I was referring to playstyle, visually I think the game looks fine, if maybe some inconsistency in things like the eyes.
As for ideas, possibly a card or species that specializes in stacking with a different species in the den, like a cuckoo bird that can stack in other birds dens.
Maybe a species that can return from the den after a certain number of turns. like 3 or 4 as that how longs games seem to have lasted to me.
Those are just off the top of my head, anyway, good luck with your game.