COLOUR.–The colour of Feathers is due to one of three causes. First, an actual pigment[4] may be present in certain corpuscles, or in diffused solution, and the tint does not then vary according to the incidence of the light. Secondly, it may arise from a pigment overlaid by colourless structures in the form of ridges or imbedded polygonal bodies; here, if the vanes are scraped or held up to the light, the pigmentary colour alone is visible.[5] Thirdly, the colour may be iridescent or prismatic; that is, a blackish {4}pigment 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.[6] 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,[7] and with them the power of flight; while in the first-named Family 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 typical _Gallinae_ the original quills are shed before the possessors are fully grown, and are succeeded by others of proportionately increased size, the power of flight being attained very early.