adult bird
Refers to a bird that has reached maturity, as opposed to a chick or juvenile. Used in ornithological and general contexts.
In this forest, many adult birds build nests in spring.
The feather color differs between chicks and adult birds.
雛 is a young, newly hatched bird, whereas 成鳥 is a fully grown adult bird.
Compound of 成 (sei, 'become' or 'mature') and 鳥 (chō, 'bird'). The reading is on'yomi, and the word is a straightforward Sino-Japanese compound.