The Alula (bastard wing) Of A Kestrel In Flight

Earlier this winter I was able to get some shots of a male American Kestrel in flight.  I was happy to get this image and a few of the others for two reasons.

Kestrels in flight have largely eluded me for eight years now.  These raptors are small, fast and usually fly erratically which makes them very difficult to track with a large lens.  The photographers best chance to get one in full flight (as opposed to at take-off) is to catch it hovering – which they’re prone to do, especially if there’s a bit of a breeze.


american kestrel 8179 ron dudley

 1/2000, f/6.3, ISO 500, 500 f/4, 1.4 tc, natural light, not baited, set up or called in

And hovering is exactly what this bird was doing.  I’ve previously posted a different but similar photo of this bird in flight but back then I hadn’t noticed something else interesting about these images.



american kestrel 8179 alula ron dudley

The alula (plural alulae) is clearly visible.  The alula is the freely moving first digit on the leading edge of the wing of modern birds.  It is the birds “thumb” and is covered by three to five small flight feathers.  The alula is commonly referred to as the “bastard wing” – one of several definitions of the word bastard is “of abnormal shape or irregular size.

Normally the alula is held flush along the surface of the wing and is very difficult to see but when landing or flying at very slow speeds (like hovering) the alula moves forward and upward which produces a small slot on the leading edge of the wing and it becomes more visible.

The function of the alula is similar to that of the slats on the leading edge of the wings of aircraft – to increase the angle of attack of the wing and produce lift, thus helping to prevent a stall during slow flight.



american kestrel 8172 ron dudley

This photo of the same hovering kestrel shows the alulae about as clearly as I’ve ever seen them (despite the fact that the image isn’t very sharp).

Many extinct and ancient relatives of modern birds had alulae, as do flies (insects of order diptera).  I find it fascinating that evolutionary selection pressure has produced this structure in such diverse and relatively unrelated groups as birds and some flying insects.   And that man has (once again) copied nature to solve a modern problem.



38 comments to The Alula (bastard wing) Of A Kestrel In Flight

  • andrew karpinen

    They generate a swirling vortex over the back of the wing to add energy to the boundary layer of air on top of the wing to prevent a stall during high angles of attack. Birds wings are absolutely incredible. Thanks for the pictures. Hope u wanted the details.

  • andrew karpinen

    They generate a swirling vortex over the back of the wing to add energy to the boundary layer of air on top of the wing to prevent a stall during high angles of attack. Birds wings are absolutely incredible. Thanks for the pictures. Hope u wanted the details.

  • Craig

    This is fantastic, thank you! My fluid dynamics professor used raptors (the fighter planes of the aviation world, as he described it) and their splayed primary flight feathers as an example of an adaptation to account for the differences in the low and high pressure on each side of the wing that creates drag-inducing vortices. (Brief explanation here:; man has yet to create a wing as adaptable as that of birds and use workarounds, like the wingtips on many modern airliners.) I had not been aware of the alulae that acheieve the same effect as slats, though.

  • Once again I’ve learned an interesting bird thing! The pictures look beautiful and clear to me. Thanks for the great shots and the intriguing information!

  • Dick Ashford

    Thanks, “teach”! As I think I once told you, I believe a collection of your images and text could/should easily be a best-selling book. I’d like to place my pre-publication order now! Thank you for helping us become more aware of the natural world. Cheers, Dick

  • Susan Manley

    Amazing shots and such good info! I had no idea about the alulae. This info was illustrated perfectly with your excellent photos.

  • Ditto Elephant’s Child comment about the 3rd image. I’ve seen the Alula in other raptors, but never as clearly as in these shots of the hovering kestrel. I enjoyed the discussion about the evolution of reptiles and birds too.

    • Thank you, Dave. Yes, the story of the evolution of birds and their relationsip with reptiles seems to be an ever-changing one as we learn more about it.

  • CharLotte Norton

    Such amazing photos! Such interesting commentary! Thanks so much for sharing!

  • PS: I particularly like the third shot which highlights the alulae so very well.

  • Education (I had not heard of alulae) and beauty in the same post. Megathanks.

  • Dick Harlow

    WOW!! What a great shot!! Wish I was still teaching, I would have used this shot for sure!
    Thanks Ron, keep these fantastic shots coming!

    • Thank you, Dick. Not too late to “unretire” from teaching… 🙂

      • Dick Harlow

        Well, I do that in my way still. Once a teacher always a teacher, but still one or at least I need to be careful.
        You are obviously using your photographs and your editing as a teaching tool, and it is darned good!!
        I’m only partially there, but working on it!

  • Wonderful! I love kestrels, they’re special little raptors.
    Thank you Ron, I’m learning so much from seeing and reading your posts. Kim

  • Carol Keeler

    I always learn so much from reading your blog. Wonderful photography as always.

  • 48dodger

    Ron….This is a great demonstration of bird anatomy! I’ve put this post to my favorites so I can link to it for discussions about the alula. You have a wonderful site here my friend.


  • I love the education I’m getting from your posts, as well as the opportunity to see such beautiful photos.

  • Thanks Ron for introducing me to this term. I knew of this “thumb”, but didn’t know the right term. What doesn’t stop to amaze me is the similarity of reptiles to birds. The few reptiles I’ve been able to get here have so many similar physical traits, it’s fascinating to wonder what is it that actually happened in the course of evolution to separate these two? Why did the birds develop feathers, as opposed the reptiles retaining their scales and terrestrial dwelling? It must have been the amount of trees there where once. There must have been so many trees at some point that some of the reptiles could no longer reach their fruits and seeds; or mountains; maybe there were so many high mountains that land dwelling reptiles could not reach or climb. Does this make any sense?

    • Maria, Darwin’s friend Thomas Huxley called birds “glorified reptiles” and it stuck, for good reason. Some of the questions you raise are still being debated but it’s sure interesting to speculate. Many authorities believe that feathers are actually modified scales, though that isn’t universally accepted.

    • 48dodger

      Most evolutionary theories will state it’s where you live and what you eat that defines who you are. I think my Mom said this to me a few times concerning my room when I was in college. Newer evidence (87 thru the 90’s) reveals that many large dinosaurs had feathers (T-Rex included)which of course ties them closer to modern birds. So, maybe a few reptiles had climbed trees and jumped thier prey on the ground, and over time (read as millions of years) learned to glide, then evolved to fly. It’s kinda strange for me to see chickens, and think “that’s what dinosaurs really looked like”…..but hey, it sure would explain a lot. So next time you see your chicken climbing up a tree, watchout, he might be trying to evolve to the next level! 🙂

      • 48dodger:
        What you say makes more sense to me. They probably already had some feathers already, and were already climbing these trees. And that might have led to the gliding. Interesting stuff. Although, at some point, the land dwelling and the tree dwelling must have differentiated considerably, so thus came the wings. Speculation, I know, but there must have been both the tree and the mountain climbers.

  • Len Boeder

    Hi Ron, Thank you again for fantastic pictures and wonderful information.

  • Julia

    Thank you for this post! I also am very keen on this wonderful trait that the kestrel owns… by way of their alulae. For such a small bird, it has so many incredible virtues! Amazing photos too!

  • Thank you Ron for once again providing cool information and incredible photos too !