This will only work if you have downloaded the app and you deleted it. Enjoy!
Step 1: Open the App Store
Open the App Store and go to the tab purchased. This may vary with your ios/android version. Android location may vary. On ios 6 go under "updates".
Step 2: Search Purchased
Go it the search purchased bar and type flappy bird in the bar. There you go! Now just download and flap on!