("Terts") -- Flight feathers borne on the basal joint (humerus) of the wing.

Feathers attached to the "upper arm" of the wing, closest to the body. In cranes, the tertials are often modified for display. When the wings are folded, the tertials look like long tail feathers. The "bustle" on the Eurasian crane consists of tertials.

flight feathers attached to upper arm (humerus)