from
The Collaborative International Dictionary of English v.0.48
Tiercel \Tier"cel\, Tiercelet \Tierce"let\, n. [OE. tercel,
tercelet, F. tiercelet, a dim. of (assumed) tiercel, or LL.
tertiolus, dim. fr. L. tertius the third; -- so called,
according to some, because every third bird in the nest is a
male, or, according to others, because the male is the third
part less than female. Cf. {Tercel}.] (Falconry)
The male of various falcons, esp. of the peregrine; also, the
male of the goshawk. --Encyc. Brit.
[1913 Webster]