I like interesting new takes on defence games and this one certainly did have quite a nice twist to it. The random bird flying movement meant that it wasn't just about accuracy. It had a bit of 'uncontrollability' to it, which actually made it very interesting.
The upgrades and the bird classes together created a very elaborate upgrading system. And the hunting gear was actually very vital to the gameplay. They were a key to earlier survival when the number of birds were low and there were enemies coming from all directions. Very well thought out.
The difficulty I think didn't quite remain steady throughout the levels progress. As the waves went on, the amount of cash increased a little too far exponentially so that by the 5th or so wave, my flock of birds was so vast and upgraded that they were pounding the enemies like flies in the final 2 or 3 waves. It felt like a pushover near the end and I think you need to keep the difficulty steady with the amount of cash given to the player. I notice that the amount of cash for the enemies goes up each wave. That could be tweaked a little to get that difficulty balance I'm talking about.
While each level is slightly different, the birds and upgrades and everything remain the same and so it becomes very repetitive after 2 levels or so. Might I suggest specific bonus bird types for certain levels. This creates a larger diversity in the levels instead of just a different map setting.
Also, it might not be necessary, but I think there should be some kind of overflow into the next level of playing. Upon finishing level 1, I have to start all over again from scratch in level 2. Now this isn't entirely a bad thing, but it kind of takes away the momentum of the game from finishing strong in the first level. Maybe an overflow - either in terms of cash or number of birds. Maybe maintain a certain percentage of army power into the next level. By retaining a certain level of power from the previous level, you ensure that you can also escalate the difficulty of each level as the game progresses. This makes it much more addictive as the entire thing becomes more of a cumulative gameplay instead of just different stand alone maps.
I like a good defence game and I certainly enjoyed this. Now take it up a notch.