There's a reason robins' eggs are blue
It seems everybody loves an omelet. Crows, jays, raccoons, weasels — even squirrels and mice — love to eat eggs. They are a complete meal and don't have a lot of defenses, other than mom. They can't run and they can't fight back.
The best defense is concealment. Some birds place their eggs in relatively hidden recesses like a tree cavity or a burrow. Woodpeckers, chickadees, bluebirds, swallows and kingfishers, among others, choose this option, and the eggs are usually white. In the nests of birds with open, cup-shaped nests, eggs tend to be masterfully camouflaged with splotches of brown and beige. Some, such as the eggs of killdeer and nighthawks, are so incredibly marked as to be invisible on the open ground from a couple of feet away. Even the eggs of red-tailed hawks and golden eagles have some brown blotches on them. Ravens always are on the lookout for a free lunch.
Then there are the sky blue eggs of the American robin. They sit in an open cup nest with not a mark on them, exposed to the world any time the parents take a short break from incubation. They might as well put out a sign that says, "Breakfast served here!"
It appears to make no sense, but ornithologists think they may have an explanation. Not all troubles come from predators. There are brood parasites, too. These are birds that lay their eggs in other bird's nests for the host to rear. In the Old World, cuckoos and honeyguides are the major brood parasites. In the New World it is cowbirds.
The eggs of cuckoos develop very fast and hatch sooner than the eggs of the host. In this way the invader gets a head start. Moreover, the young cuckoo will push anything (egg or young) out of the nest that touches its back. The young honeyguide even has temporary hooks on its beak to attack nest mates. Soon the invader has the nest to itself and all the food. Cowbird young also hatch earlier than the eggs of the host, but they merely compete with the other young, and many young of the host starve.
What's a mother to do when it finds a cowbird egg in its nest? First, she can accept the egg, which she will do if she doesn't recognize it. Consequently, she raises fewer young of her own (frequently none) along with the cowbird. Or, if she recognizes the alien egg, she can reject it by removing the egg or abandoning the nest and starting over. A few birds will even build a second nest over the top of the old, burying the cowbird egg.
The second option requires that a mother recognize the egg of the cowbird, which can be difficult. The brown-speckled eggs closely match the camouflaged eggs of many birds.
This may offer a clue to the blue of robin's eggs. Cowbird eggs look nothing like those of a robin. This may be nature's way of helping the robin easily recognize an alien egg and avoid rearing cowbirds. However, the ornithologist in me thinks this is a little too neat. I am still on the lookout for alternative explanations. What do you think?
Stewart Janes is a biology professor at Southern Oregon University. He can be reached at firstname.lastname@example.org.