It was a tough egg to crack, but scientists have finally explained why some chickens are born half male and half female.

The bodies of these hen-rooster hybrids, or gynandromorphs, have a mixture of genetically male and female cells, the research reveals.

Only about 1 in 10,000 chickens are born as gynandromorphs, which have male features—such as a rooster's comb and a defensive leg spur—on one side of their bodies and dainty, henlike features on the other.

Researchers had thought a rare genetic abnormality causes the condition. To test this theory, Michael Clinton of the University of Edinburgh and his team analyzed cells from three gynandromorph chickens.

To their surprise, the team found that the chickens' cells were normal. What was strange, however, was that male cells made up one half of the body, and female cells composed the other half.

Half-Sex Chickens Are Double Fertilized

The scientists believe gynandromorphs are created when a chicken egg becomes fertilized by two sperm.

Despite their dual nature, the hybrid birds typically have one of the sex organs, either testes or ovaries. The scientists did not test whether the chickens could actually reproduce, however.

Gynandromorphs are known to exist in other bird species, such as zebra finches, pigeons, and parrots, Clinton said by email.

It's likely that the phenomenon occurs in all birds species, he added, but it's not always obvious because males and females of many species often look similar.