In animals that reproduce by internal fertilization, as humans do, the penis is invaluable, from an evolutionary point of view.
Yet birds have evolved to not need them. Developmentally speaking, birds' penises have gone. Land fowl, which have only rudimentary penises as adults, have normally developing penises as early embryos. Later in development, however, the birds turn on a genetic program that leads their budding penises to stop growing and then wither away.