x INTRODUCTION. 
varied markings, and the nest which contains it with its infinite diversity 
of structure and position. 
Until very recently the great variety of colour in the plumage of birds 
was looked upon as so much ornament of no particular use to the species, 
but for the sole purpose of gratifying the eye and adding to the general 
harmony of animated nature. Within the last thirty years scientific 
research has shown that many of the beautiful colours on the plumage of 
birds materially affect not only their welfare, but, as will shortly be seen, 
that of their young, and consequently the very existence of their species. 
This beauty is not given aimlessly, it has a fixed and definite object—the 
benefit of the species acquiring and possessing it. The student of birds 
must therefore view each varied tint on their plumage not as so much mere 
ornament, but as a factor which is or has been essential to the safety or 
well-being of the species possessing it, which has had its origin in the 
struggle for existence to which each bird is subject, either through natural 
or sexual selection. In like manner the infinite variation of colour, and 
to some extent of form, in the eggs of birds and the endless diversity of 
their nests have had their origin in the subtle laws of variation and 
inheritance, aided by natural selection and the survival of the fittest. 
No writer has investigated this interesting subject so closely as 
Mr. Wallace ; and the views he has taken, together with the conclusions at 
which he has arrived, are probably well known to most of our readers. 
Mr. Wallace’s theory of birds’ nests is said to be far too sweeping and 
arbitrary ; and certainly it does not explain all the facts. He divides birds 
into two great classes—one in which the sexes are alike and of conspicuous 
or showy colours, and which nidificate in a covered site; and the other in 
which there is a marked difference between the colour of the sexes, the 
male being showy and the female sombre, and which nidificate in an open 
site. Nearly all known birds are supposed to come into one or the other 
of these two groups. In each of these great divisions, however, there are 
almost as many exceptions as there are cases that conform to the rule; 
and this has been taken advantage of by Mr. J. A. Allen, the well-known 
American ornithologist, who endeavours, by a critical study of the nidifi- 
cation of North-American birds, to overthrow the whole theory (Bull. 
Nutt. Orn. Club, 1878, p. 23). In treating the subject so far as birds and 
their nests are concerned, I propose to divide birds into the same two 
great groups as Mr. Wallace ; but I shall subdivide them into several mimor 
groups, which will include all the “exceptions” to either great rule. I 
purpose specially to take examples of each, as far as possible, from birds 
inhabiting our own islands, as being most interesting to the student of 
British oology. In the birds belonging to the class which build open nests 
we will notice as the first group 
Birds in which the plumage of the male is bright and conspicuous 
