How much can a bird’s egg tell us about the bird that laid it and how it lives? As it turns out, it can tell us a lot about the generalities, but little about the details.
For example, what does the color of a bird’s egg tell us? Many scientists believe that the default color of birds’ eggs is white, and indeed many birds’ eggs are white, including, in some cases, that most familiar of eggs, the hen’s egg.
The eggs of birds that nest in tree cavities, such as woodpeckers and kingfishers, also tend to be white. Scientists think this may be because the eggs are well hidden from predators, and since it’s kind of dark in those holes, the bird parents can more easily see bright white than they could a duller color.
Birds that lay their nests on the ground, such as shorebirds and game birds (for example, turkeys), tend to have eggs that are colored to blend in with their surroundings. Killdeer are technically shorebirds but are found throughout Vermont and New Hampshire, far from water, in open spaces on farms and even in parking lots. Their sandy-colored, speckled eggs are nicely camouflaged on a beach and don’t seem to do a bad job being inconspicuous in the other bare places the killdeer nest either.
But some ground-nesting birds, such as the pied-billed grebe, lay white eggs. Over time, the eggs grow dirty and stained, becoming somewhat camouflaged in that way.
Why are the eggs of so many birds, particularly thrushes, including robins, bluebirds, and hermit thrushes, a beautiful sky blue? I haven’t heard an explanation worth repeating – scientists don’t know. What scientists do know, though, is that the blue color comes from pigments closely related to the pigment that makes blood cells red, so perhaps there’s a connection there somewhere.
I remember learning as a kid that eggs are egg-shaped so that they will roll in a circle, and not in a straight line off a cliff. Sure enough, some of the pointiest bird eggs in the world are laid by birds that nest on cliffs. I’m thinking of murres, which are found on the northern coasts of North America, both Atlantic and Pacific. But there is evidence that pointy eggs and cliff dwelling don’t have to go together. For example, peregrine falcons nest on bare ledges, or on perhaps a scrape on a cliff, but their eggs are no pointier than a hen’s egg, and in some cases a bit rounder.
As it turns out, it’s those shorebirds again that in general seem to lay pointy eggs. The eggs are carefully arranged in the nest so that the pointy ends are together. That allows the eggs to be packed closer together in the nest than round eggs, an advantage if you are a small bird trying to keep a lot of eggs warm.
Having round eggs, though, does offer an advantage. A sphere, or ball shape, is the way to have the most inside (volume) with the least outside (surface area). That’s a benefit if you want to pack as much good stuff as possible inside your egg but limit its exposure to the elements.
The great long-horned owl, which nests in our area, is known for having nearly spherical eggs. This bird might take over a nest from a crow or a hawk, but also, especially in the western U.S., may lay its eggs on the bare ground of a cliff. So much for round eggs rolling off.
The relative size of a bird’s egg does tell you something about the bird that laid it but not necessarily its size. Birds whose young hatch out of the egg ready to walk, and perhaps even swim, are called “precocial.” Waterfowl, such as ducks and geese, are precocial and lay larger eggs than other birds the same size whose young are born helpless.
Becky Suomala says that she and the other biologists at New Hampshire Audubon are sometimes asked to identify eggs that people find. “It’s difficult because so many eggs have similar colorations and sizes,” she says. Sometimes she can narrow it down to a family of birds. For example, looking at her field guide to nests and nestlings, she says, “On the woodpecker page, all the eggs are white and about the same size.”
Besides size and shape, she goes by other clues, such as where an egg was found, in a nest or on the ground, in a forest or in a field. It seems that eggs can tell us a lot, but can’t always tell us the bird that made them.