I've started painting in the white underparts; trying out colours on a small area until I'm happy with them. The tricky thing here is to pitch the tone so that it's dark enough (areas that are in shadow should be darker than the sky) but not so dark that all sense of brilliant sunlight is lost. Peachy, yellowish light is reflected on to the undersides of the birds from the grassy/rocky ledge that the birds are about to land on (out of the picture) - this was a difficult colour to mix because to get it dark enough risked making the colour look muddy. I hope I've got it right; it's just a question of painting it in and hoping for the best!