Dafydd ap Gwilym had a poem about a seagull acting as messenger between the lovesick swain and his isolated sweetheart--so the idea has been in Wales for a long time, and any bird will do, really! In French songs it's the "rossignolet sauvage," while English and Scots sing "if I were a blackbird."

The Wikipedia article on the Kookaburra song, here, suggests the basis of the lawsuit: it seems the Welsh song, whose words predate "Kookaburra," may have adopted the (copyright-protected) "Kookaburra" tune. If so, I'd say that recordings of the Welsh song using that tune are indeed in violation of copyright.

Whether one cares or not is a different question--I can respect both viewpoints.