I don't know of any bird that has a vertical tailfin, but apparently aircraft need them for lateral stability. Why is this?
I did realize, however, that most fish have a vertical tailfin, or a vertical dorsal fin, or both. What exactly is going on there? Can't see how marine creatures would need lateral stability at low speeds in a thick liquid (compared to air).
If that second paragraph sounds off-topic, let me put it another way. What is it about the air, as opposed to water, that requires this? They are both fluids. One is thicker/more viscous than the other, which seems to imply that the thicker one would require less lateral stability. Yet it seems to be the opposite. I hope this will be taken as a valid on-topic aerodynamics question.
(P.S. I'm leaving out stuff like the B-2 Spirit because it uses active dynamic adjustments to stabilize. Its form is inherently unstable.)
IMPORTANT EDIT: I should have pointed out the large difference in speed. Birds of course are slow. So were the early Wright flyers. Modern airliners fly much faster, around Mach 0.85. I haven't yet looked up Reynold's Numbers, but it certainly could be the case that the faster ones need lateral stability while the slower one's don't.
1 more clarification: When I say "stability", I'm not exactly talking about yaw control. My impression is, just when the plane is flying perfectly straight, there are pressure fluctuations that will cause slips unless there is a large vertical surface area to balance out those fluctuations. I could be wrong, and ultimately I'm trying to find out why most planes seem to need this large vertical area.
1 more: Here's an example model that seems synonymous: Let's say our airplane achieves yaw with butterfly flaps at wingtips, which are aligned with the center of mass (the flaps and CoM have the same height) so there's no side-effect torques along another axis other than the yaw axis (z axis). Let's assume no backup yaw control is necessary. This craft is meant to cruise at Mach 0.85. Is a vertical fin necessary to achieve passive directional stability about the yaw axis? We should probably consider cross-winds too, even at takeoff/landing.