Use a flat brush to paint the background.
Start with your darkest color in the upper right corner.
Use light or dark blue, according to your personal preference.
I first started out darker and then changed my mind and lightened it.
As you move to the left of the painting add a bit of yellow.
Reflected light causes the background to get lighter the farther away from the light source.
I made some marks behind the bird to simulate flight movement.