great vigor; tremendous energy
Idiomatic expression describing overwhelming momentum or force, often used of a person or organization at the peak of their power. The literal image is of a force so strong it could knock down birds in flight. Compare with the variant 飛ぶ鳥を落とす勢い.
See also: 飛ぶ鳥を落とす勢い
He's working with tremendous energy right now.
あの会社は飛ぶ鳥も落とす勢いで成長している。
That company is growing with unstoppable momentum.
Variant using を instead of も; essentially the same meaning and usage, but も is slightly more emphatic ('even birds in flight').
A set phrase combining 飛ぶ鳥 (flying bird), も (even), 落とす (to make fall), and 勢い (force, momentum). The literal meaning is 'momentum that can even knock down flying birds', used figuratively for overwhelming vigor.