By Jon Friedman
Eggs are among the most enchanting and mysterious of natural objects. All birds lay eggs, and with very few exceptions, incubate them. Eggs range in size from those of the Bee Hummingbird (Mellisuga helanae) at less than 1/2 inch long to those of the ostrich at almost 7 by 5-1/2 inches. While it is true that the smallest bird lays the smallest eggs, and the largest birds lay the largest eggs, this proportion of body size to egg size does not always hold true for all birds.
The largest egg ever laid by a bird was that of the extinct Elephant Bird (Aepyornis). It measured 14-1/2 by 9-1/2 inches. It is assumed that it held two gallons of fluid and weighed in at 27 pounds. This was only about 3% of the gigantic bird’s weight. The ostrich’s egg weighs nearly 3 pounds, or about 1.7% of its body weight. The Bee Hummingbird’s egg weighs about 1/2 gram, or around 10% of its body weight. From this, you might think that larger birds lay proportionately smaller eggs. But there are exceptions to the rule. For example, the chicken-sized Kiwi lays an egg that weighs about 1 pound, or approximately 25% of its body weight.
Not all birds that are the same size and weight lay eggs of the same size. For example, the Killdeer and the American Robin are the same size, but the egg of the Killdeer is much larger. The Killdeer eggs require about 26 day to hatch. When the young are hatched, they are well-developed precocial chicks born with feathers and are able to run in a matter of a few minutes. The smaller Robin egg is incubated for only 13 days, but hatches out a helpless, naked, usually blind (eyes open several days after hatching) altricial chick in a much more primitive state than that of the Killdeer. The Robin young remain in the nest for 10 to 14 days, after which time they venture forth, but are still under the care of their parents. Within a species, the younger birds in their first year of nesting lay smaller eggs. Occasionally, a bird will lay an abnormally small, or runt, egg. This aberration usually does not hatch.
The shapes of eggs can vary. Most eggs are like those of chickens, somewhat rounded at one end and pointed at the other. The eggs of owls, ostriches, and kingfishers are round. Swallows and swifts lay elongated ones. Grebe eggs are pointed at both ends. Hummingbirds lay elongated eggs that are blunt at both ends. Shorebirds, murres and quail lay eggs that are sharply pointed at one end and rounded at the other. This exaggerated shape is an adaptation that offers some advantages. Shorebirds usually lay 4 relatively large eggs. With the pointed ends toward the center of the nest, a minimal amount of space is necessary. Quail lay large clutches of 10 or more eggs, and once again, a pear shape economizes the nest space. Murres, auks, and other alcids, i.e. guillemots, puffins, and murrelets, use cliffs as nesting sites. A pear-like shaped egg tends to roll around on its axis rather than roll off a cliff like a round one would.
Birds of closely related species usually lay similarly colored eggs. Studies of the proteins in egg whites showed that there were fewer differences in these proteins in closely related groups than in distantly related groups. Within each species, the shape and coloration of eggs is alike, and resemblances are seen in related groups of birds.
Like reptiles, many birds lay white eggs. It is generally believed that originally all bird eggs were white. Through natural selection, colors that made eggs less conspicuous were developed. Probably first ground colors and then markings such as blotches, streaks, scrawls, and speckles evolved to aid in the disguise of eggs. In support of this theory, it is noted that birds that nest in cavities where the eggs are hidden from view, lay white eggs. Examples are found among woodpeckers, swifts, parrots and kingfishers. Birds that lay white eggs in open nests, like hummingbirds, herons, owls and grebes begin incubation as soon as the first egg is laid, thereby effectively hiding the eggs from the view of predators. Some ducks, geese, grebes and many gallinaceous birds lay white eggs in ground nests. Just before they leave the nest, they cover their eggs with plants or grasses to conceal them from predators.
There are far more birds that lay colored or tinted eggs than those that lay white eggs. The pigments used to color eggshells are derived from the breakdown of hemoglobin from ruptured red blood cells. Transported to the uterus by the blood in the form of bile pigments, the color is deposited on the developing shell during the last 5 hours before the egg is laid. Quite often the markings seem to be concentrated in a wreath around the larger end of the egg. This is because the large end of the egg comes first in the final descent through the oviduct, and picks up the major portion of pigment from the cell walls. Besides providing cryptic patterns to conceal the eggs from predators, pigmentation also protects eggs from the harmful radiation of the sun.
Not all birds that are cavity nester lay white eggs. Bluebirds lay light blue eggs, but place them in an “unnecessary nest” that they build on the cavity floor. Similarly, some wrens, nuthatches, chickadees, and crested flycatchers lay colored eggs in nests built in the bottom of cavities. This is believed to indicate that they once built open nests, but have recently, in evolutionary terms, developed a hole-nesting habit.
The total number of eggs laid in one nesting is called a clutch. From one to twenty three eggs are found in a clutch. Some birds, like albatrosses, shearwaters and tropicbirds, lay four. Loons, hummingbirds, goatsuckers and most pigeons lay two eggs. Practically all other birds that lay more than two eggs experience a range of clutch size.
Food supply, latitude, nest site, and the degree of development of young at hatching also influence clutch size. Rough-legged Hawks and Snowy Owls lay larger clutches of eggs in years when lemmings (their main prey) are plentiful. Barn Owls will nest year round if meadow voles and/or other rodents are abundant. Birds that nest in the tropics generally lay smaller clutches than those that nest in more northern latitudes where the days are longer and more food is available. Ground nesters, whose eggs are more likely to be discovered by mammalian predators, generally lay larger clutches than birds that nest in trees or on remote islands. In birds that require an extensive incubation period and a long time to raise to independence, like eagles and albatrosses, only one clutch is laid in a season. Some of the smaller songbirds lay two or three clutches every season.
Birds are among the best architects in nature and use a wide variety of plant and other materials in constructing nests. Many plant parts are extensively used including the fiber and plant down as well as the occasional use of leaves, flowers, catkins, and twigs. Animal parts are also well used, especially fine fur, hair, and other bird feathers. These materials are well woven (in most cases) into tight and sturdy structures. Human hair, horse hair, dog fur, and a wide variety of soft plant materials can be put out in clean suet baskets or vinyl mesh bags. We encourage our customers who attract and feed birds in their back yards to assist the birds at this time of the year by providing a variety of nesting materials. Not only are you giving them a helping hand, but the rewards are great as we get to observe them building their nests, laying eggs, incubating them, and watching parents raise them from nestlings to fledglings – very stimulating, engaging, and entertaining for adults and educational for children. Come by and see our wonderful selection of nesting materials you can use. Do not use dryer lint. It is too short fibered, does not contribute to the structural strength of the nest, and doesn’t really benefit the birds.