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Rh fully recognised, there will always be misunderstandings upon the moult of this bird, for almost every Grouse in the country is to some extent infested with parasitic worms, and there are years when irregularity of moult is the rule rather than the exception. Moreover, it so happens that in autumn, when birds are being shot in large numbers, the survivors of the two worst months of the year for "Grouse Disease" mortality, that is, the survivors of May and June, are all convalescing; but they are convalescing with their plumage changes all retarded and put completely out of order and routine. In this way it is possible in September to kill two birds on the same day, both of which have the chestnut-coloured feathers of the winter plumage on the chin and throat; but upon examination it may be seen that in one bird the edges of these feathers are frayed and worn and the colour faded, showing that they have survived from the previous winter plumage; whereas in the other bird they are hardly free of the scaly sheaths in which they grew, and are really precocious feathers of the coming winter plumage. This is only one of the many traps which result from the deleterious influence which disease exerts upon a bird's capacity for feather growth and replacement, and so upon the regularity of its moult.

There is another point to which attention must be drawn before entering upon a systematic description of the monthly changes of feather in the cock Grouse. It is as to whether the autumn plumage of the cock can be correctly described as an "eclipse" plumage, comparable as it obviously is in character with the spring breeding plumage in the hen, but appearing just two months later and after the breeding season. In each sex the general change from winter to summer may be described as a change from a more richly pigmented, darker, black and chestnut, or rufous-chestnut plumage with rather fine transverse black markings, sometimes almost vermiculate in character, to a less richly pigmented, paler, buff or rufous-buff" or tawny-buff" plumage with characteristically broad black bars and transverse markings.

In each sex, moreover, the characteristic buff and black broad-banded summer plumage is given its special appearance on the dorsal aspect by the growth of feathers with large black centres and a few buff or tawny-buff subterminal bars of considerable width, and a terminal border or spot of the palest buff, which is a very conspicuous feature on the back of most hens, and often only less conspicuous in the cock. In the cock, however, this plumage appears just two months later, and is less beautifully developed than in the hen.