By Robert Mulvihill, National Aviary ornithologist
This is one of a series presented by the National Aviary, which works to inspire respect for nature through an appreciation of birds.
In Central and South America, a group of dull-colored quail- and grouse-like birds called tinamous may lay the most beautiful of all wild bird eggs. A veritable rainbow of colors is represented in the eggs of the nearly 50 tinamou species, including yellow, green, turquoise blue, purple, steel gray and chocolate brown. Adding to their colorful beauty, tinamou eggs have an unusually glossy surface that makes them look like glass or porcelain works of art.
But beneath the surface of all birds' eggs, the shell is just plain white, the natural color of the calcium bicarbonate from which the shell is formed. In fact, some birds lay plain white eggs, including woodpeckers and kingfishers, both of which lay their eggs inside a dark nest cavity where colors and patterns could not even be seen.
The addition of colors and markings to egg shells is common in birds that lay their eggs in the open and has a variety of possible purposes: protecting the eggs from solar radiation; adding structural strength to the shell; enabling colonially nesting seabirds, such as common murres, to recognize their eggs from among those of many other females crowded around it; and, most of all, providing camouflage.
Some of the most amazing examples of camouflaged eggs can be found among the ground-nesting shorebirds, including the endangered piping plover, which nests on sandy beaches, and the many species that nest across the vast Arctic tundra. The eggs of the American golden plover (one lives in the Grasslands Exhibit at the National Aviary) are colored and patterned in a way that closely matches the surrounding lichens and rocks. Another species of plover, the familiar killdeer, also has very cryptic eggs that it lays in a gravel "scrape" along road edges, in parking lots and even on flat gravel rooftops.
Not all eggs appear to be colored and patterned in a way that makes them inconspicuous -- case in point, the robin's bright blue eggs. Recent experimental research suggests that blue egg coloration, which comes from a pigment called biliverdin, may have evolved as a signal of health and fitness in female robins, which, in turn, elicits higher levels of parental care from their mates.
Finally, in addition to their many colors and patterns, birds' eggs naturally come in a wide range of sizes. Not surprisingly, the largest bird egg of all -- 6 by 8 inches and weighing up to three pounds -- is laid by the largest bird, the ostrich. In contrast, the smallest egg, less than half an inch in diameter and weighing just one-hundredth of an ounce, is laid by the vervain hummingbird. But, for its size, the ostrich doesn't really lay an exceptionally large egg (although it does lay a large number of eggs). In fact, one of its eggs is less than 2 percent of an adult bird's body weight. The hummingbird's egg is proportionately much larger, at 10 percent of its body weight. The world champion egg-layer is the flightless brown kiwi from New Zealand, whose single enormous egg is a whopping 25 percent of the female's body weight.
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