chicks, fledglings
鷇 centers on young birds: chicks and fledglings, the vulnerable early stage of a bird's life.
鷇 is a rare character combining 鳥 (bird) with a complex upper component that likely contributes the sound. The exact historical development is uncertain.
The lower part is 鳥 (bird), and the upper part looks like a protective covering. Picture a nest sheltering tiny chicks: 鷇 represents those young fledglings.
For コウ, imagine a fledgling's first call: a soft 'coo' sound. Coo -> コウ, the sound of a young bird.