To grow the population of your forest you will need to expand and clear spaces. This is done by clearing Expansion Foliage. Use Melodia to “Clear” away leaves and expand your environment. The new spots you unlock can either be for raising new birds, or flower areas which contain a new Food Flower.
When an Expansion Foliage is tapped the box below the "Clear" button tells you if it is space for more birds or a Food Flower. You can fit two birds in each grow spot but flower spots will not increase your forest population.