This is an amazing game and I prefer it over other similar games because there are multiple mini games to play inside the app. I only have one suggestion which I am sure would improve the game hugely.
This is that since the candies are used for upgrading the cave AND the park, it requires the original (and duel) game mode to be played more than the others to gain more candies. However, the currencies that are gained from playing the park and cave games, diamonds and green crystals respectively, are only used for buying new birds. Therefore, I think that candies should still be earned through the original game mode and used to upgrade the park, but the diamonds earned from the park should be used to upgrade the cave helmet instead of using candies for both. I am certain that this would improve the gameplay and encourage one to play the whole variety of different game modes more rather than just spending most or all of the time earning candies and in this way make the game much more fun.
This is a long review but if Ketchapp do see it, please do take an update into consideration and Iβll update my review to 5. Thank you :D