The Collaborative International Dictionary of English v.0.48:

Martlet \Mart"let\, n. [F. martinet. See Martin the bird, and
   cf. Martinet a disciplinarian.]
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   1. (Zool.) The European house martin.
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   2. [Cf. F. merlette.] (Her.) A bird without beak or feet; --
      generally assumed to represent a martin. As a mark of
      cadency it denotes the fourth son.
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