When the original site, Studies in Heraldry, was active, I invited questions on various aspects of heraldry.
I received the question "Why are martlets depicted without beak and wings?", and Luke Ueda-Sarson sent the following from Japan:
"I believe the answer is because a martlet is on old name for the type of bird called in English a Swift. [as opposed to a Martin, which is member of the swallow (hirondelle in French if I remember my school days French correctly) family that are superficially similar but taxonomically unrelated] Swifts are exceedingly fast flying birds (hence their name - swift meaning fast) - indeed the fastest reliably timed bird flight ever recorded was achieved by a Pacific Spine-tailed Swift. Thus the common appearance of such a superlative bird in heraldry is unsurprising. They are also very common birds, and can be soon throughout Europe evey summer, so are nothing exotic. Swifts have extremely small bills - so small that you really wouldn't notice them at all when drawn at the size of a typical roll of arms - they feed on flying insects by just opening their mouths as they fly, so need no protruding beak like most birds. Swifts also have very short legs indeed, and even if you were to hold a Swift in your hands, you would have difficulty in seeing its feet - they would be tucked up into their feathers. This is because swifts live by hunting flying insects and so need very aerodynamically shaped bodies - so hiding their feet is desirable. It is also because swifts are almost constantly on the wing. Indeed, swifts actually take small periods of sleep as they are flying - they almsot never come down to earth to land. When they do land, they don't land like normal birds, horizontally. This is because all of their (small) claws point forwards, they have no rear facing toes like most birds. Swifts 'land' by clinging to the sides of cliffs (or church towers), and just hang there in a vertical position. Thus drawing a swift without apparent beak and feet is quite accurate."
Luke Ueda-Sarson (part-time bird watcher and part time ancient military historian)
I added what follows
The martlet in French heraldry is called the merlette, represented by a swallow, depicted without legs, and later usually. without a beak. OED gives the meaning of martlet as swift, cypselus apus (from Greek apous, without feet), which, because it was rarely seen on the ground, was thought to have no feet. The swift in turn was confused with the swallow, thus the martlets, hirondelles, in the canting arms of Arundel(1), West Sussex.
It is often confused with the canette(2), duckling, which appears fairly often in French civic heraldry, and which is shown without feet. It often represents a stylised bird of no fixed meaning, and is sometimes called anet in French blazon.
According to Théodore Veyrin-Forrer, Précis d'héraldique, Paris, 1951, Arts Styles et Techniques, p114, "la canette représente la canne ou le canard; si elle est dépourvue du bec et des pattes, elle devient une merlette".
The reasons for the lack of beak and legs are discussed above by Luke Ueda-Sarson. In the medieval period, a further inference was that because they did not land, they did not eat, and therefore did not need beaks.
The word merlette(5) means literally a female blackbird, and in this sense it occurs several times in the civic heraldry of France. However, just as the canette may simply be shown as a stylised bird, so too can the merlette. In civic heraldry, the two terms are more or less interchangeable.
However, I feel that it was through artistic error in showing small several small charges on the shield which deprived them of beak and feet.
The peculiar case of the alérion reinforces the idea of artistic error. It is, in its present form, a completely heraldic beast, although derived initially from the eagle, and appears almost without exception only in the heraldry of France. Here, it is confined principally to the coats of Lorraine (3) and Montmorency (4).
The alérion, of which there is almost always more than one, is depicted on the shield as a small eagle displayed, without talons and beak, and with the wings abaissées. The origin is unknown, but is is highly likely that it was used to depict an eagle when there was more than one of these on a shield.There is a legend that Geoffroy de Bouillon shot three footless birds with one arrow while fighting in the Crusades, and placed them as charges in his arms. The house of Lorraine descending from him continued their use.
On seals and coins of the Ducs de Lorraine, the beaks and talons remained until the end of the thirteenth century, disappearing at the beginning of the fourteenth. This may have been because as the seals became smaller there was less room on the shield in which the engravers could work. It was not until the fifteenth century, when the beak disappeared, that the alerion assumed its present stylised form.
The definition of the alérion is itself a problem, since in medieval bestiaries it was shown as a large eagle like bird to denote the king of the birds. It has been suggested that the early heralds deliberately used it as an ironic and deliberate opposite to the traditional bird of the bestiaries, but I believe that it is more likely that, as is the case with so much in heraldry, the bird was originally an eagle and was changed by either necessity because of the space available on the shield or by copies of examples which were unclear.
The alérion is said to be a rebus on the name of Lorraine. It has also been suggested that the word derives from the Latin alerio, and that the term was first used as pun on Lorraine. The word alerio does not occur in Classical Latin, but Larousse. gives a possible derivation from Frankish *adalaro, eagle, cognate with German adler.
1 Or three martlets sable on a chief embattled gules a lion rampant of the field between two crosses crosslet argent. The arms produced here are from a black and white drawing in Wilfred Scott-Giles, "Civic Heraldry of England & Wales, Dent, 1939. Perversely, the martlets are shown with a beak, and the swallow has both beak and feet.
2 Or three pallets azure on a chief gules three canettes argent. The arms of Godreville, Seine Maritime.
3 One of the branches of Montmorency. Or a cross gules cantonned by four alérions azure. Here the charges are true alérions, without beak or feet.
4 One of the branches of Montmorency. Or a cross gules cantonned by four eaglets displayed azure overall a canton ermine. Here the charges are eaglets, derived from the eagle, and are drawn as such.
5 Thomas de Furnival appears in several early rolls. In the Galloway Roll, the blazon is d'argent a une bende et sis mereletz de gueules