82 
SEXUAL SELECTION: BIKDS. 
Part II. 
direct action of tlie conditions of life. But with many 
birds there can hardly bo a doubt that the summer 
plumage is ornamental, even when both sexes are alike. 
We may conclude that this is the case with many 
herons, egrets, &c., for they acquire their beautiful 
plumes only during the breeding-season. Moreover, 
such plumes, top-knots, &c., though possessed by both 
sexes, are occasionally a little more highly developed in 
the male than in the female ; and they resemble the 
plumes and ornaments possessed by the males alone 
of other birds. It is also known that confinement, by 
affecting the reproductive system of male birds, fre- 
quently checks the development of their secondary 
sexual characters, but has no immediate influence on 
any other characters ; and I am informed by Mr. 
Bartlett that eight or nine specimens of the Knot 
{Tringa canutus) retained their unadorned winter plu- 
mage in the Zoological Gardens throughout the year, 
from which fact we may infer that the summer plumage 
though common to both sexes partakes of the nature 
of the exclusively masculine plumage of many other 
birds.'^^ 
From the foregoing facts, more especially from 
neither sex of certain birds changing colour during 
either annual moult, or changing so slightly that the 
change can hardly be of any service to them, and from 
the females of other species moulting twice yet retain- 
ing the same colours throughout the year, we may con- 
clude that the habit of moulting twice in the year has 
In regard to the previous statements on moultiug, see, on snipes, 
&c., Macgillivray, ‘ Hist. Brit. Birds,’ vol. iv. p. 371 ; on Glareolse, 
curlews, and bustards, Jerdon, ‘Birds of India,’ vol. iii. p. 615, 630, 
683; on Totanus, ibid, p. 700; on the plumes of herons, ibid, p. 
738, and Macgillivray, vol. iv. p. 435 and 444, and Mr. Stafford Allen, 
in the ‘ Ibis,’ vol. v. i863, p. 33. 
