Egg Size and Shape
Eggs vary in Size
Birds' eggs, like the birds themselves, vary enormously in size. The largest egg from a living bird belongs to the ostrich. It is over 2000 times larger than the smallest egg produced by a hummingbird. Ostrich eggs are about 180 mm long and 140 mm wide and weigh 1.2 kg. Hummingbird eggs are 13 mm long and 8 mm wide and they weigh only half of a gram.
The extinct Elephant Bird from Madagascar produced an egg 7 times larger than that of the Ostrich!
Although large birds lay larger eggs than do small birds, small birds actually have proportionately larger eggs. It takes 60 ostrich eggs to equal the weight of one ostrich but only nine hummingbird eggs to equal the weight of the Calliope Hummingbird. The birds that lays the largest eggs, relative to body size, are the kiwis of New Zealand. Kiwis are about the size of a chicken but lay eggs that are about 140 mm long and 90 mm wide, about three times the size of a chicken egg. It takes only four kiwi eggs to equal the weight of a kiwi. Fortunately, female kiwis lay only one and occasionally two eggs per year!
Size and Hatchling Type
Species that are about the same size may lay eggs that are quite different in size. Typically, nestlings which are blind and helpless (altricial) and wholly dependent on parents, hatch from smaller eggs than nestlings that are mobile (precocial). Precocial nestlings have down-covered bodies and are active soon after hatching. They are able to leave the nest right away if they must. The Raven has altricial young. It lays eggs in a nest at the top of a spruce tree. Its eggs are smaller than those of the Long-Billed Curlew even though the two birds are similar in size. The Long-Billed Curlew, a long-legged ground-feeding bird, has precocial young. Birds that live and nest on the ground tend to produce precocial young. Precocial young require the larger food supplied by bigger eggs since they remain in the egg long after the Raven eggs have hatched.
Factors affecting Size
Within a species, egg size may differ. For example, younger birds of a species tend to lay smaller eggs than older birds of the same species. Even eggs laid by a particular bird may vary in size, both within and between clutches. Food availability is a controlling factor. A clutch is the total number of eggs laid by one bird during one nesting. If you were advising a bird, would you recommend that it lays four large eggs or five smaller eggs? Can you imagine situations where one strategy is better than the other?
Sometimes abnormal-sized eggs appear in a clutch. This is a clutch of American Avocet eggs. One of the eggs is clearly smaller than the other three. Dwarf eggs are more common within a clutch than are giant eggs but neither appear very frequently. Dwarf eggs are tiny and most often, yolkless. They are usually produced when an egg is formed around a
small piece of tissue rather than an ovum. No embryo is present and therefore no chick will hatch. These dwarf eggs are usually more spherical in shape with a thick shell and rough surface. Dwarf eggs may sometimes be found in an abandoned nest since the brood will have vacated and the unhatched dwarf egg remains.
Eggs come in all Shapes
Eggs are not all the same shape as the chicken eggs that we eat. Even the
shape of the domestic chicken egg is different from that of its wild ancestor, the Red Junglefowl. Some birds eggs are more pointed, or pyriform and, at the other extreme, some are more rounded or spherical.
Many Factors affect Egg Shape
Egg shape is determined by the internal structure of the hen. Her oviduct,
distribution of internal organs and shape of her pelvic bones all affect egg
shape. The Mallard's egg at the top of this picture is subelliptical (not
quite spherical). The Great Horned Owl's egg is spherical and the Red-tailed Hawk's egg, at the bottom, is elliptical.
Consequences of Shape
Aristotle once suggested that males came from more pointed eggs and females from more
rounded eggs. This of course, is not true. The shape of an eggshell does affect its physical
properties, however. The general spherical shape of an egg maximizes shell strength while also conserving shell materials. More rounded eggs further maximize the volume of the egg for theamount of eggshell as well as ensure heat conservation.
The Ecology of Egg Shapes
Egg shapes, over time, are often closely matched to their nest environment. Shorebirds normally lay four pointed eggs. In the nest, the eggs are oriented with their pointed ends towards the centre. This minimizes the amount of space needed to form the nest and increases the efficiency of the heat transfer from parent to egg during incubation.
Murres lay one egg on a bare nesting ledge. Their egg is very pointed. If bumped, the egg is more likely to roll in a tight arc than to roll off the cliff.