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]