New research sheds light on why some birds have lost their penises over the course of evolution.
Land fowl, such as chickens, have normally developing penises as early embryos, but only have rudimentary organs as adults.
A study in Current Biology shows that these birds initiate a genetic "programme" during development that stops the budding penises from growing.
The loss of the organ could have given hens more control over reproduction.
Co-author Dr Martin Cohn, from the University of Florida in Gainesville, said: "Our discovery shows that reduction of the penis during bird evolution occurred by activation of a normal mechanism of programmed cell death in a new location, the tip of the emerging penis."
The team discovered that a gene called Bmp4 plays a critical role in this process. In chicken development, Bmp4 switches on and the birds' developing male genitals shrink away.
In ducks and emus, which retain their penises, the gene stays switched off and the organs continue to grow.