A martlet in English heraldry is a mythical bird without feet that never roosts from the moment of its drop-birth until its death fall; martlets are proposed to be continuously on the wing. It is a compelling allegory for continuous effort, expressed in heraldic charge depicting a stylised bird similar to a swift or a house martin, without feet. It should be distinguished from the merlette of French heraldry, which is a duck-like bird with a swan-neck and chopped-off beak and legs. The Common Swift rarely lands outside breeding season, and sleeps while airborne.

The swifts are a family, Apodidae, of highly aerial birds. They are superficially similar to swallows, but are not closely related to any passerine species. Swifts are placed in the order Apodiformes with hummingbirds. The treeswifts are closely related to the true swifts, but form a separate family, the Hemiprocnidae.