The Differences between Discriminate and Indiscriminate Egg Layer Birds

A discriminate egg-laying bird only lays its eggs in specific locations, usually a nest or protected area. An indiscriminate egg-laying bird lays its eggs wherever it happens to be, without preference for any given location.

Very few birds are completely indiscriminate in their egg-laying. Most birds will choose or make a protected area for their eggs, even if it is only a scrape nest in the tundra. However, domesticated guineafowl (Numida meleagris) may lay their eggs in any location.

Both nightjars (Capimulgidae) and short-eared owls (Asio flammeus) lay their eggs directly on the ground without building any kind of nest. Short-eared owls choose an area where the vegetation has been flattened, but nightjars are completely indiscriminate.

Both the king (Aptenodytes patagonicus) and emperor (A. forsteri) penguins do not use nests at all. After they migrate to their general nesting grounds, they do not need a particular site to lay their eggs. Instead, after the female has laid the egg onto her feet, the male immediately transfers it onto his feet and places a fold of skin overtop to keep it warm. The strongly pyroform shape of the egg helps the egg to stay upright during the transfer and brooding.

Keeping the egg away from the cold air is particularly important for emperor penguins, which breed during the Antarctic winter. It takes only a few seconds of exposure to Antarctic winter temperatures to kill the unhatched chick. In a successful transfer, the egg never touches the ice, but not all transfers are successful.

Emperor penguins resist the cold by huddling as close together as they can throughout the winter. Temperatures in the center of the huddle can be as much as 18 degrees warmer than the ambient air temperature, as well as negating the wind chill. Every successful penguin breeding has managed to keep the internal temperature of the egg at 104 degrees Fahrenheit all the way from birth to hatching, even when the Antarctic winter is causing ambient temperatures which are the coldest on Earth.

Some auks, such as razorbills (Alca torda), discriminate only in choosing a high rocky ledge when they come to land to breed. The parents hardly ever leave their egg, but the egg has no other direct protection from the environment. An indirect form of protection is their shape. Like penguin eggs, the auk egg is more pyriform than most bird eggs. This keeps the egg from rolling off the narrow ledge. Razorbills lay only a single egg each year.