FROM DARKNESS TO LIGHT
To all that I have said concerning the Arctic skua in my last chapter (I
do not say it is much) I will now add what the Germans call a _Beitrag_,
on the subject of the multitudinous variety of colouring and arrangement
of markings which the plumage of this species exhibits.
Hitherto, indeed, I have spoken as if it were always of a uniformly
dusky shade, but that was because I wanted that shade (and, indeed, it
happened so to be) in the two that were chasing my tern. Otherwise they
would not have suited the part I assigned them of twin evil geniuses, or
have contrasted sufficiently with the white soul that they were seeking
to corrupt. So, till that was all over, there could be no light or
half-light skuas, but now that it is, and the effect produced, I permit
things to be as they are.
The Arctic skua, then, is supposed by ornithologists--or, at any rate,
that is how they are accustomed to speak of it--to be a bird of two
different outer appearances, independent of sex, which does not add
another one: dimorphic we are told it is, which means, or should mean,
that it is two- or double-formed, taking form here to mean colour. Two!
A hundred would be nearer the mark, I think, but I have only had the
time, or the patience, to note down fifteen, which I did very carefully,
through the glasses, as the birds stood amidst the short heather on the
ness-side. Here they are; not, perhaps, very precisely or scientifically
defined, but none the less truthfully so, for all that, and as accurate,
I think, as the fact that no two people see colours quite alike will
allow. But they, at any rate, bring out four facts, which, together,
have, I think, a distinct meaning, viz. (1) the unmistakable and, for the
most part, pronounced difference in these fifteen forms of a two-formed
species; (2) the likeness of the extremely plain, permanent form to the
plain-coloured great skua; (3) the same resemblance in the first true
plumage of the young bird; and (4) the absence in the young bird of the
two lance-like feathers which, in the old ones, project beyond the rest
of the rectrices, but which are also absent in the great skua. Well, here
they are.
(1) The neck, from just below the head, with the throat, breast, and
ventral surface, as far as the legs, a beautiful creamy white; the
rest dark, as in the ordinary dark form, but I was not careful to
note the precise shade; the crown of the head--and this, it seems, is
universal--sufficiently dark to appear black. This bird represents, I
think, the extreme of the light or ornate form, in which dark and light
are almost equally divided.
(2) The light colouring extends, speaking roughly, over the same
parts, but is very much less bright and pure. It might be described
as a dun-cream or cream-dun, the two shades seeming to struggle for
supremacy. The cream prevails on the neck, the dun on the other parts;
but even the neck is of a much duller shade than in the bird just
described (No. 1). There are parts of the breast where the original
sombre hue, a little softened, encroaches, cloudily, upon the lighter
surface. These two birds cannot, certainly, be described as more or less
handsome, merely, in the same colouring. The lighter surface, at any
rate, is plainly different in shade, also its amount and distribution,
though in a less degree.
(3) Another bird is much like this last one (No. 2), but there is, here,
a distinct, broad, dunnish space, dividing the throat and breast parts,
making, of course, a very palpable difference.
(4) Another bird--one of two standing together--is the common uniformly
dark form, except that the neck and throat just below the head is, for
about an inch, very much lighter, making a considerable approach to cream
without quite attaining it. This light part is conspicuous in the one
bird--this that I have been describing (No. 4)--but not in the other (No.
5) that it is standing by.
(5) This other one might pass for the ordinary dark form, but on
examining it through the glasses a lighter, though less salient, collar
is distinctly visible.
(6) In a third bird, not far off these two (Nos. 4 and 5) the whole
colouring, from immediately below the crown of the head--which seems
always to be black or very dark--is of a uniform brown-drab or brown-dun
colour, there being not the slightest approach to a lighter collar, or
any lightness elsewhere, except, as in every bird, without exception, on
the quill feathers of the wings as seen in flight.
(7) In another bird the breast and ventral surface is of a delicate
silvery cream, or creamy silver, something like that on the same parts
of the Great Crested Grebe. On the sides of the neck, and just below the
chin it is the same, perhaps a little less silvered; but between these
two spaces, and so between the chin and breast, a zone of faint brown or
dun, somewhat broken and cloudy, pushes itself forward from the wings,
thus breaking the continuity of the light surface by the strengthening
of a tendency which is, perhaps, just traceable even in the lightest
specimens. Besides this, a similar clouded space is continued downwards
from the back of the head, first in a diminishing quantity, and then,
again, broadening out, till it joins the upper body-colour. So that here
only a little of the nape is white, hardly more than what may still
be described as the two sides of the neck. This is a very pretty and
delicate combination.
