A field guide to mesozoic birds and other winged dinosaurs

they approach the elbow. This gives the inner wing a rounded profile and
helps bridge any gap that forms between the body and the wing due to short
or missing tertials.
Tertiary feathers, or “tertials”, are present in some birds, in which
they attach to the upper arm (humerus). Most birds generally keep the upper
arm mostly pressed against the body even when in use, and so their tertials
are either absent or reduced to contour feathers similar to those found
on the body. Only specialized soaring birds, which extend the entire arm
into a nearly straight line or maximum length, have tertial feathers which
are similar in appearance to the secondaries. Only some avians are known
with certainty to have had tertiary remiges, though they may have been
present in other euornitheans, such as Ichthyornis, which were probably
gull-like dynamic soaring birds. More primitive birds like Archaeopteryx
lacked tertiary remiges but had contour-like feathers (as well as scapulars)
partially filling the gap between the wing and the body.
The gaps between the arm and the primary, secondary, and tertial
wing feathers (together called remiges) were covered with several layers of
smaller feathers known as coverts. In all but the most primitive birds, the
minor digit also anchors pennaceous feathers, in a structure called the alula
Above: Wing anatomy of a generalized non-avian avialan bird.
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