Where'd a Bird Be, Without Alulae?

Answer - crashing, mostly.

We'd have no trouble, I think, in recognising the essential role in flight of the great driving flight feathers (remiges) or the steering, braking tail feathers (rectrices). But what would you think if I suggested that the alulae are of equal importance - even (or perhaps especially) if I called them by the more-used term 'bastard wing'?

Well, let's at least see what they look like.

Great Egret Ardea modesta alighting, Jerrabomberra Wetlands, Canberra.
The alulae can be seen half way along the top edge of each wing on the 'elbow',
little clumps of three to six feathers which can be raised as required.

The key to their importance lies in what the egret is doing - landing. This is where alulae mostly perform their essential service. My attempted explanation as to what they do will be simplistic for two significant reasons; firstly my grasp of physics is always tenuous at best, and secondly even the professional aerodynamics fraternity (they do seem to be mostly blokes) don't seem able to agree in any detail how a wing actually works, be it of bird or plane.

As the leading edge of a wing tilts up (or as the experts would have it, the 'angle of attack increases') two significant things happen. Firstly the lift increases, as a 'pressure shadow' is formed behind the spread upright wing and the wing is pushed up - this is how a bird achieves acceleration and climb. But as the area of wing facing the wind increases, so does drag, so it has to flap faster to compensate and produce nett thrust to move forward and up. Now, air turbulence over the wing further increases drag and the higher the angle of the wing, the higher the turbulence. This seems to be manageable in most situations, but there is one inevitable part of any flight where the wing is virtually at 90 degrees to the ground, which would seem inevitably to create disastrous turbulence, hence impossible drag - and a crash. This is the moment illustrated by the egret above, just before it lands.

And here, as doubtless you've divined, is where the alulae come in. They are erected from the wing like mini-wings, creating a slot through which air flows over the wing surface, nicely smooth and even and not at all turbulent. I'm not sure if the professionals don't really understand how it works, or if it's just that I can't understand their explanations - always a likely scenario - but either way I can't explain it to you. The important thing however is that counter-intuitively, rather than stall and crash the bird touches lightly down in a way that an Archaeopteryx, lacking alulae, could only have dreamt of.

Andean Condor Vultur gryphus, Torres del Paine NP, Chilean Patagonia.
Everything is spread - wings, tail, feet - to maximise drag, and the alula visible on the left wing
is erected to minimise the turbulence created.

So when did the alula appear on the billions of wings that daily drop their cargoes lightly to earth? Well certainly a very long time ago. We know this thanks to a remarkable fossil from near the World Heritage listed ancient walled town of Cuenca in Spain. While not doubting the city's beauty or historical significance, to me it is most significant for the superbly preserved fossil of Eoalulavis hoyasi, the only known species of its genus. This small bird died 115 million years ago with a belly full of tiny shrimps - and a clear alula!

Eoalulavis hoyasi, with alula indicated.
(In searching for an image to share with you, I came upon this in one of the most unlikely places
I could imagine ever finding myself - a bizarre web site dedicated to 'proving' that evolution has never happened!
As I can't find the image anywhere else on the web, I can only assume that they pinched it, without acknowledgement,
from a book. If this picture is yours, I apologise for being unable to give you due credit.
However it's too good not to use!)

However Eoalulavis ('dawn alula bird', incidentally) left no surviving descendants, so either the alula arose more than once, or it arose a very long time ago - before the separation of the ancestral Eoalulavis and the ancestor of modern birds. Either way it underlines the importance of the alula to successful flight. The feathers of the alula are apparently controlled by the first of the three remaining vestigial finger bones in a bird's wing (technically number 2, as numbers 1 and 5 have long disappeared).

Here are some more illustrations of alulae erected by birds in the process of landing; in some of these pics you may need to click on the photo to enlarge it to see the alula clearly.

Welcome Swallow Hirundo neoxena, alighting to feed dependent young, Canberra.

Australian Pelican Pelecanus conspicillatus skating to a halt on the Georgina River, Camooweal, far west Queensland.
In this case not only wings (with alula visible), tail and feet are splayed to slow the bird down, but apparently the
huge bill as well!

Great-winged Petrel Pterodroma macroptera landing at sea off Wollongong, New South Wales.
(Old scanned slide - sorry.)

The other primary situation where the alula comes into its own, for the same reason - that is to reduce turbulence over near-vertical wings - is when a bird is hovering. This Australian Kestrel Falco cenchroides is illustrating the point quite nicely.

Australian Kestrel hovering near Canberra. Again, all feathers are working hard here, including the obvious alulae.

I find this fascinating; I can only hope you do too, and that you'll pay close attention to the next landing bird you see. If so, Glory Alula!

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