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HETEROCHROSIS IN ESSEX BIRDS AND IN
THEIR EGGS.
A Practical Consideration.
By WILLIAM E. GLEGG.
(With 1 Plate.)
IT was my original intention to have dealt with only one
phase of abnormal colour in the plumage of Essex birds,
namely, that which is described as albinism, but as this soon led
me to the extreme in the other direction, namely, melanism,
I felt that the most satisfactory result would be obtained by the
adoption of the present title.
Although the object of the paper is to deal with the subject
as it relates to the County of Essex I have found it necessary in
the first instance to present some of the chief facts which have
been determined regarding the subject as a whole, that is, not
merely from an Essex aspect. Had the writer confined himself to
what can be gleaned from local data, the subject would not have
been intelligible, as so little is known of heterochrosis. Further,
some knowledge of the nature of colour of plumage becomes
necessary. The paper is, therefore, divided into the four
following parts :—
1. To present the main features of the subject as a whole.
2. To outline the nature and causes of colour in plumage.
3. To deal with abnormal colouration in Essex birds.
4. To deal with abnormal colouration in the eggs of Essex
birds.
Part 1.—The Main Features of Heterochrosis.
Heterochrosis, or deviation from normal colouration, pro-
bably presents itself in many aspects, but there are four phases,
namely, albinism, xanthism, erythrism and melanism, of which
something is known, and moreover the acceptance of these defini-
tions facilitates consideration.
Albinism is the term which is used to describe the condition
of birds whose plumage has become white or partially so, or
which has a tendency in this direction. This condition is said to
result from a partial or total absence of the normally present
black pigment in the feathers and other parts. Prof. Sir J. A.
Thomson states that when albinism is complete it implies that