On the other hand, most birds that nest in cavities (like birdhouses) or in trees or shrubs, lay brightly colored eggs which could be easily spotted if they were on the ground. In this case, the brighter the egg, the healthier the chick; since an egg's brightness indicates plenty of nutritious food - hence a healthy mother.
BIRDS DINE ON COLOR. A bird's intensity of coloring (such as the bright red of a male Cardinal) usually comes from nutrients and pigments ingested in its food. That's a reason why birds seek out the freshest food. It's also a sign of the male bird's fitness to propagate, to rear baby chicks and to defend a good territory (thus, a nutritious food supply).