Male birds have testes and sperm ducts and male swans, ducks, geese and ostriches have a penis. However, most birds make do with a small amount of erectile tissue known as a papilla. To reduce weight for flight most female birds only have one ovary - usually the left, which produces extremely yolky eggs. The eggs are fertilised in the upper part of the oviduct (equivalent to the fallopian tube and uterus of mammals) and as they pass down it albumin (the white of the egg), the membrane beneath the shell and the shell are laid down over the yolk. Finally the egg is covered in a layer of mucus to help the bird lay it (see diagram 13.15).
Most birds lay their eggs in a nest and the hen sits on them until they hatch. Ducklings and chicks are relatively well developed when they hatch and able to forage for their own food. Most other nestlings need their parents to keep them warm, clean and fed. Young birds grow rapidly and have voracious appetites that may involve the parents making up to 1000 trips a day to supply their need for food.
Diagram 13.15 - Female reproductive organs of a bird