The Zoologist/4th series, vol 4 (1900)/Issue 703/On the Moult and Colour Changes of the Corncrake (''Crex pratensis''), Bonhote
ON THE MOULT AND COLOUR CHANGES OF
THE CORNCRAKE (CREX PRATENSIS).
By J.L. Bonhote.
Having for several years successfully kept, and on one occasion bred, the Corncrake (Crex pratensis) in confinement, and having also had several wild caught specimens through my hands during the same period, the following results of my observations on the moult and colour change of this species may prove of some interest. In common with many other birds, the Corncrake has two moults in the year, the chief one taking place in July and August, as soon as the breeding season is over, and involving a change of every feather; the other taking place in February, while they are still in their winter quarters, and involving a change of all the feathers except the tail and wing quills. In the autumn moult the primaries and secondaries in both sexes are thrown off simultaneously, and for about ten days the birds are totally incapable of flight. On two occasions I have had wild birds brought in which had been captured when in this condition.
The males can be easily distinguished from the females during the summer months, as the breast, neck, and sides of the head are during that time of a delicate slaty grey colour. The plumage of the female hardly differs throughout the year.
At the autumn moult, however, the males lose the slate-colour on the breast and neck, and assume a plumage similar to that of the females.
The plumage assumed after the spring moult is precisely similar in both sexes to that which has just been discarded; in the male, however, the feathers on the breast and head, as soon as they are fully grown, begin to assume, by a change of colour, the bluish tinge of the breeding-season dress, and, as summer advances, the edgings of these feathers in both sexes wear off by abrasion.
A change somewhat similar to this is found in the Great Northern Diver (Colymbus glacialis), and probably in other species as well. It is comparatively difficult to procure an individual of C. glacialis in full winter plumage (as described by Mr. Cecil Smith in his 'Birds of Somerset,' p. 540), which resembles somewhat that of the immature birds; for as soon as the feather is formed, or even while growing, it commences, by a change of colour in the feather itself, to assume the distinguishing features of the breeding plumage for the following year; it is therefore possible to find in the same individual worn feathers of last year's breeding-dress, new feathers that have not yet changed but are of a uniform bluish grey, and those which have already by change of colour assumed the white spots and dark ground of the next year's breeding dress.
Returning to the Corncrake, the similarity of the moult in both sexes, with the subsequent although practically simultaneous assumption of the breeding plumage by the male, tends to show that, in this instance, the reason of the moult is not for the assumption of the breeding dress, for in that case it ought only to be undergone by the male. The evidence before us seems to point to the fact that the moult has no connection with the colour-change, but is due to some ulterior cause of which we are at present ignorant.
The method by which this change of colour is brought about is very hard to determine, but the following notes of a microscopical examination of feathers in process of change, taken from a living bird a few minutes before examination, may be of interest.
If a feather be taken which is fully grown and in process of changing, and be microscopically examined by transmitted light, with a low power of about 66 diameters (with a higher power it was difficult to get satisfactory results by reflected light), we find that, whether the part of the feather be blue or brown, there is absolutely no difference in the arrangement of the rami or radii, and that both are opaque and show no colour. If these same parts be then examined by reflected light, the brownish part appears dull, the rami and radii being both of a light-brown colour, but the radii considerably darker; the rami on the bluish part appear bright and of a whitish colour, while the radii are clear and apparently colourless.
Lastly, we may briefly consider the order in which the feathers come on the nestling. The first feathers show themselves when the bird is about three days old, and are those which immediately surround the ear, about six in number on each side. Next come the two ventral tracts, beginning at their lower end and gradually stretching up the neck. These are followed by the scapulars and tracts over the thighs, which in turn are almost immediately followed by the dorsal tract. This tract arises simultaneously over the greater part of its length, and then spreads both upwards and downwards. The remainder of the small feathers on the head, legs, and vent are the next to grow, and finally, after a short interval, the primaries, secondaries, and tail quills, followed by their coverts.
The young birds are able to fly about seven weeks after hatching; they are similar in plumage to the female, but the brown edgings to the feathers of the back are much broader; they do not moult till the following February, when they assume the adult dress in a similar manner to their parents.
- See Zool., th ser., vol. i., p. 35.