Most insects and some fishes are "explosive" breeders, which refers to producing as many young as possible in one massive effort. Birds produce fewer young, reproduce several times during their lifetime, and are the only vertebrates that exclusively lay eggs. A few species of mammals, such as the platypus, lay eggs; but most mammals produce live young. Many lizards and snakes bear live young; whereas, other species reproduce via eggs.

Domesticated birds bred for egg production, such as chickens, are capable of producing over 100 eggs per year; but if all eggs hatched, these birds would be unable to raise this number of young. This method of egg laying is termed indeterminate and is achieved by removing eggs before incubation occurs.

By contrast, the ancestor of domestic chickens, the red jungle fowl, produces a clutch of five to seven eggs. Hence, in the wild, birds generally are determinate layers, meaning that clutch size is relatively fixed. By the way, why don't chickens like people? -- Because people beat eggs.

Size of clutches may be related to food availability. Songbirds in more northerly latitudes have longer summer days to raise young compared to songbirds in more southerly latitudes; breeding birds can find greater amounts of food with longer days and, hence, raise larger numbers of young. Birds with precocial young (which means they can walk at birth), such as geese, appear to be somewhat limited in numbers of eggs produced because of nutritional limitations on the female. In contrast, birds with altricial young (helpless in a nest after hatching), such as robins, are probably limited by the capabilities of parents to feed young.

Optimal clutch size is one that ensures the largest number of young surviving to breeding age, which is a fundamental observation termed the "Lack hypothesis." However, what triggers when a species stops laying an optimal number of eggs and begins incubating eggs is not completely understood.

In birds with precocial young, incubation begins once the last egg is hatched. In many birds with altricial young, such as songbirds and hawks, incubation is initiated before the last egg is laid, resulting in young of different sizes.

Three general trends occur with egg size. First, egg size is related to body size of birds. The largest known egg was that of the extinct elephant bird from Madagascar, with an egg over 13 inches in length and holding nearly 2 gallons. The smallest known bird egg is that of the Jamaican emerald hummingbird, measuring less than half an inch.

Second, larger birds have smaller eggs, relative to body size. For instance, an ostrich egg is 1.8 percent of an adult female's body weight, but the much smaller house sparrow has an egg 13.7 percent of its body weight. Third, bird eggs with precocial young have larger eggs (10 to 15 percent of adult female body weight) versus those of similar-sized birds with altricial young (5 percent of weight).

Why aren't eggs round? Diameter and muscular tension in the female's oviduct plays a major role in the shape of eggs -- the deeper the pelvis, the rounder the egg. Most eggs have the familiar oval shape, with one end slightly broader and flattened.

Birds that nest on the ground or on cliffs tend to have eggs that are more pointed at one end to prevent rolling and allow packing of eggs during incubation. Birds that nest in cavities, such as woodpeckers, have more spherical eggs.

Chickens have been bred to produce white or brown eggs, but eggs of wild birds come in a variety of colors, including cream, gray, brown, blue, green, red or even spotted, which helps camouflage them. Originally, bird eggs were probably white, much like those of living reptiles. Cavity-nesting species, such as woodpeckers, generally have white eggs.

These eggs are out of direct sunlight; thus, egg color is not necessary to protect the developing embryos within eggs from ultraviolet radiation. A whitish egg in a dark cavity also may be more visible to the incubating parent compared to a darker or spotted egg.