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  • Conservationists identified a bobolink with split-sex characteristics at a preserve northwest of Chicago, noting it as the first documented case for this species.
  • The bird exhibited male plumage on its right side and female plumage on its left, a condition known as bilateral gynandromorphism caused by early embryonic cell division errors.
  • Despite the rare physical anomaly, the individual successfully established a territory, mated, and raised offspring before departing for wintering grounds in South America.

A bobolink exhibiting a striking genetic anomaly has been documented at a nature preserve northwest of Chicago, offering researchers and birdwatchers a rare opportunity to study bilateral gynandromorphism in the wild. The bird, which displays male plumage on its right side and female plumage on its left, represents the first confirmed case of this condition in the species. Conservationists observed the individual during the spring breeding season at the Hill ‘N Dale Preserve in Barrington Hills, approximately 40 miles from downtown Chicago.

Luke Dahlberg, a conservation seed program manager with Citizens for Conservation, initially noticed the unusual bird while photographing male bobolinks returning from their South American wintering grounds. At first glance, the tan-gold coloration appeared to be an irregular molt pattern, as males typically adopt female-like coloring outside of breeding season. However, upon closer inspection during a subsequent visit, Dahlberg realized the bird possessed two distinct halves divided by a straight vertical line. Ornithologists later confirmed the classification as a bilateral gynandromorph.

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Cameron Rutt, a conservation ecologist with the Field Museum in Chicago, assisted in identifying the bird, which was informally dubbed “Black and Tan.” Rutt captured roughly 1,000 images of the individual, noting that its appearance shifted dramatically depending on its orientation. When flying right, the bird appeared entirely male; when flying left, it appeared entirely female. This visual dichotomy highlights the precise nature of the genetic split, which affects feather pigmentation and structure along a clear midline.

Bilateral gynandromorphism is an exceptionally rare phenomenon in avian biology. Scientists have recorded fewer than 100 individual birds with this condition across approximately 45 species worldwide. The underlying cause is believed to stem from errors during female meiosis, the cell division process that produces egg cells. Female birds possess one Z and one W sex chromosome. If the separation of these chromosomes fails during development, an egg cell may retain both or lose one, leading to a mix of genetically male (ZZ) and genetically female (ZW) cells in the resulting embryo.

Brian Peer, an ecologist at Stephen F. Austin State University, explains that if such an anomalous egg is fertilized by sperm carrying Z chromosomes, the developing embryo can contain both cell types. As the bird grows, these distinct genetic lines manifest as separate physical characteristics on either side of the body. While laboratory studies provide some insight into this mechanism, field observations remain limited, making each documented case valuable for understanding developmental biology in wild populations.

Despite its unusual appearance, the bobolink behaved like a typical male during the breeding season. It sang, performed courtship flights, and established a territory in an effort to attract a mate. Unlike previous cases of gynandromorphic birds, such as a northern cardinal observed in Illinois between 2008 and 2010 that failed to pair up, this individual succeeded in reproduction. Reports indicate the bird nested, produced eggs, and successfully raised fledglings before departing the preserve.

The successful breeding of this split-sex bobolink challenges assumptions about the fitness costs associated with such genetic anomalies. While gynandromorphism is often viewed as a developmental defect, this case demonstrates that individuals can maintain normal behavioral patterns and reproductive success. The bird has since left the area for its wintering grounds, but conservationists hope it will return next spring, providing further opportunities for study.

This discovery follows other recent sightings of rare genetic variations in wildlife, including a split-color lobster caught off Cape Cod earlier this year. Although that crustacean was fully female and not a gynandromorph, its orange-and-brown coloration resulted from similar cellular mosaicism. These events underscore the diversity of biological outcomes arising from early developmental errors, offering glimpses into the complex mechanisms governing sex determination and physical development in animals.

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  • Smithsonian Magazine↗An 'Extremely Rare' Half-Male, Half-Female Bird Temporarily Took Up Residence Outside of Chicago