How do birds coordinate their turns?

Question

I am building a plane that looks like an Eagle, so I was watching YouTube videos so I can see how to realistically fly it.

One thing I don't understand is how do birds like this coordinate their turns? The bird's tail is flat and flexes a little, but not very much. You can see the wings warping (Wright Brothers style) to go into and out of turns. But with no rudder, it seems like there's both no way to counter the adverse yaw; and with no vertical fin, nothing to bring the tail around while in the steady-state turn.

I was watching the wing tips and have a theory. It appears the inner wing tip feathers are more splayed out than the outer ones. I'm thinking that might increase drag on the inner wing tip, which both counters adverse yaw and brings the nose around during a steady state turn.

Answer 1

Birds coordinate their turn by rotating their tail along the roll axis. This tilts the lift vector of the tail and makes it work like a rudder.
The roll angle of the bird itself is controlled by wing warping.

You can see this in the video especially when the bird is operating in gliding configuration and the tail is providing positive (upward) lift. During gliding phases, rolling the tail (w.r.t. the body) to inside of the turn reduces slip / increases skid, and rolling to the outside of the turn increases slip / reduces skid). You can see constant small corrections of the tail roll angle, as well as the tail pitch angle to correct for pitch instability.

Answer 2

You might take a look at Prandtl wings, and research by Al Bowers. The lift distribution on most bird wings are different than an most planes. This different lift distribution can mean that the tip of the wing is at such an AoA to actually get a "thrust" out of it -- this is actually what you would see with a winglet. (See 17:54 in the below video.) With the right lift distribution, instead of adverse yaw, you can get proverse yaw.

Birds can warp their wing to customize the effective angle of attack across the wingspan. This is similar to washout on a plane wing, except a bird can change this dynamically in flight. This can be (is) exploited to adjust the drag on each side to cause yaw without a "rudder" -- potentially by causing "thrust" at a wingtip by very heavily "washing out" the wing.

I highly recommend watching the following video of a talk by Al Bowers (NASA researcher) on this topic, he does a much better job of explaining it than I ever could: