martlet
/ˈmɑːtlɪt
/noun
1 Heraldry A bird like a swallow without feet, borne as a charge or a mark of cadency for a fourth son.What the martlet was originally is a matter for dispute. Some claim it was the martin, for in some mediaeval documents it is written as "martenette"....
- The fifth martlet was added because when the charges were placed on a shield, the base looked a bit empty.
- The martlet in French heraldry is called the merlette, represented by a swallow, depicted without legs, and later usually. without a beak.
1.1 literary A swift or house martin.I believe the answer is because a martlet is on old name for the type of bird called in English a Swift....
- The martlet, answering to our cliff swallow, is not so strong and ruddy a looking bird as our species, but it builds much the same and has a similar note. It is more plentiful than our swallow.
Origin
Late Middle English (denoting a swift): from Old French merlet, influenced by martinet (see martin).