Runes allow customizing birds based on your play style preferences. Equipping runes increases their power even further. The following traits can be enhanced by having the runes equipped on a bird.

Damage
Health
Critical Hit Damage
Critical Hit Chance

There are various rune types and their power depends on how ancient they are. The runes are as following:

Common
Rare
Epic
Legendary

Please keep in mind that using the same colored runes does not grant additional benefits. Each rune boosts an effect, and you need to check if it is going to boost the effect you would like to enhance. Please keep in mind that Legendary runes are the strongest. 

IMPORTANT: Equipping a rune on a bird doesn’t require additional in-game resources, however, removing a single rune from a bird will require 1000 in-game coins. Once this is done the rune should back to your runes inventory, but in some cases it may be transferred to your game inbox.