/INTRODUCTION. XV 
tube. In Australia there is a very conspicuous black-and-white Pigeon 
(Carpophaga luctuosa) which always, it is said, prefers to build its slight 
nest on a branch over water. How is it possible for even winged enemies 
to take such a citadel by storm, or harm the parent safely swinging in her 
wonderful cradle? ‘This is undoubtedly the true reason that these nests 
are hung so conspicuously ; the eggs, the young, and the brooding (often 
brightly coloured) parent are all equally safe in such a structure, where 
concealment could serve no special end in shielding them from their 
natural enemies. It seems also a most interesting fact that these domed 
pensile nests are not always so conspicuous as might be imagined. As an 
example, I will take the by no means brilliantly arrayed little Sericornis 
citreogularis of Australia, whose nest is built in the dense and humid 
forests, where the trees are covered with moss, often accumulated in large 
masses at the extremities of the drooping branches. In these masses of 
moss, or suspended to them, the little bird places its nest with so much 
skill that it is impossible to say which are really nests, and which are mere 
festoons of moss, until each is minutely examined. Our next group 
consists of 
Birds in which both sexes are dull in colour, and which build 
covered nests from motives of safety other than concealment.—|[ do 
not think that the fact of dull-coloured females sitting in covered nests can 
be taken as a serious objection to the law of bright-coloured females sitting 
in covered nests; for, as Darwin remarks (‘ Descent of Man, ii. p. 168), 
other advantages may be gained irrespective of concealment, such as 
shelter, greater warmth, or in hot countries protection from the sun, or 
sudden changes of temperature, or, in the case of many domed pensile 
nests built on slender branches, protection from certain enemies, as is the 
case with the Indian Ploceus baya, which makes the entrance to its bottle- 
shaped pendulous nest inverted, so as to baffle the approaches of tree- 
snakes and other enemies. Another explanation may be that these plain- 
coloured birds of both sexes are the descendants of some showy ancestor 
that built in a covered nest ; and this peculiarity has been transmitted to 
an entire genus, and retained, even in cases where the plumage of the bird 
has assumed a more sombre tint, through the laws of Inheritance. It is 
easy to believe that if no direct and special cause for a change arises, the 
nesting-economy will remain unchanged even if the plumage of the bird 
or of any of its descendants changes to less showy tints. 
The Swift (Cypselus apus) and the Sand-Martin (Hirundo riparia) are 
both dull-coloured birds, the sexes are alike, and they build nests concealed 
in holes. But in these cases the colour of the plumage does not influence 
the conditions of nidification. The Swift makes its rude nest in a hole, it 
has, so far as we can determine, no means of protecting itself or its eggs 
from enemies, and consequently retires to such a site where it can rear its 
