Thursday, May 30, 2013

IR Reflective Feathers and Facial Disks

While creating owl highlights this morning for our Eaglecrest site, I came across some very close-up footage of a fledgling owl's face. As the capture above shows, the young owl has a lovely ring of IR reflective feathers around its facial disk. I was curious, so I did a little research. If owls can't see IR light, why would they have IR reflective feathers? Although I wasn't able to find much on IR and feathers, it seems most likely that the ring is produced by melanin, which fluoresces under IR light. Melanin not only produces colors ranging from dark black to reddish brown and pale yellow, it also makes feathers stronger, stiffer, and more resistant to wear.

While many birds have some amount of melanin in their feathers, especially stiff wing primaries, owls instead have porphyrin, a pigment that doesn't stiffen feathers. Soft, serrated feathers aid an owl's silent flight by breaking up turbulence. An angry peregrine falcon whooshes by you, but you won't hear an owl coming (cue creepy music here). Owls are the only birds known to use porphyrin in their wing feathers.

The rigidity of a material also impacts the way it reflects sound. Rigid materials reflect sound better and more quickly than softer ones. To quote: "The speed of sound in a medium is determined by a combination of the medium’s rigidity and its density. The more rigid the medium, the faster the speed of sound." A wall or canyon might produce an echo, but a pillow won't. I tried it myself, just to make sure.

So soft feathers are wonderful for silent flight, but they don't reflect sound nearly as well as rigid feathers. I am speculating that our little barn owl has a ring of stiff, melanin-producing feathers around its facial disk. The stiff feathers channel sound better and more quickly than soft ones would - absolutely crucial for an animal that hunts in little or no light. The contrast between the owl's porphyrin and melanin laden feathers produces a ring effect that might not be noticeable in a bird with more melanin. It is interesting to speculate that 'echoes' produced by the irregularly shaped ring might also aid directional hearing. Changes in the speed of sound might help owls gauge prey distance and location much as bats use echolocation (aside: this might be the most hilarious video about echolocation ever made).

We know that an owl's facial disk is special in many ways, and that barn owls in particular seem to be well-adapted for low to no-light hunting. This looks like another example of owl adaptation in action.

Thanks to the camera operators at Eaglecrest for the wonderful video! The following things helped me learn about this topic: