The first pair of birds was done in watercolor. I started with the main shapes of the birds, then adding more layers of details as I progressed. With watercolor, you can add darker tones on top (this is called glazing), but only add highlights when you mix the paint with gouache white. Birds often use similar postures when they sit or rest, which makes them great subjects for these sort of stylized illustrations.
For the second pair of birds I worked with gouache: Adding gouache to the mix shows how different this kind of paint reacts. You can easily correct mistakes, or change colors completely. It has a very graphic look. I added details with PH Martins Bleedproof White here, but of course you can use white gouache, too.