(8) Close beside this last bird (No. 7) is a uniformly dark brown one; and
(9) Not far off, on the other side of it, one which exhibits the same
sort of general effect, in a dark, smoky dun. This latter bird would
generally pass as representing the dark form, and, with fluctuations
in either direction, dark or light, it does represent the common form.
Nevertheless, it is both light and varied compared with the extreme or
uniform dark brown form beside it (No. 8), which appears to me to be the
least common one of all, less so than the extreme light one (No. 1) at
the other end. When I say uniform, however, I do not mean to include the
crown of the head or tips of the wings, which are always darker than the
rest of the plumage, nor yet that lighter shade which is on the primary
quills of every individual, but only seen in flight. These exceptions
must always be understood, and, moreover, the expression uniform is not
to be construed with mathematical accuracy, but only as conveying the
general effect upon the eye.
(10) A bird that from the dark crown of the head to the dark tips of the
wings is, above and below, a uniform dark, browny dun, yet some washes
lighter than the uniformly brown one (No. 8) that I have spoken of.
(11) A bird that, from the dark crown to the dark wing-tips, is, above
and below, a uniform light fawny dun.
(12) A bird that would be the extreme light form (No. 1) that I have
first described, were it not that, both on the throat and breast, the
cream is encroached upon by cloudy barrings of a soft greyey-brown (or
something between the two) which extend also over the under surface of
the wings. Moreover, a toning of the darker colour of the general upper
surface encroaches a little upon the cream of the nape.
(13) A bird exhibiting the uniform, dusky-dunnish colour of the common
form (a shade lighter, perhaps, on the under surface), but with a cream
patch on each side of the neck, just below the head. These patches are
not, perhaps, of the brightest cream, but they are very conspicuous,
whether the bird is seen standing or flying--in fact, the salient feature.
(14) A bird that would be the extreme light form (No. 1), but for a
distinct collar of soft brown dividing the cream of the neck and throat
from that of the breast.
(15) A bird that is yellowish dun on the neck and throat, mottled-brown
on the breast, and a fine cream on the ventral surface.
Moreover, all these birds differed to a greater or less extent in those
lighter markings of the quill feathers, both on the upper and under
surface, some being lighter and some darker; following, in this respect,
the general colouring. This feature, however, is only apparent when the
birds fly, and I found it too laborious to include.
I can say with certainty, I think--judging by the lance-like projecting
feathers of the tail, absent in the young bird, and by every other
indication--that all the individuals here described by me, were birds of
mature plumage. They were all established in one locality, and I was able
to compare most of them with each other. I think, therefore, that though
there might, perhaps, be some difference of opinion in regard to some of
my colour terms--as where would there not be?--yet that the variation
between the different forms is properly brought out. Without my seeking
it, the list includes the two extreme forms, as I believe them to be, of
dark and light; the former represented by a uniformly dark-brown bird,
the latter by one having the whole under surface of the body, as well as
the sides and nape of the neck, of a beautiful cream colour, by virtue of
which, and of the salient contrast exhibited between this and the dusky
upper surface, it is extremely handsome, not to say beautiful--one of
the handsomest of all our birds in my opinion. Both the extreme forms
are uncommon, but only, I think, as compared with all the intermediate
shades, not with any one of them. Also the extreme light, or handsome,
form seems to me to be commoner than the extreme plain one. Should not a
bird like this be described as multi-morphic rather than as dimorphic?
I believe that there exists as perfect a series between the two extreme
forms as between the least eye-like and the most perfect eye-feather in
the tail of the peacock--to take the well-known illustration given by
Darwin to enforce his arguments in favour of sexual selection. The eye,
however, insensibly masses the less saliently distinguished individuals
together, so that those in whose plumage the light colour is more _en
évidence_ than the dark, go down as the light form, and _vice versâ_.
Moreover, the more _prononcé_ a bird is, in one or another direction,
the more it is remarked; so that, perhaps, the intermediate shadings are
forgotten, on the same principle as that by which extreme characters,
in any direction, are more appreciated than less extreme ones, by the
breeders of fancy birds--pigeons, poultry, etc. The uniform brown form,
however, as being less striking (though extreme at one end) is not, I
believe, so much noticed as those various dunnish shades, which have, in
my view, been classed all together, as the dark variety.
