I feel like the starting units of ducks and chickens get unforgivingly expensive the first time you update them. I've tried to upgrade mana regen several times, and it hasn't been enough to make these units feasible to churn out on a frequent basis. This is especially the case with level 15 or so - considering how fast the frogs keep coming, waiting that long to get 30 or 35 mana means you won't have a big enough force to push them back. At the same time, all of the birds are also the fastest units, meaning that if you just created a bunch of the other units, the frogs will still push them back anyway. I think that would probably be the biggest key to balancing out the game - either that, or making mana regen and costs much more forgiving in general.
Otherwise, great fun!