Egg Colors and Shapes

The Chicago Peregrine Program inspired me to write a quick blog on the colors and shapes of eggs. Bald eagles have white eggs, peregrine falcons have eggs that range from light cream through brick red, and red-tailed hawks have pale eggs that are lightly splotched with brown. How and why do the birds we watch lay differently-colored and shaped eggs?

Eggs of American Birds

Eggs of American Birds

In general, female birds inherit egg colors and patterns from their female parents. Egg-shell is made primarily of calcium carbonate, a white material, so the default color of all eggs could be considered white. As an egg moves down a female bird’s oviduct, it squeezes or presses against glands that produce colored pigments. Color may be applied relatively evenly or in drips and drabs depending on the species and the speed of the egg. If the egg is stationary or moving very slowly through the female’s oviduct, it may be solid, blotched, or spotted. If it is in motion, it will be streaked.

Coloring eggs carries a metabolic cost, so why aren’t all bird eggs white? It’s believed that birds with white or nearly white eggs hide their eggs from predators without the use of color. They might nest in cavities like barn owls, cover their eggs in vegetation like geese, or begin incubation immediately, like bald eagles. Since the eggs aren’t visible to predators, camouflage colors and/or cryptic markings don’t provide a survival advantage. Birds that lay colored eggs tend to nest in places or ways that are more visible to predators. Peregrine falcons, for example, don’t usually begin full incubation until after their third egg is laid. The red color and light speckling helps conceal peregrine eggs when Mom and Dad aren’t sitting on them and could make the eggs harder for nest invaders like raccoon to find. Ten or 20 seconds might buy enough time for enraged parents to drive nest intruders away.

Predators aren’t the only problem birds face. Some birds commonly dump or lay eggs in the nests of other birds. Splotched, spotted, or streaked eggs may help individual birds recognize their own particular markings and reject eggs that don’t match. So why don’t Canada geese, which egg-dump, lay patterned or marked eggs? In this case, non-parental eggs probably don’t really impact the survivability of parental eggs. Canada geese are precoccial, so young require less parental investment once the eggs hatch. Canada geese also time hatching quite tightly, so an egg dumped at the wrong time won’t survive.

How about egg shape? Peregrine falcons, Bald eagles, and Red-tailed hawks lay differently colored eggs, but the eggs of all three species are elliptical or oval in shape. We used to think that egg shape was influenced by clutch size, egg stacking, calcium availability, and/or ‘the roll factor’ – ie, heavily tapered eggs roll in a tight circle instead of rolling off ledges. But in 2017, Professor Mary Caswell Stoddard and her team found that the shape of an egg correlates with the hand wing index, a measure of the shape of the wing. Fast, frequent flyers have longer, narrower, pointier wings and longer, narrower, pointier eggs. Slower, less-frequent fliers have shorter, broader, more rounded wings, and shorter, broader, more rounded eggs. How weird is that?

Why would flying ability influence egg shape? The maximum size or width of a stretched oviduct is constrained by a bird’s body size. Faster, frequent flyers have reduced body sizes and abdominal cavities relative to slower, less-frequent fliers. Their muscular, streamlined body plans and narrower oviducts can’t accommodate large, round eggs, but their eggs still have to carry enough nutrients to support embryonic development. These birds maximize egg volume by forming elliptical (oval) or asymmetrical (one round, blunt end and one narrow, pointier end) eggs that can pass through their narrow oviducts while still carrying the nutrients their embryos need to develop and grow.

How do they do it? While I tend to think of eggs as being shaped by their stiff outer shells, an egg’s inner membrane determines its shape. Birds that form elliptical or asymmetrical eggs lay down a membrane that is thicker on the big end and thinner on the pointy end. As an egg moves through a bird’s oviduct prior to eggshell development, the thinner end is squeezed and elongated to produce an asymmetrical egg.

In general, egg color and shape is influenced by survival. Female birds that produce more young will out-compete female birds that don’t. Egg-color and shape may be influenced by overall health (healthier birds tend to lay more vibrant eggs), metabolic cost, the need to hide from predators, the need to identify one’s own eggs, and the shape imposed on an egg by the parent bird’s body plan. In all cases, our parents have demonstrated egg-ceptional egg-care and we look forward to young birds each spring!

Things that helped me learn and write about this topic: