greater or less transference to the female of the
characters acquired by the males through sexual selection. In the several
British finches, the two sexes differ either very slightly or considerably;
and if we compare the females of the greenfinch, chaffinch, goldfinch,
bullfinch, crossbill, sparrow, etc., we shall see that they differ from one
another chiefly in the points in which they partially resemble their
respective males; and the colours of the males may safely be attributed to
sexual selection. With many gallinaceous species the sexes differ to an
extreme degree, as with the peacock, pheasant, and fowl, whilst with other
species there has been a partial or even complete transference of character
from the male to the female. The females of the several species of
Polyplectron exhibit in a dim condition, and chiefly on the tail, the
splendid ocelli of their males. The female partridge differs from the male
only in the red mark on her breast being smaller; and the female wild
turkey only in her colours being much duller. In the guinea-fowl the two
sexes are indistinguishable. There is no improbability in the plain,
though peculiarly spotted plumage of this latter bird having been acquired
through sexual selection by the males, and then transmitted to both sexes;
for it is not essentially different from the much more beautifully spotted
plumage, characteristic of the males alone of the Tragopan pheasants.
It should be observed that, in some instances, the transference of
characters from the male to the female has been effected apparently at a
remote period, the male having subsequently undergone great changes,
without transferring to the female any of his later-gained characters. For
instance, the female and the young of the black-grouse (Tetrao tetrix)
resemble pretty closely both sexes and the young of the red-grouse (T.
scoticus); and we may consequently infer that the black-grouse is descended
from some ancient species, of which both sexes were coloured in nearly the
same manner as the red-grouse. As both sexes of this latter species are
more distinctly barred during the breeding-season than at any other time,
and as the male differs slightly from the female in his more strongly-
pronounced red and brown tints (10. Macgillivray, 'History of British
Birds,' vol. i. pp. 172-174.), we may conclude that his plumage has been
influenced by sexual selection, at least to a certain extent. If so, we
may further infer that nearly similar plumage of the female black-grouse
was similarly produced at some former period. But since this period the
male black-grouse has acquired his fine black plumage, with his forked and
outwardly-curled tail-feathers; but of these characters there has hardly
been any transference to the female, excepting that she shews in her tail a
trace of the curved fork.
We may therefore conclude that the females of distinct though allied
species have often had their plumage rendered more or less different by the
transference in various degrees of characters acquired by the males through
sexual selection, both during former and recent times. But it deserves
especial attention that brilliant colours have been transferred much more
rarely than other tints. For instance, the male of the red-throated blue-
breast (Cyanecula suecica) has a rich blue breast, including a sub-
triangular red mark; now marks of nearly the same shape have been
transferred to the female, but the central space is fulvous instead of red,
and is surrounded by mottled instead of blue feathers. The Gallinaceae
offer many analogous cases; for none of the species, such as partridges,
quails, guinea-fowls, etc., in which the colours of the plumage have been
largely transferred from the male to the female, are brilliantly coloured.
This is well exemplified with the pheasants, in which the male is generally
so much more brilliant than the female; but with the Eared and Cheer
pheasants (Crossoptilon auritum and Phasianus wallichii) the sexes closely
resemble each other and their colours are dull. We may go so far as to
believe that if any part of the plumage in the males of these two pheasants
had been brilliantly coloured, it would not have been transferred to the
females. These facts strongly support Mr. Wallace's view that with birds
which are exposed to much danger during incubation, the transference of
bright colours from the male to the female has been checked through natural
selection. We must not, however, forget that another explanation, before
given, is possible; namely, that the males which varied and became bright,
whilst they were young and inexperienced, would have been exposed to much
danger, and would generally have been destroyed; the older and more
cautious males, on the other hand, if they varied in a like manner, would
not only have been able to survive, but would have been favoured in their
rivalry with other males. Now variations occurring late in life tend to be
transmitted exclusively to the same sex, so that in this case extremely
bright tints would not have been transmitted to the females. On the other
hand, ornaments of a less conspicuous kind, such as those possessed by the
Eared and Cheer pheasants, would not have been dangerous, and if they
appeared during early youth, would generally have been transmitted to both
sexes.
