Western Meadowlark + "Birds w/o Eggs" EOR bonus

Birds like Western Meadowlark instruct all players to play an egg. If the active player chooses to activate this ability, are all other players required to play an egg? The rules say that activating is optional, but once activated, must players lay an egg if possible? This matters for the "birds with no eggs" end of round bonus from the European expansion. I looked in as many rules locations I could find (including this forum's FAQ) but couldn't find an answer.