Understand the alula
WHEN talking about a bird’s wing, most birders think about primaries and secondaries, and maybe the tertials and coverts. Few pay attention to the smallest part of the wing: the alula.
When looking at the skeletal structure of a wing, it is useful to make comparison’s with a human hand. The main bend of the wing, or carpal joint, corresponds to the wrist.
The outer part of the wing, which the primaries attach to, correspond to the hand bones and two digits in humans. A third digit is what has become the alula, and it has long been thought this is the thumb, but there is evidence that it could be derived from the second digit, our forefinger.
The alula is often referred to as the ‘bastard wing’ and literally means ‘small wing’. It consists of several feathers, attached to the small movable digit bone just by the wrist.
These may also be covered by one or more alula coverts. Most passerines have three or four alula feathers, cuckoos may have five or six, while hummingbirds have only two. The alula can be extended and is used when a bird is steeply descending in flight. It also enables a bird to produce extra wing lift when moving at slow speeds, preventing stalling.
It is also used in other ways by some birds. Striped Cuckoo of Central and South America is often called ‘Four-winged Cuckoo’ because of its habit of flexing its large black alulae outwards as it walks along the ground, probably using them to flush out insects. Pavonine Cuckoo behaves similarly, while Pheasant Cuckoo partially extends its primaries and alulae.
Research into Snow Bunting breeding suggests that older male birds, with greater reproductive potential, have larger black alulae which contrast with their white body feathers and are used by a female to assess a male’s condition. Buff-breasted Sandpipers have a display in which wings are raised with the alulae extended. Interestingly, observations of Surf and Whitewinged Scoters showed that they could dive underwater, using their feet, with wings crossed over the back and the alulae extended like small wings.
Some birds retain a claw on the alular digit, notably Hoatzin, which also has a claw on the second digit, as well as several other groups of birds including divers, ratites, gamebirds, finfoots, New World vultures, Secretarybird, screamers and some waders. Chris Harbard