Looks like it was done on several layers.
The background is probably just a cloudy sky with his own adjustments.
The birds are probably just bird brushes and maybe the trees as well.
He may have drawn the houses in black and just cropped the window holes and drew some window lines in them...behind which it looks like a later with a fill colour of yellow.
Must say good composition and good idea and probably took the person some time to put it all together.
It would take quite some time to explain this is detail..you would need to know all about layers and adjustments.
Eddie