The Bird Watcher in the Shetlands/Chapter 3
CHAPTER III
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 enroached 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 multimorphic rather than as dimorphic? I believe that there exists as perfect as 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 versa. 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.
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.
- ↑ 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."