HETEROCHROSIS IN ESSEX BIRDS AND THEIR EGGS. 173
specially resistent. White examples of the Grey Parrot (Psitta-
cus erithacus) have been several times recorded, but generally
the red tail is retained and white Bullfinches usually retain
the red of the under surface. Miss G. Lister has an albinistic
Chaffinch, a female, taken at Chilworth on 6th January, 1905,
which retains the green of the rump, and a similar instance will
be found under Essex.
Mr. Pycraft argues that this lack of colour is due to weakness
in the secretion of pigment for the reason that between absolutely
white and albino varieties (cases where the iris, beak and legs
are affected) and the normal plumage almost every gradation in
shade may be met. This idea suggests that we are dealing with
something of a pathological nature, which is supported by the
remarkable statement that the absence of colour does not affect
the pattern of the feather, as, for example, in white Peacocks
all the characteristic markings can be traced. Another state-
ment which would appear to affect the immediate question is
that, although some white varieties appear to be congenital and
also to turn up in the offspring of normally coloured birds through
successive generations, yet frequently birds, which assume a white
dress, gain their normal plumage at the next or at a subsequent
moult. Further, partially affected nestlings have been known
to revert to the normal plumage. Fortunately we are in the
position to quote instances of both of the foregoing. The Hon.
Walter Rothschild has recorded examples in the Swallow. In
the summer of 1891 four white birds of this species were hatched
at Aylesbury and were allowed to escape. In 1892 the same pair
of birds—it is not stated how they were proved to be the same
birds—reared one white bird out of an otherwise normal brood ;
in 1893 they produced two white and three normal young ; and
in 1894 two white young, male and female, and two normally
coloured birds. Another nest in 1894, in the same town, also
contained two white young, females, and two or three normal
birds. It is suggested that the parents of these were probably
related to the first pair. In 1895 this pair produced two broods,
the first consisting of three white and two normally coloured birds,
which all flew away on 29th June, the second of one white—stated
to be a female and now in the Tring Museum—and four normal
birds. The view that certain birds have a predisposition to
beget white young is further supported by the case of a nest of