INTRODUCTION. 
xiii 
(resembling each other as much as they differ from those of all 
other genera), will need to be told that they are closely allied, 
much as the eggs differ in colour. 
With regard to birds of the same species, although the eggs 
of most of them are subject to more or less variety, some are 
constant, or nearly so, in their colouring, whilst others display 
an almost unceasing variety. Amongst the former, are those 
chiefly of one colour, to which may be added a few others, 
which seldom vary much. These are the Golden Oriole, Lesser 
Whitethroat, the various species of Titmice, Reed Bunting, 
Goldfinch, and Black Grouse. The eggs, on the other hand, 
which are subject to the greatest variety, are those of the Water 
Birds, the Gulls, Terns, Guillemots, &c. ; and of the land birds, 
the Red Grouse, the Rook and Carrion Crow, Yellow Bunting, 
and House Sparrow, and above all, the Tree Pipit. There is, 
notwithstanding, in all these a character, by which a little ex¬ 
perience will for the most part enable us to determine the 
species ; and though we might not readily believe the varieties 
of the eggs of the Tree Pipit to belong to the same species, we 
should have no difficulty in referring them to the right genus. 
In other branches of Natural History the larger the series of 
specimens we have for comparison the more likely we are to 
arrive at a correct conclusion as to species. In Oology it is 
otherwise. The more we see of the eggs of the Waders and 
Water birds the more difficult it becomes to point out with 
certainty the demarcation of species, or to fix upon those which 
are the most typical. Colour and form alone are but unsafe 
guides. 
In places where those materials are to be met with which 
instinct has taught individuals of the same species of bird to 
make use of in the construction of their nests, we shall usually 
find the same adopted. I know of no other bird which seems 
so much to consult its own taste in this respect, as the Common 
Wren. The materials of its nest are as different as the situa¬ 
tions in which it is placed. It may be found built entirely of 
clover, even in places where moss is abundant; its interior is 
