Birds for scale!
MORTAL KOMBAT TM & © Warner Bros. Entertainment Inc. (s24)
The bird is essentially lerping between two states: Flapping and Gliding, all of which are being done via WPO animation in the material. Also, I modeled this bird myself and as you can clearly see, it's perfect.
They do move in herds. Location is being driven through a super basic Niagara emitter which also sends a random value into each bird's material instance to offset their animation timing.