A songbird that spent the spring and summer at a nature preserve outside Chicago turned out to be a biological rarity: a bilateral gynandromorph, its body split cleanly down the middle between male and female.
Luke Dahlberg, a conservation seed program manager with Citizens for Conservation, first photographed the bird in May at the group's Hill 'N Dale Preserve in Barrington Hills, roughly 40 miles northwest of downtown Chicago. One side of the bobolink showed the buttery-yellow nape and black-and-white body typical of breeding males; the other displayed the streaky, camouflaged brown plumage of females. He nicknamed it "Black and Tan."
Gynandromorphism arises from a genetic anomaly during egg formation: if an egg cell retains an extra chromosome and is then fertilized by two sperm carrying male chromosomes, the resulting embryo can develop with a genetically male half and a genetically female half, divided by a near-vertical line. Smithsonian Magazine reported the condition has been documented in fewer than 100 individual birds across roughly 45 of the world's more than 11,000 bird species.
I had a better chance of photographing a unicorn. It's extremely rare in nature.
Luke Dahlberg, Citizens for Conservation
What made the sighting more remarkable, according to the Chicago Sun-Times, is that the bird didn't just survive the season — it paired with a mate, nested and successfully raised fledglings before departing the preserve on its winter migration. Ecologists including Cameron Rutt of Chicago's Field Museum and Brian Peer of Stephen F. Austin State University said reproductive success in a gynandromorphic bird is especially unusual, since split hormone systems often complicate mating behavior.
Dahlberg said he hopes the bird returns to Hill 'N Dale next spring, though bobolinks migrate as far as South America each winter, so there is no guarantee. Bobolink populations have been declining across their North American grassland range for decades, making any individual bird's story of interest to conservationists even before its genetics became a talking point.
Word of the sighting drew birders to the preserve throughout the summer hoping for a glimpse, turning a routine seed-collection stop into something of a minor pilgrimage site. Citizens for Conservation said it plans to document any future sightings and share updates through its regular preserve newsletters and social media channels.
