A small songbird spotted in an Illinois nature preserve this spring turned out to be something far more unusual than an oddly colored bird.
The bobolink appeared almost divided down the middle.
Its right side displayed the striking black-and-white breeding plumage associated with a male bobolink, while its left side showed the softer brown and golden coloring typically seen in females.
Experts identified the bird as a bilateral gynandromorph — an extraordinarily rare condition in which an animal develops both male and female characteristics on opposite sides of its body.
The bird, nicknamed “Black and Tan,” may be the first gynandromorphic bobolink ever documented.
An Ordinary Day of Bird Photography Turns Extraordinary
Luke Dahlberg, conservation seed program manager with the Illinois nonprofit Citizens for Conservation, first photographed the unusual bird on May 10 at Hill ’N Dale Preserve near Barrington Hills.
Dahlberg had been photographing male bobolinks as the birds returned to their northern breeding grounds.
At first, nothing seemed especially extraordinary.
While reviewing his photographs later, however, he noticed a bird whose plumage didn’t look quite right.
One side appeared similar to the dark breeding plumage of a male. The other looked much more like a female.
Initially, Dahlberg considered a much simpler explanation: perhaps the bird was going through an unusual molt.
Then he saw it again.
As additional bird experts became involved, it became apparent that the color pattern wasn’t simply an odd collection of feathers.
The bird appeared to be divided almost perfectly from one side of its body to the other.
What Is a Bilateral Gynandromorph?
Gynandromorphism occurs when an animal develops a combination of male and female biological characteristics.
In a bilateral gynandromorph, those characteristics can be divided along the body’s midline — with one side showing male traits and the other showing female traits.
In Black and Tan’s case, the distinction was particularly easy to see because male and female bobolinks look dramatically different during the breeding season.
A breeding male bobolink has mostly black underparts, a white back and rump and a pale golden or straw-colored patch behind its head.
Females are much less conspicuous, with warm buff and brown coloring and darker streaking.
Black and Tan appeared to display both patterns on the same bird.
A Bird Even Experts Rarely See
Citizens for Conservation contacted bird experts, including ornithologist and conservation ecologist Cameron Rutt of Chicago’s Field Museum, to help evaluate the unusual visitor.
Rutt confirmed that the bird’s appearance was consistent with bilateral gynandromorphism.
The phenomenon is exceptionally uncommon in wild birds.
According to Rutt, only about 45 of the world’s approximately 11,000 bird species have been mentioned in scientific literature as exhibiting gynandromorphism, representing fewer than 100 individual birds.
Black and Tan is believed to be the first documented example in a bobolink.
Finding one at all would have been remarkable.
But this bird provided something researchers don’t often get with such a rare animal — time.
Black and Tan Stayed Around
Many unusual wildlife sightings last only seconds.
An animal passes through.
Someone takes a photograph.
Then it disappears.
Black and Tan was different.
The bobolink established a territory at Hill ’N Dale Preserve and could be observed repeatedly for much of the breeding season.
That gave birders and researchers an unusual opportunity to watch how a gynandromorphic bird behaved in the wild.
And despite its divided appearance, Black and Tan’s behavior looked remarkably similar to that of a male bobolink.
Observers saw the bird singing, defending or occupying a territory, performing courtship behavior and interacting with females.
It was also observed carrying insects toward an area where nesting activity was occurring.
Did Black and Tan Become a Parent?
This is where careful wording becomes important.
Some reports have described Black and Tan as mating and raising fledglings.
Observers clearly documented the bird associating with a female, participating in nesting behavior and bringing food toward the nesting area.
But researchers did not capture the bird for genetic or reproductive testing.
That means its exact reproductive biology — including whether it biologically fathered any chicks — was not established through DNA testing.
Rather than disturb or sacrifice such an extraordinarily rare wild bird to answer those questions, researchers chose to observe it naturally.
The observations themselves still provide valuable information because scientists have so few opportunities to watch gynandromorphic birds behaving over an extended period in the wild.
An Amazing Bird Even Without the Rare Condition
Black and Tan would be remarkable even if it were an ordinary bobolink.
Bobolinks are among the world’s most impressive migratory songbirds.
