Gray Catbird nest with eggs. Photo by Flickr user JMK Birder.

In my last post I covered Killdeer eggs and nests, focusing on their pyriform shape and mottled coloration. Here I’d like to talk a bit about egg pigmentation in more detail, since variation in patterns and colors is so fascinating in itself!

We saw with Killdeer that the spotted coloration of the eggs helped them blend in better with their surroundings, but what about eggs that aren’t marked at all? Well, white eggs, as we might assume, don’t have much camouflage potential unless placed in a white background, which is essentially limited to very light sand or gravel. White eggs, therefore, need to be disguised in other ways. They can be covered by things like feathers or vegetation, which is what many waterbirds do–wet leaves or seaweed can even stain parts of the eggs brown! They can also be laid in burrows or cavities where they won’t be seen anyway, which is what many woodpeckers, parrots, and owls, among other species, do.

White eggs, since they contain no pigmentation derived from blood cells, cost the least amount of energy and nutrients to produce, and as long as the eggs are either hidden in the ways described above or well-guarded by the parents the female can benefit from not expending energy on pigmentation. A group that helps prove the guarding/camouflage relationship is gulls: larger species like Herring Gulls and Great Black-backed Gulls have more variable egg colorations and a higher proportion of lighter eggs than smaller species, probably since their size and aggression helps protect the eggs more than camouflage. Keep in mind that pigmentation probably is not an egg-by-egg decision made by the female–although in many species each egg might have significantly different densities of spots, this is likely a mechanical result of the mother’s energy stores at the time of shell formation, or pure chance; also, a species that lays white eggs, such as the Barn Owl, should never have eggs that are pigmented, while a species that lays blue eggs, such as the Eastern Bluebird, has been known to lay white eggs (less than 5% of the time).

Various egg types (from left to right): scrawled (Bronze-winged Jacana), wreathed (Sharp-shinned Hawk), dotted (American Coot), capped (Eurasian Golden-Plover), streaked (Great Crested Flycatcher), overlaid (American Woodcock), and splashed (Royal Tern). Photo by Carrol L. Henderson; University of Texas Press.

Page from Eyewitness Handbooks: Birds’ Eggs by M. Walters; photography by H. Taylor, Natural History Museum, London. Published by Dorling Kindersley, Inc.

Eggs that are pigmented derive their patterns from the way the egg moves in the mother’s uterus from the isthmus, where the shell is formed. Pigments are secreted from the lining of the oviduct onto the shell being created, and as the egg shifts and twists in the uterus, spots or streaks are formed, appearing most distinctly as scrawls like the Bronze-winged Jacana egg above and especially the Common Murre eggs (described briefly in my last post for their pyriformity) on the left. The varying darkness of blotches we saw in the Killdeer eggs (or in the American Woodcock egg above) can be attributed to the stage of shell development at which the pigment was emitted. If the egg moves quickly through the uterus as pigment is oozing from the oviduct lining, coloration tends to form in longitudinal (vertical) streaks like the Great Crested Flycatcher’s egg; if the egg moves slowly, more significant spotting occurs. Since the egg enters the uterus with its larger, blunt end first, this end of the egg often has more markings than the top, and sometimes the egg can look “capped” or “wreathed,” like two of the eggs above.

A question that remains for me is why certain birds lay eggs that are a solid color that doesn’t seem to support camouflage, like the Eastern Bluebird’s blue eggs mentioned earlier. Since this species is a cavity nester, blue pigmentation shouldn’t help hide the egg, and also doesn’t support the common hypothesis that white eggs help parents see their eggs in dark cavities. Some other species with blue eggs, such as the Gray Catbird and Hodgson’s Shortwing (found in parts of Asia), put their nests in relatively deep and dense foliage, where the egg is well-hidden. As far as I can tell, the reasons behind blue pigmentation remain a mystery!