owl
鴞 centers on a specific kind of bird: an owl. The meaning is narrow and refers directly to the animal.
鴞 combines 号 (likely phonetic) with 鳥 (bird), forming a character for a bird whose name sounds like 号. The exact historical development is uncertain.
The left side 号 can be thought of as a call sign, and the right side 鳥 is a bird. Picture an owl hooting its distinctive call: 号 + 鳥 = the calling bird, an owl.
For ヨウ, imagine an owl letting out a long, echoing 'yoooou' hoot through the night: yoooou -> ヨウ, the owl's call.