In addition to the effects of the partial transference of characters from
the males to the females, some of the differences between the females of
closely allied species may be attributed to the direct or definite action
of the conditions of life. (11. See, on this subject, chap. xxiii. in the
'Variation of Animals and Plants under Domestication.') With the males,
any such action would generally have been masked by the brilliant colours
gained through sexual selection; but not so with the females. Each of the
endless diversities in plumage which we see in our domesticated birds is,
of course, the result of some definite cause; and under natural and more
uniform conditions, some one tint, assuming that it was in no way
injurious, would almost certainly sooner or later prevail. The free
intercrossing of the many individuals belonging to the same species would
ultimately tend to make any change of colour, thus induced, uniform in
character.
No one doubts that both sexes of many birds have had their colours adapted
for the sake of protection; and it is possible that the females alone of
some species may have been modified for this end. Although it would be a
difficult, perhaps an impossible process, as shewn in the last chapter, to
convert one form of transmission into another through selection, there
would not be the least difficulty in adapting the colours of the female,
independently of those of the male, to surrounding objects, through the
accumulation of variations which were from the first limited in their
transmission to the female sex. If the variations were not thus limited,
the bright tints of the male would be deteriorated or destroyed. Whether
the females alone of many species have been thus specially modified, is at
present very doubtful. I wish I could follow Mr. Wallace to the full
extent; for the admission would remove some difficulties. Any variations
which were of no service to the female as a protection would be at once
obliterated, instead of being lost simply by not being selected, or from
free intercrossing, or from being eliminated when transferred to the male
and in any way injurious to him. Thus the plumage of the female would be
kept constant in character. It would also be a relief if we could admit
that the obscure tints of both sexes of many birds had been acquired and
preserved for the sake of protection,--for example, of the hedge-warbler or
kitty-wren (Accentor modularis and Troglodytes vulgaris), with respect to
which we have no sufficient evidence of the action of sexual selection. We
ought, however, to be cautious in concluding that colours which appear to
us dull, are not attractive to the females of certain species; we should
bear in mind such cases as that of the common house-sparrow, in which the
male differs much from the female, but does not exhibit any bright tints.
No one probably will dispute that many gallinaceous birds which live on the
open ground, have acquired their present colours, at least in part, for the
sake of protection. We know how well they are thus concealed; we know that
ptarmigans, whilst changing from their winter to their summer plumage, both
of which are protective, suffer greatly from birds of prey. But can we
believe that the very slight differences in tints and markings between, for
instance, the female black-grouse and red-grouse serve as a protection?
Are partridges, as they are now coloured, better protected than if they had
resembled quails? Do the slight differences between the females of the
common pheasant, the Japan and gold pheasants, serve as a protection, or
might not their plumages have been interchanged with impunity? From what
Mr. Wallace has observed of the habits of certain gallinaceous birds in the
East, he thinks that such slight differences are beneficial. For myself, I
will only say that I am not convinced.
Formerly when I was inclined to lay much stress on protection as accounting
for the duller colours of female birds, it occurred to me that possibly
both sexes and the young might aboriginally have been equally bright
coloured; but that subsequently, the females from the danger incurred
during incubation, and the young from being inexperienced, had been
rendered dull as a protection. But this view is not supported by any
evidence, and is not probable; for we thus in imagination expose during
past times the females and the young to danger, from which it has
subsequently been necessary to shield their modified descendants. We have,
also, to reduce, through a gradual process of selection, the females and
the young to almost exactly the same tints and markings, and to transmit
them to the corresponding sex and period of life. On the supposition that
the females and the young have partaken during each stage of the process of
modification of a tendency to be as brightly coloured as the males, it is
also a somewhat strange fact that the females have never been rendered
dull-coloured without the young participating in the same change; for there
are no instances, as far as I can discover, of species with the females
dull and the young bright coloured. A partial exception, however, is
offered by the young of certain woodpeckers, for they have "the whole upper
part of the head tinged with red," which afterwards either decreases into a
mere circular red line in the adults of both sexes, or quite disappears in
the adult females. (12. Audubon, 'Ornith. Biography,' vol. i. p. 193.
Macgillivray, 'History of British Birds,' vol. iii. p. 85. See also the
case before given of Indopicus carlotta.)
