KEY POINTS

  • The condition is known as gynandromorphy
  • It occurs when an egg with two nuclei is fertilized
  • The bird's coloring is expected to be even starker during breeding season

Biologists at the Powdermill Nature Reserve say they have discovered a songbird that is male on one side of its body and female on the other.

The rose-breasted grosbeak was discovered during normal banding operations, Annie Lindsay, the Pennsylvania nature reserve’s Avian Research Center bird banding program manager told LiveScience in an email.

The condition is known as gynandromorphy. Scientists said it occurs when an egg with two nuclei is fertilized. The condition, which is rare, is found in other animals as well, including butterflies, reptiles and crustaceans. Gynandromorphs also differ from hermaphrodites, which have the genitals of both sexes.

"The entire banding team was very excited to see such a rarity up close, and are riding the high of this once-in-a-lifetime experience," Lindsay told ScienceAlert.

The Avian Research Center has been banding birds since 1961 and amassed 800,000 records. In that time, it has found only five other specimens, Lindsay said, the last one 15 years ago.

Lindsay told Science News the bird was found Sept. 24.

“It was spectacular. This bird is in its nonbreeding [plumage], so in the spring when it’s in its breeding plumage, it’s going to be even more starkly male, female,” Lindsay said.

As with many species, the male grosbeak is mor colorful than the female with a scarlet splash on its throat, a white breast and black head. Females have more earthy tones. She said the contrast will be even starker during breeding season.

Scientists say they don’t know much about gynandromorphs and it was unclear whether the bird would be able to reproduce.

"In female songbirds, the left ovary is the functional ovary, and because this bird's left side is the female side, it may be able to produce viable eggs," Lindsay explained. "However, the bird would also need to behave as a female to attract a male mate, and that isn't something we are able to observe during normal banding operations."

Feather samples were taken for further DNA analysis.