Cocks (almost) don't have a penis, a trait common to 97% of bird species, but they can grow one when the expression of the Bmp4 gene is prevented. The expression of this gene causes the percursor of the phallus in the chick embryo to undergo apoptosis (cell death) and Bmp genes are also involved in 3 other bird traits: feather development, toothlessness and beak shape. In penis-less bird species, copulation requires a sex maneuver nicknamed the cloacal kiss (in French) which requires a full cooperation of the female (3 min of tender parrot sex). In species where males have a penis, like ducks, females are less lucky: the coevolution of the rather convoluted morphology of male and female genitalia has been hypothezised to occur through sexual conflict [many previouslies]. The evolutionary mechanisms that drove phallus reduction in most birds species are still unknown.