Finally, with respect to our present class of cases, the most probable view
appears to be that successive variations in brightness or in other
ornamental characters, occurring in the males at a rather late period of
life have alone been preserved; and that most or all of these variations,
owing to the late period of life at which they appeared, have been from the
first transmitted only to the adult male offspring. Any variations in
brightness occurring in the females or in the young, would have been of no
service to them, and would not have been selected; and moreover, if
dangerous, would have been eliminated. Thus the females and the young will
either have been left unmodified, or (as is much more common) will have
been partially modified by receiving through transference from the males
some of his successive variations. Both sexes have perhaps been directly
acted on by the conditions of life to which they have long been exposed:
but the females from not being otherwise much modified, will best exhibit
any such effects. These changes and all others will have been kept uniform
by the free intercrossing of many individuals. In some cases, especially
with ground birds, the females and the young may possibly have been
modified, independently of the males, for the sake of protection, so as to
have acquired the same dull-coloured plumage.
CLASS II.
WHEN THE ADULT FEMALE IS MORE CONSPICUOUS THAN THE ADULT MALE, THE YOUNG OF
BOTH SEXES IN THEIR FIRST PLUMAGE RESEMBLE THE ADULT MALE.
This class is exactly the reverse of the last, for the females are here
brighter coloured or more conspicuous than the males; and the young, as far
as they are known, resemble the adult males instead of the adult females.
But the difference between the sexes is never nearly so great as with many
birds in the first class, and the cases are comparatively rare. Mr.
Wallace, who first called attention to the singular relation which exists
between the less bright colours of the males and their performing the
duties of incubation, lays great stress on this point (13. 'Westminster
Review,' July 1867, and A. Murray, 'Journal of Travel,' 1868, p. 83.), as a
crucial test that obscure colours have been acquired for the sake of
protection during the period of nesting. A different view seems to me more
probable. As the cases are curious and not numerous, I will briefly give
all that I have been able to find.
In one section of the genus Turnix, quail-like birds, the female is
invariably larger than the male (being nearly twice as large in one of the
Australian species), and this is an unusual circumstance with the
Gallinaceae. In most of the species the female is more distinctly coloured
and brighter than the male (14. For the Australian species, see Gould's
'Handbook,' etc., vol. ii. pp. 178, 180, 186, and 188. In the British
Museum specimens of the Australian Plain-wanderer (Pedionomus torquatus)
may be seen, shewing similar sexual differences.), but in some few species
the sexes are alike. In Turnix taigoor of India the male "wants the black
on the throat and neck, and the whole tone of the plumage is lighter and
less pronounced than that of the female." The female appears to be
noisier, and is certainly much more pugnacious than the male; so that the
females and not the males are often kept by the natives for fighting, like
game-cocks. As male birds are exposed by the English bird-catchers for a
decoy near a trap, in order to catch other males by exciting their rivalry,
so the females of this Turnix are employed in India. When thus exposed the
females soon begin their "loud purring call, which can be heard a long way
off, and any females within ear-shot run rapidly to the spot, and commence
fighting with the caged bird." In this way from twelve to twenty birds,
all breeding females, may be caught in the course of a single day. The
natives assert that the females after laying their eggs associate in
flocks, and leave the males to sit on them. There is no reason to doubt
the truth of this assertion, which is supported by some observations made
in China by Mr. Swinhoe. (15. Jerdon, 'Birds of India,' vol. iii. p. 596.
Mr. Swinhoe, in 'Ibis,' 1865, p. 542; 1866, pp. 131, 405.) Mr. Blyth
believes, that the young of both sexes resemble the adult male.
[Fig. 62. Rhynchaea capensis (from Brehm).]
The females of the three species of Painted Snipes (Rhynchaea, Fig. 62)
"are not only larger but much more richly coloured than the males." (16.
Jerdon, 'Birds of India,' vol. iii. p. 677.) With all other birds in which
the trachea differs in structure in the two sexes it is more developed and
complex in the male than in the female; but in the Rhynchaea australis it
is simple in the male, whilst in the female it makes four distinct
convolutions before entering the lungs. (17. Gould's 'Handbook to the
Birds of Australia,' vol. ii. p. 275.) The female therefore of this
species has acquired an eminently masculine character. Mr. Blyth
ascertained, by examining many specimens, that the trachea is not
convoluted in either sex of R. bengalensis, which species resembles R.
australis so closely, that it can hardly be distinguished except by its
shorter toes. This fact is another striking instance of the law that
secondary sexual characters are often widely different in closely-allied
forms, though it is a very rare circumstance when such differences relate
to the female sex. The young of both sexes of R. bengalensis in their
first plumage are said to resemble the mature male. (18. 'The Indian
Field,' Sept. 1858, p. 3.) There is also reason to believe that the male
undertakes the duty of incubation, for Mr. Swinhoe (19. 'Ibis,' 1866, p.
