Silent Flight and Owl Feather Structure Plus John Blakeman

Photograph by John Blakeman


When I opened my email box I was very pleased to find this grand photograph from our raptor man in Ohio John Blakeman--

Donna,
Here's a macro photo of a great horned owl's leading edge left wing feather, showing the structures that silence the wing.

John Blakeman

Thanks John, for the terrific macro shot of the fimbrae, those fringes or serrated looking feather edges on the leading edge of the primaries which are thought to turn big turbulence into any number of mini-turbulences thus decreasing sound. As to exactly how, now there lies a question amongst scientists.

Actually I've been thinking about owl feathers all day, as the deeper you dig on this, like in most science, the more disagreement about what is what, the how, and why, begins to rear its head.

But first some factoids in which I haven't discovered much disagreement--yet.

As we know, birds molt and owls are no exception. They molt in a specific pattern after their young are raised for the season and it usually takes no longer than three months to complete the process ending with all new feathers. Obviously if you are a bird you don't want so many feathers missing at any one time that it affects your ability to do your bird business.

In the case of owls on the U.S. side of the globe, everybody except Barn Owls molt their flight feathers from the inside towards the outside. Barn Owls, always being different, I mean they started out living in caves before barns were available after all, molt their feathers from the center first and replace them on both sides as they go along from there.

As to tail feathers they drop a few at a time. Again the exception, in this case some of the small owls drop all their tail feathers at once. (Now what is the evolutionary advantage of that I wonder?)

Okay, they've got new feathers now but just how do they contribute to silent flight. First off if we're going to be picky, they don't have silent flight, they have flight in which the vibration is more than 2000 hertz so we, the owl, and the prey likely can't hear it, except as pointed out previously-- moths and bats may hear them just fine.

Not only do the fimbrae turn big turbulence from major to any number of mini-turbulences but the secondary feathers across the wing are particularly soft so they may be absorbing sound or turbulence, actually possibly two different things, and the trailing edge of the wing is described as "ragged". Ragged? So far I've not found a macro shot of just specifically in this case what ragged is.

That's really how the digging started.

And, oh dear, the disagreement is major.

First off remember I'd said that owls don't seem to have much in the way of down feathers? That is only true if you decide that the feathers on the legs of Great Horned Owls and the legs and feet of Snowy owls for instance aren't down feathers. Those sections of those owls are certainly covered with fluffy feathers that look like down in the long view, but in some cases they're named by some scientists to be something called something else besides down.

More digging.

So what is the purpose of those feathers? Well, the no-brainer answer by many for the reason Snowies have long fluffy feathers on their feet is they hang out in the Arctic where there aren't a lot of trees to perch on so it helps keep their feet warm while they're standing in snow.

Seems plausible right? And may well be true, but then why do some number of larger owls have extremely fluffy legs as well as some feathering, perhaps of a shorter nature like the Great Horned Owls, on their feet where there are plenty of trees? They don't all hang out in the Arctic after all.

I dig some more.

Another why? Some say, because all that fluffy feathering on their appendages helps decrease sound while in flight. Just how any feather is definitively decreasing the sound of flight is still under hot debate by the way, so that's just one more cherry for the top.

Dig, dig, dig.

Aha! I find an abstract of a paper (a very long abstract) that is comparing the morphology (as it relates to size) of the feathers and their parts of pigeons, Columba livia, and Barn Owls, Tyto alba pratincola. Pigeons being very noisy flyers (not a bad thing for pigeons, being flock birds who practice tactics against predators--my supposition-- or we wouldn't have any) and Barn Owls, being members of the owl silent squad. The two species also weigh about the same so weight isn't a differentiating factor for the feathers. I must say the birds are certainly shaped differently, at least when it comes to birds, but you have to give the science guys a break somewhere.

So far , I've learned that the vanes on those secondary feathers on the owls that supposedly help damp sound and the secondaries on pigeons which one would surmise don't, start out by the skin with the same radius but in the owls, they diminish in size more rapidly as they go than does the pigeon vane.

How might that affect the integrity of the feather in the owl? It makes it less stiff and well--softer, wavier--gulp, dare I say it? It gives the feather the beginnings of FLUFFY.

More as it susses itself out.

Donegal Browne