Chap. XIII. 
DOUBLE ANNUAL MOULT. 
S] 
summer-plumage^ whicli seems, wlien there is no change 
of colour, the most probable cause of a double moult. 
Secondly, there are birds, for instance certain species of 
Totanus and other grallatores, the sexes of which re- 
semble each other, but have a slightly different summer 
and winter plumage. The difference, however, in colour 
in these cases is so slight that it can hardly be an 
advantage to them ; and it may, perhaps, be attributed 
to the direct action of the different conditions to which 
the birds are exposed during the two seasons. Tliirdly, 
there are many other birds the sexes of which are 
alike, but which are widely different in their summer 
and winter plumage. Fourthly, there are birds, the 
sexes of which differ from each other in colour; but 
the, females, though moulting twice, retain the same 
colours throughout the year, whilst the males undergo 
a change, sometimes, as with certain bustards, a great 
change of colour. Fifthly and lastly, there are birds 
the sexes of which differ from each other in both 
their summer and winter plumage, but the male un- 
dergoes a greater amount of change at each recurrent 
season than the female — of which the .Ruff (Machetes 
jpugnax) offers a good instance. 
With respect to the cause or purpose of the differences 
in colour between the summer and winter plumage, this 
may in some instances, as with the ptarmigan,'^^ serve 
during both seasons as a protection. When the dif- 
ference betw^een the two plumages is slight it may 
perhaps be attributed, as already remarked, to the 
The brown mottled summer plumage of the ptarmigan is of as 
much importance to it, as a protection, as the white winter plumage ; 
for in Scandinavia, duiing the spring, wdien the snow has disappeared, 
this bird is known to suffer greatly from birds of prey, before it has 
acquired its summer dress : see Wilhelm von Wright, in Lloyd, ‘ Game 
Birds of Sweden,’ 1867, p. 125. 
VOL. If. 
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