298.) found the females before the close of the summer associated in
flocks, as occurs with the females of the Turnix.
The females of Phalaropus fulicarius and P. hyperboreus are larger, and in
their summer plumage "more gaily attired than the males." But the
difference in colour between the sexes is far from conspicuous. According
to Professor Steenstrup, the male alone of P. fulicarius undertakes the
duty of incubation; this is likewise shewn by the state of his breast-
feathers during the breeding-season. The female of the dotterel plover
(Eudromias morinellus) is larger than the male, and has the red and black
tints on the lower surface, the white crescent on the breast, and the
stripes over the eyes, more strongly pronounced. The male also takes at
least a share in hatching the eggs; but the female likewise attends to the
young. (20. For these several statements, see Mr. Gould's 'Birds of Great
Britain.' Prof. Newton informs me that he has long been convinced, from
his own observations and from those of others, that the males of the above-
named species take either the whole or a large share of the duties of
incubation, and that they "shew much greater devotion towards their young,
when in danger, than do the females." So it is, as he informs me, with
Limosa lapponica and some few other Waders, in which the females are larger
and have more strongly contrasted colours than the males.) I have not been
able to discover whether with these species the young resemble the adult
males more closely than the adult females; for the comparison is somewhat
difficult to make on account of the double moult.
Turning now to the ostrich Order: the male of the common cassowary
(Casuarius galeatus) would be thought by any one to be the female, from his
smaller size and from the appendages and naked skin about his head being
much less brightly coloured; and I am informed by Mr. Bartlett that in the
Zoological Gardens, it is certainly the male alone who sits on the eggs and
takes care of the young. (21. The natives of Ceram (Wallace, 'Malay
Archipelago,' vol. ii. p. 150) assert that the male and female sit
alternately on the eggs; but this assertion, as Mr. Bartlett thinks, may be
accounted for by the female visiting the nest to lay her eggs.) The female
is said by Mr. T.W. Wood (22. The 'Student,' April 1870, p. 124.) to
exhibit during the breeding-season a most pugnacious disposition; and her
wattles then become enlarged and more brilliantly coloured. So again the
female of one of the emus (Dromoeus irroratus) is considerably larger than
the male, and she possesses a slight top-knot, but is otherwise
indistinguishable in plumage. She appears, however, "to have greater
power, when angry or otherwise excited, of erecting, like a turkey-cock,
the feathers of her neck and breast. She is usually the more courageous
and pugilistic. She makes a deep hollow guttural boom especially at night,
sounding like a small gong. The male has a slenderer frame and is more
docile, with no voice beyond a suppressed hiss when angry, or a croak." He
not only performs the whole duty of incubation, but has to defend the young
from their mother; "for as soon as she catches sight of her progeny she
becomes violently agitated, and notwithstanding the resistance of the
father appears to use her utmost endeavours to destroy them. For months
afterwards it is unsafe to put the parents together, violent quarrels being
the inevitable result, in which the female generally comes off conqueror."
(23. See the excellent account of the habits of this bird under
confinement, by Mr. A.W. Bennett, in 'Land and Water,' May 1868, p. 233.)
So that with this emu we have a complete reversal not only of the parental
and incubating instincts, but of the usual moral qualities of the two
sexes; the females being savage, quarrelsome, and noisy, the males gentle
and good. The case is very different with the African ostrich, for the
male is somewhat larger than the female and has finer plumes with more
strongly contrasted colours; nevertheless he undertakes the whole duty of
incubation. (24. Mr. Sclater, on the incubation of the Struthiones,
'Proc. Zool. Soc.' June 9, 1863. So it is with the Rhea darwinii: Captain
Musters says ('At Home with the Patagonians,' 1871, p. 128), that the male
is larger, stronger and swifter than the female, and of slightly darker
colours; yet he takes sole charge of the eggs and of the young, just as
does the male of the common species of Rhea.)
