seagull
鷖 centers on a specific water bird: the seagull. The meaning is narrow and concrete, with no abstract extensions.
鷖 combines 殹 (likely phonetic) with 鳥 (bird) to denote a specific bird, the seagull. The exact historical development of 殹 is uncertain.
The top 殹 looks like a complex shape, but the bottom 鳥 clearly signals a bird. Picture a seagull with a distinctive, intricate wing pattern matching the top part.
For エイ, imagine a seagull gliding over the bay and letting out a long 'A' call: A -> エイ, like the bird's cry echoing across the water.