The Zoologist/4th series, vol 6 (1902)/Issue 736/Variations in Colouring of Stercorarius crepidatus

by Edmund Selous

 

VARIATIONS IN COLOURING OF STERCORARIUS
CREPIDATUS.

By Edmund Selous.

 

The Arctic Skua (Stercorarius crepidatus) is usually described as being dimorphous—that is to say, of two forms in regard to its plumage, one dark and the other light. I have lately, whilst in the Shetlands, written down the description of various birds as they stood on the heath, after carefully watching them through the glasses, at a very moderate distance. I might have added greatly to the list, hardly any two individuals being alike in the same degree that the individuals of most other birds are—at least this appeared to me to be the case. But my time for this was limited, and my list consists of fifteen birds only. It is as follows:—

(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 cases; but I was not careful to note the precise shade. The crown of the head—and this, I think, is universal—sufficiently dark to appear black. This bird represents, I think, the extreme of the light form in which dark and light are almost equally divided.

(2) The light colouring extends, speaking roughly, over the same parts, but it 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—I say this after due comparison—be described as more or less handsome in the same colouring. The lighter surface, at any rate, is plainly different in shade, as 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.

(4) Another bird—one of two standing together—is the common form (that is, dark), except that the neck and throat just below the head for about an inch is very much lighter, making a considerable approach to cream, without quite obtaining it. This light part is conspicuous in the one bird, but not in the other (No. 5) 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 forehead and 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 that which—as in all the birds—becomes visible on the quill-feathers of the wings in flight.

(7) In another bird the breast and ventral surface is of a delicate silvery cream or creamy silver, something like that 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 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 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. (N.B.—When I say uniform, 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.)

(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 uniform 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, 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 (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 conspicuous 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.

So far as I can be sure—judging by the lance-like projecting feathers of the tail, absent in the young bird, and by every other indication—all the individuals here described were old birds in 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 some of my colour-terms may not be quite accurate—in describing colours there is generally some difference of nomenclature—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 British birds, in my opinion. Both the extreme forms are uncommon, whilst of the many forms between them hardly any two seem to me to be quite alike. The extreme forms are, or much more so; and this would make them more numerous than any one of the others, though less so than all of these collectively. 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 multimorphous rather than as dimorphous? 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, as pointed out by Darwin, and exhibited in the Natural History Museum at South Kensington. The eye, however, insensibly masses the less saliently distinguished individuals together, so that those in whose plumage the light colouring 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, &c. 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. Still, with all this, I confess it is a puzzle to me how a bird, the individuals of which differ so greatly and indefinitely, can have come to be considered as merely exhibiting two forms of colouring.

As far as I remember, all the nestling birds which I have seen have been merely brown, without any admixture of cream under the fluff; but I have not seen very many. When older and able to fly, but still young, all that I have seen have had a colouring of their own—for their plumage has borne a considerable resemblance to that of the Great Skua (Stercorarius catarrhactes), 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 lance-like feathers (or feather) in the tail. This mottled brown is the only kind of colouring which I have seen in these immature but comparatively advanced birds. Certainly, compared to the old ones, there were but few of these to be seen on my late visit. Had there been only one, however, that exhibited the ordinary light or dark form of plumage, or any sensible approach towards it, I believe I should have noticed it, as I was for seventeen days on the spot. My impression is that in the still younger birds this 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, having been originally a plain homely-coloured bird, like 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 seems here excluded, or, at any rate, extremely doubtful; and, if it be proposed that the lighter (or darker) birds have the more vigorous constitutions, I can only say that I believe it would be extremely difficult to produce any kind of evidence in favour of the suggestion. Without evidence, such a view is a mere supposition, and therefore not worth while considering. The main facts 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 form most resembles the young bird in colouring (which is my own experience), whilst the young bird resembles, more than any old one, an allied plainer species, this seems to make it more than likely that the passage is from the plain to the lovely, and not from the lovely to the plain. Supporting and emphasizing this, we have the absence of those lance-like feathers in the tail of the young bird 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 anyone, and it goes hand in hand with an increased and ascending scale of beauty in colour. All this seems to me to point towards sexual selection, 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 has said for it, nor seen anything that has appeared to me to be inconsistent either with his facts or his arguments.