I will specify the few other cases known to me, in which the female is more
conspicuously coloured than the male, although nothing is known about the
manner of incubation. With the carrion-hawk of the Falkland Islands
(Milvago leucurus) I was much surprised to find by dissection that the
individuals, which had all their tints strongly pronounced, with the cere
and legs orange-coloured, were the adult females; whilst those with duller
plumage and grey legs were the males or the young. In an Australian tree-
creeper (Climacteris erythrops) the female differs from the male in "being
adorned with beautiful, radiated, rufous markings on the throat, the male
having this part quite plain." Lastly, in an Australian night-jar "the
female always exceeds the male in size and in the brilliance of her tints;
the males, on the other hand, have two white spots on the primaries more
conspicuous than in the female." (25. For the Milvago, see 'Zoology of
the Voyage of the "Beagle," Birds,' 1841, p. 16. For the Climacteris and
night-jar (Eurostopodus), see Gould's 'Handbook to the Birds of Australia,'
vol. i. pp. 602 and 97. The New Zealand shieldrake (Tadorna variegata)
offers a quite anomalous case; the head of the female is pure white, and
her back is redder than that of the male; the head of the male is of a rich
dark bronzed colour, and his back is clothed with finely pencilled slate-
coloured feathers, so that altogether he may be considered as the more
beautiful of the two. He is larger and more pugnacious than the female,
and does not sit on the eggs. So that in all these respects this species
comes under our first class of cases; but Mr. Sclater ('Proceedings of the
Zoological Society,' 1866, p. 150) was much surprised to observe that the
young of both sexes, when about three months old, resembled in their dark
heads and necks the adult males, instead of the adult females; so that it
would appear in this case that the females have been modified, whilst the
males and the young have retained a former state of plumage.)
We thus see that the cases in which female birds are more conspicuously
coloured than the males, with the young in their immature plumage
resembling the adult males instead of the adult females, as in the previous
class, are not numerous, though they are distributed in various Orders.
The amount of difference, also, between the sexes is incomparably less than
that which frequently occurs in the last class; so that the cause of the
difference, whatever it may have been, has here acted on the females either
less energetically or less persistently than on the males in the last
class. Mr. Wallace believes that the males have had their colours rendered
less conspicuous for the sake of protection during the period of
incubation; but the difference between the sexes in hardly any of the
foregoing cases appears sufficiently great for this view to be safely
accepted. In some of the cases, the brighter tints of the female are
almost confined to the lower surface, and the males, if thus coloured,
would not have been exposed to danger whilst sitting on the eggs. It
should also be borne in mind that the males are not only in a slight degree
less conspicuously coloured than the females, but are smaller and weaker.
They have, moreover, not only acquired the maternal instinct of incubation,
but are less pugnacious and vociferous than the females, and in one
instance have simpler vocal organs. Thus an almost complete transposition
of the instincts, habits, disposition, colour, size, and of some points of
structure, has been effected between the two sexes.
Now if we might assume that the males in the present class have lost some
of that ardour which is usual to their sex, so that they no longer search
eagerly for the females; or, if we might assume that the females have
become much more numerous than the males--and in the case of one Indian
Turnix the females are said to be "much more commonly met with than the
males" (26. Jerdon, 'Birds of India,' vol. iii. p. 598.)--then it is not
improbable that the females would have been led to court the males, instead
of being courted by them. This indeed is the case to a certain extent with
some birds, as we have seen with the peahen, wild turkey, and certain kinds
of grouse. Taking as our guide the habits of most male birds, the greater
size and strength as well as the extraordinary pugnacity of the females of
the Turnix and emu, must mean that they endeavour to drive away rival
females, in order to gain possession of the male; and on this view all the
facts become clear; for the males would probably be most charmed or excited
by the females which were the most attractive to them by their bright
colours, other ornaments, or vocal powers. Sexual selection would then do
its work, steadily adding to the attractions of the females; the males and
the young being left not at all, or but little modified.
CLASS III.
WHEN THE ADULT MALE RESEMBLES THE ADULT FEMALE, THE YOUNG OF BOTH SEXES
HAVE A PECULIAR FIRST PLUMAGE OF THEIR OWN.
