Whilst the majority of birds mate by pressing together the cloacas, a number of groups have evolved a penis. These too are erectile, and can show some intriguing features. The ducks are one of the main groups of birds to evolve a penis (arguably because copulation takes place on the water) but that of the Argentine Lake Duck achieves heroic dimensions of c.20 cm (half the body length) and bristles with spines (possibly to remove sperm of competitors and consistent with a promiscuous system of group mating). The bird penis differs, however, in a number of respects that include transport of the sperm along an external groove (or similar folds) and inflation by lymphatic fluids (as against blood). Given the awkwardness of typical bird copulation, with cloaca to cloaca contact, it seems surprising that the penis in birds has so rarely evolved (or if inherited from an ancestral form has been lost multiple times). There are several possibilities: the nature of the cloaca combines sexual products with waste so potentially increasing the risk of infection with either sexually transmitted diseases or pathogens. Alternatives are linked to risk from predators during copulation (keep it short!) or problems of balance.
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