Chapter VI: Appendix: (A) Plumage and Soft Parts

The following description of the plumage and soft parts of _Pygoscelis Adeliae_, which is perfectly correct, is taken from the zoological report of the _Discovery_ Expedition.

Soft Parts.

"_Bill_, when first hatched, blackish. A week old, black terminally, deep red at the gape and along the cutting edges. Immature of the first year, blackish. Adult, brick-red, the upper bill black terminally, and the mandible black along the cutting edge.

"_Iris_, brown; varying between reddish brown and greenish brown.

"_Eyelids_, black throughout the first year; pure white in the adult at fourteen months and onwards.

"_Feet_, flesh red; dusky when first hatched, brightening in the first week or two. Immature and adult, pale flesh pink above, black beneath (in some cases piebald beneath).

"_Claws_, brown."

* * * * *

In the majority of the chicks the down is uniformly dark and sooty, but here and there, in progeny of quite normal parents, one may find nestlings of so pale a grey as to be almost silvery white, with blackish heads, possibly a reversion to an earlier type, and, at any rate, suggestive of the young of the Emperor penguin, which perhaps represents the oldest stock of all. According to Dr. Bowdler Sharp, the colour of the head is in all cases blacker in the earlier stages than the rest of the body.(8)

(8) This was invariable at Cape Adare.

As the chick ages the colour of its down changes, and all of it takes on a dull rusty brown colour. As it moults the abdomen and thighs change first, and white feathers appear in place of the down. Then come changes on the head, round the bill, and at the tail; the upper breast, neck, and back being the last parts to moult.

The feet, which in the young nestling have been almost black, change in colour to a brick-red that shows up very markedly against the rusty brown down, looking as if the legs were raw and inflamed. Later the permanent flesh colour is acquired, with black plantar surfaces. The nails are black at first, and later change to brown.

When the nestling down is shed, the resulting plumage is that of the adult, except that the throat is white instead of black. The upper part of the head and neck are bluish black, the throat, fore-neck, breast, and abdomen being a pure dazzling metallic white, a sharp line separating the white from the black areas. The flippers are the same bluish "tar" black on the back and white beneath.

In addition to the distinctive pure white plumage of the throat, the immature bird differs from the adult in one very marked particular, which is that the eyelids are black, as in the chick, and do not acquire the staring whiteness which is so distinctive of the adult Adelie penguin, increasing, as it does, the white area of the sclerotic so that the bird has the appearance of being perpetually surprised or very angry.

The iris is a rich reddish brown in the adults, but variable in the young.

At Cape Adare the light grey "silvery" coloured chicks mentioned by Dr. Wilson were by no means uncommon; in fact, quite a large proportion of the chicks had very light-coloured down. This is shown in some of the specimens I brought back to the British Museum.

(B) Variations.

Variations occasionally are met with in the plumage and soft parts of Adelies. The least rare of these consist of tufts of white feathers amongst the black plumage of the head. Several specimens so marked were seen at Cape Adare during the summer of 1911-12.

When these white tufts were present the feathers comprising them were usually longer than the black feathers among which they appeared, so that they stuck out in an untidy manner, and were very conspicuous.

In marked distinction to the slight variations above described were the three "Isabelline" varieties that I preserved, and are now to be seen in the British Museum collection. As these variations are very startling, and of the greatest interest, I give below a full description of their plumage and soft parts.

_First specimen_ captured on the Cape Adare rookery on November 4, 1911.

_Iris_, light brown. _Eyelids_, white. _Bill_, light brown. _Feet_, white. _Claws_, light brown.

The whole of the area covered by black feathers in the normal bird was covered by those of a very light fawn, somewhat darker on the neck and shoulders than elsewhere. _Sex_, male.

_Second specimen_ captured on November 14, 1911.

_Iris_, light brown. _Eyelids_, white. _Bill_, light brown; mandible, blackish on dorsum; maxilla, blackish on cutting edges. _Feet_, white on both surfaces. _Claws_, light brown.

In place of the black feathers of the normal bird, there was a fawn-coloured plumage, darkest on head and neck; lightest at bottom of back, back of flippers, and on shoulders.

_Sex_, female.

_Third specimen_ captured on December 23, 1911.

_Iris_, light brown. _Feet_, browny white. _Claws_, brown. _Bill_, brown; very dark on dorsum of mandible. _Eyelids_, white with a pink tinge.

In place of the black feathers of the normal bird, this specimen had those of a very light cream colour: in fact very slightly darker than the white area but deepening in shade to light fawn on the head, neck, and shoulders.

_Sex_, male.

The second specimen had mated with a normal cock. In each case the Isabelline birds were very much more docile than the normal forms. For instance, they did not struggle when picked up, as the others would have done, and the third specimen, when brought into our hut, gazed around with curiosity and apparent contentment, and showed not the least resentment at its captivity. A normal bird would have struggled and fought to the last extremity. Each bird was killed with chloroform.

So carefully did we keep the entire rookery under observation that I do not think it likely there were any more Isabelline forms. Thus we can conclude 3/750000 roughly represents the proportion of Isabelline forms among the species.