In this class the sexes when adult resemble each other, and differ from the
young. This occurs with many birds of many kinds. The male robin can
hardly be distinguished from the female, but the young are widely
different, with their mottled dusky-olive and brown plumage. The male and
female of the splendid scarlet ibis are alike, whilst the young are brown;
and the scarlet colour, though common to both sexes, is apparently a sexual
character, for it is not well developed in either sex under confinement;
and a loss of colour often occurs with brilliant males when they are
confined. With many species of herons the young differ greatly from the
adults; and the summer plumage of the latter, though common to both sexes,
clearly has a nuptial character. Young swans are slate-coloured, whilst
the mature birds are pure white; but it would be superfluous to give
additional instances. These differences between the young and the old
apparently depend, as in the last two classes, on the young having retained
a former or ancient state of plumage, whilst the old of both sexes have
acquired a new one. When the adults are bright coloured, we may conclude
from the remarks just made in relation to the scarlet ibis and to many
herons, and from the analogy of the species in the first class, that such
colours have been acquired through sexual selection by the nearly mature
males; but that, differently from what occurs in the first two classes, the
transmission, though limited to the same age, has not been limited to the
same sex. Consequently, the sexes when mature resemble each other and
differ from the young.
CLASS IV.
WHEN THE ADULT MALE RESEMBLES THE ADULT FEMALE, THE YOUNG OF BOTH SEXES IN
THEIR FIRST PLUMAGE RESEMBLE THE ADULTS.
In this class the young and the adults of both sexes, whether brilliantly
or obscurely coloured, resemble each other. Such cases are, I think, more
common than those in the last class. We have in England instances in the
kingfisher, some woodpeckers, the jay, magpie, crow, and many small dull-
coloured birds, such as the hedge-warbler or kitty-wren. But the
similarity in plumage between the young and the old is never complete, and
graduates away into dissimilarity. Thus the young of some members of the
kingfisher family are not only less vividly coloured than the adults, but
many of the feathers on the lower surface are edged with brown (27.
Jerdon, 'Birds of India,' vol. i. pp. 222, 228. Gould's 'Handbook to the
Birds of Australia,' vol. i. pp. 124, 130.),--a vestige probably of a
former state of the plumage. Frequently in the same group of birds, even
within the same genus, for instance in an Australian genus of parrakeets
(Platycercus), the young of some species closely resemble, whilst the young
of other species differ considerably, from their parents of both sexes,
which are alike. (28. Gould, ibid. vol. ii. pp. 37, 46, 56.) Both sexes
and the young of the common jay are closely similar; but in the Canada jay
(Perisoreus canadensis) the young differ so much from their parents that
they were formerly described as distinct species. (29. Audubon, 'Ornith.
Biography,' vol. ii. p. 55.)
I may remark before proceeding that, under the present and next two classes
of cases, the facts are so complex and the conclusions so doubtful, that
any one who feels no especial interest in the subject had better pass them
over.
The brilliant or conspicuous colours which characterise many birds in the
present class, can rarely or never be of service to them as a protection;
so that they have probably been gained by the males through sexual
selection, and then transferred to the females and the young. It is,
however, possible that the males may have selected the more attractive
females; and if these transmitted their characters to their offspring of
both sexes, the same results would follow as from the selection of the more
attractive males by the females. But there is evidence that this
contingency has rarely, if ever, occurred in any of those groups of birds
in which the sexes are generally alike; for, if even a few of the
successive variations had failed to be transmitted to both sexes, the
females would have slightly exceeded the males in beauty. Exactly the
reverse occurs under nature; for, in almost every large group in which the
sexes generally resemble each other, the males of some few species are in a
slight degree more brightly coloured than the females. It is again
possible that the females may have selected the more beautiful males, these
males having reciprocally selected the more beautiful females; but it is
doubtful whether this double process of selection would be likely to occur,
owing to the greater eagerness of one sex than the other, and whether it
would be more efficient than selection on one side alone. It is,
therefore, the most probable view that sexual selection has acted, in the
present class, as far as ornamental characters are concerned, in accordance
with the general rule throughout the animal kingdom, that is, on the males;
and that these have transmitted their gradually-acquired colours, either
equally or almost equally, to their offspring of both sexes.
