These birds would be at it for 10-20 minutes," says ecologist Tim Birkhead of the red-billed buffalo weaver and its remarkable false phallus. A male uses his organ to rub females up the right way and improve his sperm's chance of success1. By massaging his member the male also reaches what looks like an orgasm - a first in the bird world.

Few birds are blessed with a phallus; most couples achieve fertilization by pushing together their rear ends for a functional exchange of fluids. So 19th-century reports of a mock member in the buffalo weaver sent Birkhead and his team from the University of Sheffield, UK on a field trip to drought-struck Namibia.

Catching the birds in the act was a tough job, recounts Birkhead: "In three years we saw eight matings." Living communally in a large stick nest, a frisky pair would occasionally emerge and fly to a nearby tree. "I'd run after them, sweating profusely with my binoculars steaming up," he says. The amorous pair would start bouncing up and down - over numerous consecutive bouts.

Compared to the 1-2-second tryst most birds manage, their staying power is unique. Yet, being caught up in the action, entry of the elusive organ was hard to make out. Even in captivity "they performed beautifully," but the view was blocked, says Birkhead.

So his team glued a piece of cardboard to an unlucky bird's member. The crude contraception did not prevent mating, suggesting that the buffalo weaver organ is actually a weapon in sperm wars. By choosing a male who rubs longest or best, females may be selecting top-quality sperm. "He persuades her of his suitability," thinks Birkhead.

Sperm competition - in which males' sperm vie to fertilize female eggs - is rife amongst animal groups, explains Geoff Parker, a behavioural ecologist at the University of Liverpool.

Testes size is one sure sign that competition is intense. Cod, for example, whose eggs and sperm are released in a fertilization free-for-all, have huge ones. In contrast, seahorses have testes "so small you can hardly find them," says Parker - because the male exclusively carries and fertilizes the eggs.

Whereas males strive to outdo each other in volume, females may also have a say - by storing or selecting sperm from favoured partners. "It's difficult to measure," admits Parker, because the numbers of offspring fathered have to be separated from sperm dose.

Paternity testing revealed that female buffalo weavers sire birds from multiple males, suggesting that sperm competition is hot. Time spent courting must be shown to predict sperm transfer or success to really back up the idea, says Parker.

The buffalo-weaver's 'false penis' first appeared in 1831 reports from a German anatomist. The 1.5-cm appendage lacks blood vessels and has a twisted furrow down its length. Males in communal nests have longer ones than those that live alone, showing that size is a factor in social success.

But for males at least, the phallus is for more than foreplay. At the critical moment, the males enter an orgasmic state: "It shuddered and its eyes glazed over," reports Birkhead. To confirm their observation, the team went to the lengths of manually stimulating the organ. "It's a mystery," Birkhead says - but a unique and stimulating first for birds.