Every year, they make a round-trip migration of roughly 12,500 miles, traveling between breeding areas in northern North America and wintering grounds in southern South America.
Over the course of its lifetime, one bobolink may travel a distance equivalent to circling the Earth several times.
They are grassland birds, nesting in prairies, meadows, hayfields and other areas with tall vegetation.
Their nests are built on the ground and can be hidden beneath dense grasses.
During spring and early summer, males are particularly noticeable because they perform display flights while producing a distinctive bubbling song.
A Species Facing a Bigger Challenge
Although Black and Tan’s unusual biology is what attracted attention, the story also highlights a broader issue facing ordinary bobolinks.
Their population has declined substantially.
The Cornell Lab of Ornithology reports that Bobolinks declined by an estimated 63 percent between 1966 and 2023 based on North American Breeding Bird Survey data.
The 2025 State of the Birds report lists the species as an Orange Alert Tipping Point species, meaning it has lost more than half of its population during the past 50 years and has experienced accelerating declines.
Loss and alteration of grassland habitat are among the challenges facing bobolinks and other grassland birds.
That makes the location where Black and Tan appeared especially relevant.
From Horse Farm to Wildlife Habitat
Hill ’N Dale Preserve was formerly part of a large horse farm.
Citizens for Conservation acquired the approximately 246-acre property in 2022 with plans to permanently protect the land and restore native prairie, wetland and other natural habitat.
The organization specifically identified grassland birds — including bobolinks and Henslow’s sparrows — among the wildlife expected to benefit from the property’s protection and restoration.
The preserve also helps connect other protected lands in the region, creating a larger corridor for wildlife.
Restoration is continuing today.
Citizens for Conservation is gradually replacing portions of the former agricultural landscape with native vegetation while carefully managing areas already being used by nesting grassland birds.
More than 80 bobolinks were recorded at Hill ’N Dale last year, according to the organization.
And among them this year was one bird nobody expected.
Black and Tan Has Moved On — For Now
Black and Tan is no longer being observed at Hill ’N Dale.
That isn’t necessarily surprising.
Bobolinks are migratory birds, and after breeding they eventually begin the enormous journey south toward their wintering grounds in South America.
Dahlberg hopes the unmistakable bird will return next spring.
If it does, researchers may get another rare opportunity to observe the same individual through another breeding season.
And unlike most wild birds, there shouldn’t be much difficulty recognizing it.
One side looks male.
The other looks female.
Together, they make Black and Tan one of the most unusual bobolinks ever documented.
An Extraordinary Discovery in an Ordinary Field
Stories about rare wildlife often involve remote rainforests, deep oceans or expeditions into places few people ever visit.
Black and Tan was discovered in a grassland preserve outside one of America’s largest metropolitan areas.
No animal had to be captured to make the discovery.
No expedition was organized.
A conservation worker simply photographed the birds living on protected land and noticed that one of them was different.
That may be one of the most fascinating parts of the story.
There are still unusual animals, behaviors and biological mysteries waiting to be discovered in places people pass every day.
Sometimes all it takes is someone paying close enough attention to notice.
Sources & Further Reading
WTTW Chicago — “This ‘Wicked Cool’ Half-Female, Half-Male Bird Landed in Suburban Chicago — Offering a Rare Glimpse Into One of Nature’s Strangest Phenomena,” August 5, 2026. Primary local reporting based on interviews with Luke Dahlberg, Citizens for Conservation Executive Director Abigail Derby Lewis and Field Museum ornithologist Cameron Rutt.
WBEZ Chicago — “Just How Rare Is This Half-Female, Half-Male Bird Sighting?” August 11, 2026. Interview and background featuring Luke Dahlberg and ornithologist Brian Peer.
Citizens for Conservation — Hill ’N Dale Preserve. Primary information about the preserve’s acquisition, habitat protection, restoration plans and importance for grassland birds and other wildlife.
Cornell Lab of Ornithology — All About Birds — Bobolink. Species information covering appearance, behavior, migration, habitat and conservation status.
Field Museum — Cameron Rutt. Background on the ornithologist and conservation ecologist who helped evaluate and document the unusual bird.