Birds need to be levelled up to increase their coin yield. To level up a bird, you need to raise and hatch your bird, and feed it the required amount of Gnats it needs to turn the wing gold. The progress bar will indicate how much Gnats is needed for this :) Each bird species must be merged 5 times to add each of the 4 wings to your bird. Gnats is produced by Gnats flowers. Simply tap a bird and drag it with your finger through the Gnats to collect it!