DOUBLE ANNUAL MOULT. 
81 
Cir. 
4P - Sin. 
of'?. 1T ! er 'P luma o e J which seems, when there is no change 
^ e °lour 5 the most probable cause of a double moult, 
lot ly ’ ^ ler<: are birds, for instance certain species of 
„ c mp antl 0l ' 101 ' grallatores, the sexes of which re- 
an'l ’ 6 eacl1 otlier > but have a slightly different summer 
tl' VlIl ^ ei Pi uma " e . The difference, however, in colour 
a< j ese cases is so slight that it can hardly be an 
to J^ a § e to them ; and it may, perhaps, be attributed 
tli, 1*° c i' rec t action of the different conditions to which 
“ e birds 
tt 
iero 
are exposed during the two seasons. Thirdly, 
a ]jj. are many other birds the sexes of which are 
Hat j’ e ’ but which are widely different in their summer 
St Xp "* nter plumage. Fourthly, there are birds, the 
^ \V'Vt 1 f ilw dfIT.M* vn nnnT\ A"f l'l D V in mlnnr • Tvnt. 
ath ^ Ibroughout the year, whilst the males undergo 
e p i;ln S e , sometimes, as with certain bustards, a great 
tli,? colour. Fifthly and lastly, there are birds 
tliei? eXeS 0t which differ from each other in both 
d e SUn »ner and winter plumage, but the male un- 
°f which differ from each other in colour ; hut 
ouiales, though moulting twice, retain the same 
goes ; 
greater amount of change at each recurrent 
^ an ^be female— of which the Huff ( Machetes 
offers a good instance. 
iu c , res pect to the cause or purpose of the difference 
Uw ,° Ur between the summer and winter plumage, this 
cln? n ? s °uie instances, as with the ptarmigan, 74 serve 
fe ten g both seasons as a protection. When the dif- 
p 6f i Ce between the two plumages is slight it may 
a P s he attributed, as already remarked, to the 
'PUoL brow u mottled Bummer plumage of the ptarmigan is of as 
' 3t ' in p ( , ^ Jr, rtanee to it, as a protection, as the white winter plumage ; 
lnav ' a ! during the spring, when the snow has disappeared, 
Waited u hnown to suffer greatly from birds of prey, before it has 
b h<l s of 15 S SUtQ mer dress : see Wilhelm von Wright, in Lloyd, 1 Game 
V 0L ^ edei V 1S67, p. 125. 
