。
An eagle was flying high up in the sky.
Literal
Eagle [subject-が] one-bird sky high flying-[state-ていた].
Descriptive past-continuous. 一羽 (いちわ) is the counter for birds — 羽 (わ, 'feather' reading) is the bird counter. 飛ぶ ('to fly'). 空高く ('high in the sky') uses the adverbial form 高く. ~ていた is the past continuous / state describing the bird's flight as witnessed.