This is a seagull I made for The Pathless. I made it without using skeletal meshes or even blendshapes, to keep it cheap. I used a couple tricks to get nice animation regardless! Tech-art break down in thread: #techart #gamedev #UnrealEngine #ThePathless
To get this level of animation fidelity, I first modeled 4 keyframe poses for a flap cycle, kind of like how you would make blendshapes. 
Then I projected each pose to UVs, and transferred them to 4 UV sets on a base mesh in a straight glide pose. I made sure that the poses only differed on horizontal and vertical axes, since the depth is not captured with this technique.
In Unreal, I made a material that blends between each UV set in a loop, using RGBA curves from a Color Curve asset as blend weights. Luckily only 4 poses are needed, since you can only use 4 UV sets per mesh in most realtime projects.
Then I convert the UV coordinates to vertex positions. UE4 materials move verts by offsetting them, but I need to set their positions directly here. You can do this by first scaling all the verts to 0, and then "offsetting" from there. We only set 2 axes since UVs are 2D.
Plugging that into the world position offset gives you a nice flap cycle. But I also blend between flaps and a simple glide anim where I shear the wings up and down to break the monotony of the cycle. I just use some noise to lerp between the flap and this glide shear effect:
This technique has limitations- you can't use UV sets for textures or color (I use vertex colors to shade the model instead), the poses are 2D and you only get 4. But for some things like distant birds that's all you need.
With this trick you can make distant flocks of birds that are cheap enough to use in large quantities, but wont break down if you get too close. Check out more about The Pathless at thepathless.com Or read our devblog for more tech art tricks! giantsquidstudios.com/Blog
