Oh, sorry. Well, in my opinion the hardest to identify is the crow - trying to make sure you shoot a crow and not a rook or raven etc so, Clicky. And swans and geese because of the amount of different ones so clicky. Oh and gulls because they all look so damn similar clicky.
The rest of the species if you don't know what they look like just google them, but in my opinion that covers all the tricky ones.