By Kennedy Zittel, naturalist
Bird eggs come in all sorts of colors and patterns, from brilliant blues to muted browns, from plain to polka dots! We may be familiar with bright blue robin’s eggs – but there are many other species such as Eastern Bluebirds, Gray Catbirds, and even Starlings that also lay blue-green eggs. Red-winged blackbirds have pale greenish-blue eggs with dark irregular markings. Tree Swallows lay light pink eggs that gradually fade to white overtime. Black-capped Chickadees’ eggs are white with reddish-brown spots. Speaking of red, Peregrine Falcons lay lava-red colored eggs!
Despite the large variety of colors and patterns that bird eggs can be, all species use just two pigments to produce those colors. Protoporphyrin (produce reddish-brown) and biliverdin (produce bluish-green). By using more of one pigment and less of the other – eggshells can become different colors, shades, and patterns. By using neither pigment a plain white egg can be produced.
Why do eggs come in such a large range of colors and patterns?
Scientists are unsure about the exact reasoning behind every species’ eggshell appearance. One obvious reason is camouflage. Many ground nesting birds (like Killdeer) will lay eggs that are speckled, splotchy, or streaky to help blend in with grasses and rocks on the ground. Many shorebirds (like the Piping Plover) lay speckled eggs to blend in with sand and pebbles along the beach. Many cavity nesting birds (like Eastern Screech Owls) will lay plain white eggs, as they are already hidden. Of course, there are always exceptions.
There are many other reasons for why eggs vary in appearance – such as mimicry, increased laying, and sunlight.
- Some bird species that lay eggs in other bird’s nests can mimic (copy) the appearance of other eggs within the nest. Some parasitic species can even mimic multiple species of bird eggs.
- There are a few ground nesting birds that lay brightly colored eggs – clearly not designed to camouflage! This may attract more females to lay in the same area. The more eggs found in one area, the more likely theirs will survive if a predator comes around (they can’t eat all the eggs, right?)
- Sunlight also plays a role in appearances. Too light of eggs can allow too much sunlight to enter, but too dark can cause the embryo to heat up too much.
To make understanding egg appearances even more complicated, coloring in a single species can also change. Some species seem to “run out of ink” as they lay eggs, with the colors fading with each egg laid. Some birds have even been known to completely switch color and/or pattern at random. For example, it has been documented that some Eastern Bluebirds – who typically lay blue eggs – have begun to lay plain white ones.
There seems to be so much that goes into every color, fleck, stripe, and splotch on a bird’s egg!
Image of Robin eggs from Wikipedia