In regard to the young birds, I only remember those nestling ones which
had feathers under the fluff, as brown, without any admixture of cream.
But I had not, at that time, these matters in my mind, and, moreover, I
did not see many. When older, however, and able to fly, all that I have
seen have had a distinct colouring of their own--for their plumage has
borne a considerable resemblance to that of the Great Skua (_Stercorarius
catarriactes_), being mottled on the back with two shades of brown, a
darker and a lighter one. I got the effect of this when I watched young
birds flying or standing, and one day I caught one whose wing had been
injured, and saw that it was so. This resemblance is increased by such
birds wanting the two lance-like feathers in the tail. As I say, this
mottled brown is the only kind of colouring which I have seen in these
immature but comparatively advanced birds, and my impression is that,
in the still younger birds, such mottling was either absent or not so
noticeable. At any rate, I have no clear recollection of it.
My own explanation of all these facts is that _Stercorarius
crepidatus_--by my faith, 'tis a pretty name, though not wholly
deserved--having been, originally, a plain homely-coloured bird, like
his relative, the great skua, is being gradually modified, under the
influence of sexual selection, into a most beautiful one, as represented
by the extreme light or half-cream form. Natural selection, in the more
general sense, seems here excluded, or, at any rate, extremely doubtful;
and if it be suggested that the lighter birds have the more vigorous
constitutions, that they are fuller of verve and energy, to which they
owe their cream colouring, I, for my part, can only say "Prodigious!" (or
think it), like Dominie Sampson. But I can assure all those who hold this
unmanageable view--for really there is no dealing with it--that the one
sort came not a whit nearer to knocking my cap off than did the other.
But, leaving shadows, the main facts here suggest choice in a certain
direction. There is a gradation of colour and pattern, connecting two
forms--one plain, the other lovely. This suggests a passage from one
to the other, and if the plain mature form--I mean the uniform brown
one--most resembles the young bird in colouring--which to me it seems to
do--whilst the young bird resembles, more than any old one, an allied
plainer species, this makes it more than likely that the passage has
been from the plain to the lovely, and not from the lovely to the plain.
Supporting and emphasising this, we have the absence, in the tail of the
young bird, of those lance-like feathers which give so marked a character
to, and add so infinitely to the grace of, the old one. Of what use can
this thin projection, an inch or so beyond the serviceable fan of the
tail, be to the bird? Seeing how well every other bird does without it,
can we suppose it to be of any service? Its beauty, however--which one
misses dreadfully in the young flying bird--is apparent to any one, and
it goes hand in hand with an ascending scale of beauty in colour. All
this seems to me to point strongly towards sexual selection as the agency
by which these changes have been, and are being, effected;[1] since I am,
personally, a believer in the reality of that power, having never heard
or read anything against it, so convincing to my mind as what Darwin said
for it, nor seen anything that has appeared to me to be inconsistent
either with his facts or his arguments.
[Footnote 1: It is a strong enforcement, I think, of this view, that in
another variable species of skua--_Stercorarius pomatorhinus_--the same
two feathers give the bird "the grotesque appearance of having a disk
attached to its tail."]
No doubt if the varied coloration of the Arctic skua is really to be
explained in this way, the lighter-coloured forms, especially the
extreme one, in which the whole under surface is cream, ought to be
on the increase, whilst the dark ones should ultimately die out or
remain, perhaps, as a separate species, the intermediate tintings having
disappeared. It is very difficult to form an idea of the relative number
of individuals constituting any one form, because one unconsciously
compares such form with a great many others instead of with each
separately; but, whereas I remember various repetitions of the extreme
light or half-cream variety, I have not the same clear recollection in
regard to birds exhibiting other shades and proportions of cream. It was
the opinion, moreover, of the man engaged to protect the sea-birds during
the breeding season on Unst, that the light birds, by which he meant
the ones more markedly so, were increasing in numbers. It would appear,
therefore, that the process one might expect, were sexual selection
the agency here at work, is in operation, and, for the rest, it is no
use being in a hurry. A little patience, the "rolling" of "a few more
years"--say a million--will settle the matter either one way or the
other.