In the song sparrow they vary to such an extent
that in some cases they are reduced to narrow lines; in others so
enlarged as to cover the greater part of the breast and sides of the
body, sometimes uniting on the middle of the breast into a nearly
continuous patch."
Mr. Allen then goes on to particularise several species in which such
variations occur, giving cases in which two specimens taken at the same
place on the same day exhibited the two extremes of coloration. Another
set of variations is thus described: "The white markings so common on
the wings and tails of birds, as the bars formed by the white tips of
the greater wing-coverts, the white patch occasionally present at the
base of the primary quills, or the white band crossing them, and the
white patch near the end of the outer tail-feathers are also extremely
liable to variation in respect to their extent and the number of
feathers to which, in the same species, these markings extend." It is to
be especially noted that all these varieties are distinct from those
which depend on season, on age, or on sex, and that they are such as
have in many other species been considered to be of specific value.
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