The mating game

Birds, like all animals, are driven by the urge to reproduce. Choosing the right partner is of vital importance, so birds have evolved rituals and displays that help them assess the opposite sex. Usually the female gets to choose and the male strives to impress her. She has to choose a partner of the same species, and this is why most birds, especially males, have distinctive calls and markings. Next, the male must prove he is a good catch using every trick, from bringing gifts to singing love songs, dancing, or battling with his rivals.

Fighting fit. Male birds don’t win mates simply by looking beautiful – they have to prove their superiority over rivals, even if it means a fight to the death. Male golden pheasants slash at each other with the vicious spurs on the feet, but they usually settle their disputes without serious injury.

Building a bower. As a substitute for showy feathers, bowerbirds collect colourful objects and arrange them around a stack of twigs. Satin bowerbirds are particularly choosy, collecting only blue objects. Females mate with the males who produce the most artistic displays.

Gift giving. Some male birds give presents to prospective mates. Male herons bring nest material, offered with much ceremonial bowing, and snowy owls offer freshly killed lemmings. Gifts of food are of more than symbolic value. They allow the female to assess how well her partner will feed future offspring, and they provide her with the valuable nutrients she needs to produce eggs.

A brief affair. Courtship is often a long and complex affair, but mating itself is usually very brief. The male flutters onto the female’s back and the two birds press their genital openings together, allowing sperm to pass from male to female.

Eggs

All birds lay eggs rather than giving birth to live young as mammals do. This is because birds need to keep their weight down in order to fly, so mothers must get rid of their offspring as soon as they can. An egg, therefore, serves as an external womb, containing all the nutrients that a chick will need to develop. Parents simple keep their eggs warm, protect them from predators, and wait for them to hatch. We think of eggs as fragile, but in fact they are surprisingly tough: ostrich eggs are strong enough to stand on.

Inside an egg. A freshly laid egg consists of little but egg white and yolk. At first the chick is a tiny pink speck on the yolk, called a germ spot or embryo. Drawing on the food in the yolk and the water in the egg white, the embryo grows and a recognizable chick begins to take shape.

Extraordinary eggs. Eggs are surprisingly varied and many are very different from the chicken eggs we eat. They range in size from hummingbird eggs the size of your little fingernail, to ostrich eggs, which are bigger than pineapples. Shapes range from cones to spheres (spherical eggs are strongest), and textures vary from rough and chalky to smooth and shiny. Many eggs are coloured or speckled, either for camouflage or so that the mother can tell her own eggs from impostors, which other birds may have sneaked into her nest.

A difficult lay. Although the largest birds lay the largest eggs, small birds lay big eggs in proportion to their body size. An ostrich’s egg is a hundredth of its weight, but a hummingbird’s is more than a tenth of its weight. Kiwis lay proportionately the biggest eggs, at a quarter of the adult bird’s weight.

Hot seat. Birds appear to be doing little while sitting on their eggs, but they are expending up to 25% of their energy just keeping the eggs warm. Incubating eggs is especially important for birds in cold places, such as snowy owls. Their bellies have a special patch of almost bare skin which they press against the eggs to keep them warm.

Belly blanket. Male emperor penguins balance an egg on their feet and tuck a paunchy flap of belly over the top to keep it from contact with the icy ground or air. They can go for months without eating while the mothers are away looking for food.

Hatching out. Breaking out of an egg is such hard work that it takes hours or even days. To make the first crack, the chick pushes its “egg tooth” – a hard spike on its beak that it loses after hatching – against the shell with all its might.

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