”Many kinds of birds have spotted eggs … Eggs do not grow spotted, but have spots ‘applied’ as they pass through the oviduct, sliding against special pigment-laden pores (which is why the markings so often look streaky).”
So birds color their eggs (beyond the single color brown, white, blue) much like humans do. We paint an egg, applying pigments with brushes. Birds paint their eggs, too, daubing paint on them as the eggs leave their butts. Who knew?
source: Crow Planet: essential wisdom from the urban wilderness by Lyanda Lynn Haupt