Rare Half-Male, Half-Female Bobolink Spotted Near Chicago

A bobolink with a striking biological split recently drew birdwatchers to a nature preserve outside Chicago. The bird’s right side has male characteristics, while its left side has female characteristics, separated by a nearly straight vertical line down its body. Smithsonian Magazine reports that the bird was observed at the 246-acre Hill ’N Dale Preserve in Barrington Hills, about 40 miles northwest of downtown Chicago. Luke Dahlberg, conservation seed program manager for Citizens for Conservation, first noticed it in photographs taken in May while documenting male bobolinks in their breeding plumage. At first, the bird appeared to have tan-gold feathers across its entire body. Dahlberg initially considered the possibility that it was undergoing an unusual molt. When he returned to the preserve several weeks later and saw the same bird, he recognized that its two sides displayed different plumage: the darker, patterned coloring associated with males on one side and the female coloration on the other. Ornithologists identified the bird as a bilateral gynandromorph. The condition is linked to a genetic anomaly that produces male and female characteristics in different parts of an animal’s body. The bird, nicknamed “Black and Tan,” behaved like a male bobolink, singing, making courtship flights and defending a breeding territory. Cameron Rutt, a conservation ecologist with Chicago’s Field Museum, photographed the bobolink roughly 1,000 times after being called in to help document it. According to the report, the bird eventually nested, reproduced and raised fledglings before leaving the preserve. Dahlberg hopes it will return the following spring. The report describes the sighting as possibly the first documented gynandromorphic bobolink. Such split-sex birds are exceptionally uncommon: Rutt estimates that gynandromorphism has been recorded in fewer than 100 individual birds across roughly 45 of the world’s more than 11,000 bird species. Scientists suspect the condition in birds may arise from an error during the formation of an egg, followed by fertilization involving two sperm. The resulting embryo can develop a mixture of genetically male and female cells, though researchers say the precise cause remains incompletely understood.