Passage IV

NATURAL SCIENCE: This passage is adapted from the book Eggs: Nature's Perfect Package by Robert Burton (©1987 byRobert Burton).

Birds are 'glorified reptiles' and some biologists believe that they are indeed no more than reptiles with feathers. It is not surprising, therefore, that birds' eggs are similar to those of reptiles. The main interest in birds' eggs from an evolutionary point of view is the adaptation of the eggs of different species to particular lifestyles.

In both reptile and bird eggs, there is a large yolk which not only supplies the needs of the embryo but will also sustain the newly hatched animal until it can feed. In birds' eggs, after the egg cell—consisting mainly of a ball of yolk-has been fertilized, it moves down the oviduct where it collects the albumen or 'egg white' and takes on its shell. The albumen is mainly a filler for protecting the embryo, but it also supplements the yolk as a valuable source of protein and a reservoir of water. Between the albumen and the shell are two parchment-like shell membranes. When they are, first formed they fit quite loosely, but during the next stage, known as 'plumping', water and salts fill out the albumen until the membranes are taut. The shell is then attached firmly onto the outer membrane, as can be seen when peeling a hard-boiled egg.

The innermost layer of albumen is thick and forms a hammock slung between the ends of the shell to keep the yolk suspended in the centre. Within it are two twisted cords called chalazae, which act as bearings to allow the yolk and embryo to rotate freely when the egg is turned by the parent bird in the nest. Chalazae are presumably unnecessary in reptile eggs because they are never turned, and some, such as those of tortoises, will not hatch if they are turned accidentally.

The shell takes 15 to 16 hours to form in a domestic hen. It consists of crystals of calcite, a form of calcium carbonate, strengthened with protein fibres which also attach it to the underlying membrane. The amount of calcium needed for each egg is about 2 grams. As no more than 25 milligrams are circulating in the blood of the hen, the balance must be mobilized from her reserves. She cannot absorb enough calcium from her food while making the egg, so she has to remove it from her bones. Birds have developed a special kind of bone, called medullary bone, which females lay down in the marrow cavity of existing bones during the weeks before laying commences and use as a source of calcium when the eggshells are being formed. Some birds even have difficulty in laying down this reserve because their diet is deficient in calcium. Shortage of calcium affects the breeding of scavenging birds, such as vultures, which have a diet of pure meat; to overcome this problem some species switch to hunting small animals, which they swallow whole, during the breeding season. Another example of the ingenious methods used by birds to provide calcium for their eggs can be seen on the Arctic tundra, where insect-eating sandpipers swallow the bones of long-dead lemmings; as a result there is more calcium in each clutch of eggs than there is in the adult's entire skeleton.

The shell of a hen's egg is peppered with 10,000 tiny pores, just visible to the naked eye as small depressions in the shell. These pores are used for the passage of oxygen, carbon dioxide and water vapour. Unlike a lung, however, the flow of gases cannot be regulated and relies purely on diffusion through the pores. Nevertheless, the flow must be finely attuned to the needs of the embryo. As development proceeds, more oxygen is required, but the loss of water must be kept low. The eggshell's role in regulating gas flow and water loss is shown by the arrangement of pores in different eggs—quite simply, diffusion depends on the size and number of the pores. Of all the eggs studied, from the 1-gram eggs of warblers to the 1.5-kilogram ostrich eggs, the weight of the egg and hence its metabolic requirements is directly proportional to the total cross-sectional area of the pores.

Birds' eggs are uniform in shape when compared with the eggs of reptiles, but there are significant variations from the standard oval. Owls' eggs are noticeably more spherical than normal, while those of swifts and swallows are longer and more elliptical, although it is not obvious why this should be. Many waders have eggs shaped like old-fashioned spinning tops, blunt atone end and tapering sharply at the other. The reason for this is clear: there are usually four in a clutch and, being large for the size of the parent, they fit under its body better when packed points inward in the nest.