114 CONTROLLED NATURAL SELECTION 
For these several reasons, it is not necessary 
for the females of the above birds, although 
making open nests, to be cryptically coloured. 
This completes the list of British Birds, with 
the exception of the Phalaropes and Dotterel, 
in which the female sex is the more brightly 
coloured one. In these birds, the male in- 
cubates the eggs in an open nest, and so only 
the female retains the conspicuous colouring 
that is used for the protection of young. 
One other fact is of some interest: many cases have 
been observed of females assuming male plumage; this 
change of plumage has been found to be associated with 
sterility; whatever the mechanism by which this is 
brought about, it is clearly, according to the theory, an 
advantage to the species. Males are more conspicuous 
than females because they are less valuable, therefore a 
sterile female, because she is less valuable than a fertile 
one, becomes conspicuous. 
It will be well to review the facts which 
the theory correlates regarding the colour of 
British Birds: it accounts for 
1. The presence of conspicuous colouring 
in both sexes. 
2. The more conspicuous colouring of the 
male in these birds. 
3. The presence of conspicuous colouring 
in the male only. 
