HETEROCHROSIS IN ESSEX BIRDS AND THEIR EGGS. 177
which turn out yellow instead of green. It is said that the arti-
ficially induced yellow feathers reappear with each moult.
Erythrism is defined as the abnormal replacement of other
colours, generally green or yellow, by red. As is the case with
xanthism not much is known about erythrism, but it has been
shown that it can be produced artificially. A brief reference
to this is all that is necessary here. Canaries fed when young
with cayenne in some cases become a rich orange colour and
in others the plumage becomes crimson. White fowls have
been similarly affected when fed with cayenne pepper. The
natives of the Amazonian region feed the Common Green Parrot
of the region (Chrysotis festiva) with the fat of large siluroid fishes,
and the birds become beautifully variegated with red and yellow
feathers. Hepatic Cuckoos and the rufous examples of the-
Partridge, once known as Perdix montana, are presumably
erythristic examples of their species.
Melanism is the term used to describe the condition of birds
whose plumage has become or is darker than that which is
associated with the normal condition of the species. It is as-
sumed that this condition results from an excess of black or dark
pigment. Melanistic specimens of many birds are stated to have
been described, such as the Bullfinch and Skylark, but one of the
most notable instances is the Snipe, which in this phase was de-
scribed as a distinct species, Scolopax sabinii. As compared to
albinism, however, melanism in birds is almost unknown, but
we must bear in mind that the latter would not be so noticeable as
the former. In any case, if little is known of albinism still less
is known of melanism. As we have already stated it has so far
been found impossible to produce white examples, but it is other-
wise with melanism. Bullfinches, as is well known, may become
black if fed on hemp-seed. Mr. Pycraft makes the definite
statement that, as in the case of the absence of pigment, the
characteristic pattern of the feather is preserved by melanistic
birds, although often this can be demonstrated only in certain
lights.
Part 2.—The Nature and Causes of Colour in Plumage.
During the course of this outline of heterochrosis we have
strayed across the frontier of a closely allied subject. Colour,
as it relates to the plumage of birds, is by no means easy to treat,
as the subject is involved and extensive, so that an attempt to