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5𠖫 Egg Shape Nests & Incubation 5.2.1 . . . Nesting Behavior 5.2.2 . . . Parental Duties 5.2.3 . . . Nest Construction 5.2.4 . . . Hummingbird Nests 5.2.5 . . . Mud Nests 5.2.6 . . . Nesting in Burrows 5.2.7 . . . Egg Shape 5.2.8 . . . Incubation 5.2.9 . . . Killdeer Nest Protection 5.2.10 . . . Brood Parasitism 5.2.11 . . . Feeding Young 5.2.12 . . . Fledglings

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Eggs are restricted to a narrow range of shapes. The structure and strength of the shell depends on the arch-like curvature of a sphere or an ellipse (see Egg Shell, 5.1). However, within this narrow range, one interesting shape has served as an adaptation for birds who build no nests but lay their eggs on bare ground or rocky ledges. Most birds that we are familiar with have the typical "egg-shaped" eggs . . . like those of the domestic chicken in our refrigerators. The eggs are not spherical, probably for ease of laying; however a sphere is the most economical shape to contain a given volume. Greater Roadrunner eggs are nearly spherical and because of this less calcium is lost to the female as she produces eggs. It may not represent a dramatic advantage, but even small adaptations help to ensure evolutionary success.

Greater Roadrunner © Don Baccus, donb.photo.net

Greater Roadrunner nest with egg and chicks © Greg Clark; mirror-pole.com

Cupped within a basket-like nest, young Greater Roadrunner chicks and an egg are protected from a fall. Round eggs must be cupped or contained within the architecture of the nest or burrow simply because they roll. The more round the egg, the less shell is required to contain the contents due to the geometry of spheres and ellipsoids. It is more economical to produce round eggs. It requires less shell material to contain a given volume as a sphere than any other shape . . . however, spheres can roll away.

Bird Rock; Santa Cruz Island, California

Many seabirds depend on isolation for protection of eggs against predators. Islands or simply exposed rocks like "Bird Rock" off the coast of Santa Cruz Island, California (above) offers protection from predators, like the Island Fox, but provide no nesting materials. Birds nesting on these islands must lay their eggs on bare rock, sometimes on narrow ledges; and spherical eggs would be a clear disadvantage.

Eggs laid on rock ledges are characteristically conical at one end. These eggs roll in circles, reducing their chances of rolling off the ledge. Killdeer typically lay their eggs on beaches or rocky stream beds. Although not as extreme as some of the seabird eggs, like the murres, Killdeer eggs have the characteristic conical end of an egg that will not roll in a straight line but in a circle. Killdeer © Don Baccus, donb.photo.net

Killdeer eggs; Santa Cruz Island, California

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(Revised 1 March 1999)