4 MOULT CHAP. 



iiieut 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 tlie 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 Acci^ntres 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 Phalaro2ni.s (Zimicolae) and 

 Edectus (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 anade 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 tlie 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 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 in- 

 creased size, the power of flight being attained very early. 



■* Albinism is due to tlie absence of pigment ; melanism, xanthochroism and 

 ery thrism are terms im2)lying an abnormal proi)ortion of black, yellow, or red in the 

 plumage. They may be caused by food. 



^ In. some cases at least Rails and Moor-hens do the same. 



