How to say "martlet" in Chinese?
martlet /märtlət/

If there's no official translation, you can always use transliteration.  For example 瑪勒鳥 (Mǎ Lēi bird)

Or you can call it '無足燕' , based on the description "swallow without feet"

Edit: 

seems like user3306356 had found the official translation of martlet-- '无足鸟'(no feet bird) ; But I wonder, if it is a kind of swallow, why not call it '無足燕'(no feet swallow)? 

学名岩燕 (scientific name:rock swallow), may be '岩燕' is a better choice?