Another point is more doubtful, namely, whether the successive variations
first appeared in the males after had become nearly mature, or whilst quite
young. In either case sexual selection must have acted on the male when he
had to compete with rivals for the possession of the female; and in both
cases the characters thus acquired have been transmitted to both sexes and
all ages. But these characters if acquired by the males when adult, may
have been transmitted at first to the adults alone, and at some subsequent
period transferred to the young. For it is known that, when the law of
inheritance at corresponding ages fails, the offspring often inherit
characters at an earlier age than that at which they first appeared in
their parents. (30. 'Variation of Animals and Plants under
Domestication,' vol. ii. p. 79.) Cases apparently of this kind have been
observed with birds in a state of nature. For instance Mr. Blyth has seen
specimens of Lanius rufus and of Colymbus glacialis which had assumed
whilst young, in a quite anomalous manner, the adult plumage of their
parents. (31. 'Charlesworth's Magazine of Natural History,' vol. i. 1837,
pp. 305, 306.) Again, the young of the common swan (Cygnus olor) do not
cast off their dark feathers and become white until eighteen months or two
years old; but Dr. F. Forel has described the case of three vigorous young
birds, out of a brood of four, which were born pure white. These young
birds were not albinos, as shewn by the colour of their beaks and legs,
which nearly resembled the same parts in the adults. (32. 'Bulletin de la
Soc. Vaudoise des Sc. Nat.' vol. x. 1869, p. 132. The young of the Polish
swan, Cygnus immutabilis of Yarrell, are always white; but this species, as
Mr. Sclater informs me, is believed to be nothing more than a variety of
the domestic swan (Cygnus olor).)
It may be worth while to illustrate the above three modes by which, in the
present class, the two sexes and the young may have come to resemble each
other, by the curious case of the genus Passer. (33. I am indebted to Mr.
Blyth for information in regard to this genus. The sparrow of Palestine
belongs to the sub-genus Petronia.) In the house-sparrow (P. domesticus)
the male differs much from the female and from the young. The young and
the females are alike, and resemble to a large extent both sexes and the
young of the sparrow of Palestine (P. brachydactylus), as well as of some
allied species. We may therefore assume that the female and young of the
house-sparrow approximately shew us the plumage of the progenitor of the
genus. Now with the tree-sparrow (P. montanus) both sexes and the young
closely resemble the male of the house-sparrow; so that they have all been
modified in the same manner, and all depart from the typical colouring of
their early progenitor. This may have been effected by a male ancestor of
the tree-sparrow having varied, firstly, when nearly mature; or, secondly,
whilst quite young, and by having in either case transmitted his modified
plumage to the females and the young; or, thirdly, he may have varied when
adult and transmitted his plumage to both adult sexes, and, owing to the
failure of the law of inheritance at corresponding ages, at some subsequent
period to his young.
It is impossible to decide which of these three modes has generally
prevailed throughout the present class of cases. That the males varied
whilst young, and transmitted their variations to their offspring of both
sexes, is the most probable. I may here add that I have, with little
success, endeavoured, by consulting various works, to decide how far the
period of variation in birds has generally determined the transmission of
characters to one sex or to both. The two rules, often referred to
(namely, that variations occurring late in life are transmitted to one and
the same sex, whilst those which occur early in life are transmitted to
both sexes), apparently hold good in the first (34. For instance, the
males of Tanagra aestiva and Fringilla cyanea require three years, the male
of Fringilla ciris four years, to complete their beautiful plumage. (See
Audubon, 'Ornith. Biography,' vol. i. pp. 233, 280, 378). The Harlequin
duck takes three years (ibid. vol. iii. p. 614). The male of the Gold
pheasant, as I hear from Mr. Jenner Weir, can be distinguished from the
female when about three months old, but he does not acquire his full
splendour until the end of the September in the following year.), second,
and fourth classes of cases; but they fail in the third, often in the fifth
(35. Thus the Ibis tantalus and Grus americanus take four years, the
Flamingo several years, and the Ardea ludovicana two years, before they
acquire their perfect plumage. See Audubon, ibid. vol. i. p. 221; vol.
iii. pp. 133, 139, 211.), and in the sixth small class. They apply,
however, as far as I can judge, to a considerable majority of the species;
and we must not forget the striking generalisation by Dr. W. Marshall with
respect to the protuberances on the heads of birds. Whether or not the two
rules generally hold good, we may conclude from the facts given in the
eighth chapter, that the period of variation is one important element in
determining the form of transmission.
