A field guide to mesozoic birds and other winged dinosaurs

longest digit is called the major digit. The third finger is called the minor
digit. The minor digit is usually long and slender in primitive birds, but in
many powered fliers, it became reduced and, in modern birds as well as
some extinct lineages, fused to the major digit. It is likely that the minor
digit was joined to the major digit by skin and soft tissue even in some more
primitive birds. Claws (unguals) are a primitive trait for birds--the ancestral
bird had claws on each of its three digits. The relative sizes and curvatures
of the wing claws varied considerably among early birds, and they may have
been used for everything from preening to climbing to predation and fighting.
In more advanced birds, some or all of the wing claws were lost. This
reduction of wing claws occurred in at least two separate lineages (enantiornitheans
and euornitheans), both times in species with increased flapping
ability. It’s possible that most primitive avialans used their wing claws
primarily for climbing up sufficiently high to launch into the air. When
flapping flight advanced to the point that it was possible for these birds to
launch from the ground, the role of the claws was reduced. Claws may have
been retained as vestigial organs in some lineages; in fact, even some modern
birds retain small claw sheaths on their alular digits.
The outer wing is composed of primary feathers, which attach to
the major digit (both the metacarpals and phalanges). In flying birds, the
vanes of these feathers are typically the most highly asymmetrical, differing
in breadth on either side of the quill. In many birds, the primaries overlap
each other to create a smooth border; however, in some species the outer
primaries are separated or
notched for greater maneuverability.
In some primitive birds
(those without a fused hand,
or carpometacarpus), the flexibility
of the digits in the wing
could have allowed for additional
maneuverability by altering
the wing’s curvature and
the spacing of the feathers.
Secondary feathers,
which attach to the rear forearm
bone (ulna), are usually
broader and more rounded
than primaries. The secondaries
typically form the entirety
of the inboard wing, and point
Above: Basic feather anatomy.
slightly toward the body as
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