4 MOULT CHAP. 
ment may lie beneath a surface, which, whether polished, ridged, 
or pitted, acts as a series of prisms, causing the hue to vary 
according to the relative position of the spectator’s eye and the 
light. This is seen in a remarkable degree in Humming-birds.' 
Not uncommonly the vanes of feathers have an appearance like 
watered silk, due to very indistinct transverse striations. In 
regard to plumage generally, it may be noticed that the markings 
on a feather frequently indicate the age of a bird. In some the 
immature plumage is characterised by light-coloured tips to the 
feathers, which are lost as maturity is reached. _ In other groups, 
and especially in most of the Accipitres or Diurnal Birds of Prey, 
the markings of the immature bird are generally longitudinal, 
and in the adult transverse. In nearly all these cases the change 
is effected at the first moult. Females and young are usually duller 
than males, but in some cases, such as Phalaropus (Limicolae) and 
Eclectus (Psittaci), the hen-birds are the more brightly coloured. 
Moult.—Referring to p. 2, it should be remarked that, after 
the production of a feather, the formative substances become for 
a while dormant, but awake to renewed activity, if accidental 
or periodical loss needs to be made good; and so we naturally 
arrive at the phenomena of the annual Moult, which is often 
“double,” that is, occurring towards autumn, and again in spring. 
Though some Birds do not lose their quill-feathers the first 
year, they normally gain a winter plumage—differing in colour 
from the summer garb—by moulting or shedding their feathers. 
The wing-quills, and even those of the tail, are ordinarily discarded 
in pairs, though not quite simultaneously ; but most Anatidae 
(Swans, Geese and Ducks), and apparently the Phoenicopteridae 
(Flamingos), lose all the former at once,” and with them the 
power of flight; while in the first-named Famjly the males of 
many species assume for several weeks a dress resembling that of 
the female, and are said to undergo an “eclipse.” Young birds 
moult, as a rule, somewhat later than adults, but in the typicai 
Gallinae the original quills are shed before the possessors are 
fully grown, and are succeeded by others of proportionately in- 
creased size, the power of flight being attained very early. 
1 Albinism is due to the absence of pigment; melanism, xanthochroism and 
erythrism are terms implying an abnormal proportion of black, yellow, or red in the 
plumage. They may be caused by food. 
* Tn some cases at least Rails and Water-hens do the same. 