With birds it is difficult to decide by what standard we ought to judge of
the earliness or lateness of the period of variation, whether by the age in
reference to the duration of life, or to the power of reproduction, or to
the number of moults through which the species passes. The moulting of
birds, even within the same family, sometimes differs much without any
assignable cause. Some birds moult so early, that nearly all the body
feathers are cast off before the first wing-feathers are fully grown; and
we cannot believe that this was the primordial state of things. When the
period of moulting has been accelerated, the age at which the colours of
the adult plumage are first developed will falsely appear to us to be
earlier than it really is. This may be illustrated by the practice
followed by some bird-fanciers, who pull out a few feathers from the breast
of nestling bullfinches, and from the head or neck of young gold-pheasants,
in order to ascertain their sex; for in the males, these feathers are
immediately replaced by coloured ones. (36. Mr. Blyth, in Charlesworth's
'Magazine of Natural History,' vol. i. 1837, p. 300. Mr. Bartlett has
informed me in regard to gold pheasants.) The actual duration of life is
known in but few birds, so that we can hardly judge by this standard. And,
with reference to the period at which the power of reproduction is gained,
it is a remarkable fact that various birds occasionally breed whilst
retaining their immature plumage. (37. I have noticed the following cases
in Audubon's 'Ornith. Biography.' The redstart of America (Muscapica
ruticilla, vol. i. p. 203). The Ibis tantalus takes four years to come to
full maturity, but sometimes breeds in the second year (vol. iii. p. 133).
The Grus americanus takes the same time, but breeds before acquiring its
full plumage (vol. iii. p. 211). The adults of Ardea caerulea are blue,
and the young white; and white, mottled, and mature blue birds may all be
seen breeding together (vol. iv. p. 58): but Mr. Blyth informs me that
certain herons apparently are dimorphic, for white and coloured individuals
of the same age may be observed. The Harlequin duck (Anas histrionica,
Linn.) takes three years to acquire its full plumage, though many birds
breed in the second year (vol. iii. p. 614). The White-headed Eagle (Falco
leucocephalus, vol. iii. p. 210) is likewise known to breed in its immature
state. Some species of Oriolus (according to Mr. Blyth and Mr. Swinhoe, in
'Ibis,' July 1863, p. 68) likewise breed before they attain their full
plumage.)
The fact of birds breeding in their immature plumage seems opposed to the
belief that sexual selection has played as important a part, as I believe
it has, in giving ornamental colours, plumes, etc., to the males, and, by
means of equal transmission, to the females of many species. The objection
would be a valid one, if the younger and less ornamented males were as
successful in winning females and propagating their kind, as the older and
more beautiful males. But we have no reason to suppose that this is the
case. Audubon speaks of the breeding of the immature males of Ibis
tantalus as a rare event, as does Mr. Swinhoe, in regard to the immature
males of Oriolus. (38. See footnote 37 above.) If the young of any
species in their immature plumage were more successful in winning partners
than the adults, the adult plumage would probably soon be lost, as the
males would prevail, which retained their immature dress for the longest
period, and thus the character of the species would ultimately be modified.
(39. Other animals, belonging to quite distinct classes, are either
habitually or occasionally capable of breeding before they have fully
acquired their adult characters. This is the case with the young males of
the salmon. Several amphibians have been known to breed whilst retaining
their larval structure. Fritz Muller has shewn ('Facts and arguments for
Darwin,' Eng. trans. 1869, p. 79) that the males of several amphipod
crustaceans become sexually mature whilst young; and I infer that this is a
case of premature breeding, because they have not as yet acquired their
fully-developed claspers. All such facts are highly interesting, as
bearing on one means by which species may undergo great modifications of
character.) If, on the other hand, the young never succeeded in obtaining
a female, the habit of early reproduction would perhaps be sooner or later
eliminated, from being superfluous and entailing waste of power.
The plumage of certain birds goes on increasing in beauty during many years
after they are fully mature; this is the case with the train of the
peacock, with some of the birds of paradise, and with the crest and plumes
of certain herons, for instance, the Ardea ludovicana. (40. Jerdon,
'Birds of India,' vol. iii. p. 507, on the peacock. Dr. Marshall thinks
that the older and more brilliant males of birds of paradise, have an
advantage over the younger males; see 'Archives Neerlandaises,' t
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