Local researchers and bird-lovers were in awe this spring when they spotted a rare bird with both male and female characteristics at a nature preserve in suburban Barrington.
The mysterious bird was a bobolink, and was identified as a bilateral gynandromorph: The bird’s body was divided down the middle, with one side having male characteristics, and the other side female.
Luke Dahlberg, a conservation seed program manager with Citizens for Conservation and a longtime birder, first encountered the bird May 10 at the organization’s Hill ’N Dale Preserve. Due to its dual color, he named it “Black and Tan,” and experts say its discovery can help provide a greater look into the condition.
Dahlberg was photographing bobolinks as they returned to the area for the summer when he noticed one bird that looked different from the others.
At first, he thought the unusual coloring might have been caused by molting.
When he returned to the same spot weeks later, he saw the bird once again and could immediately tell the bird was divided. In nature, male bobolinks have distinctive black-and-white breeding plumage, while females have more muted brown and golden coloring. Black and Tan displayed both patterns, making the bird immediately recognizable.
“It was just straight down the middle,” Dahlberg said.
Dahlberg consulted other bird experts, including members of the Illinois Ornithological Society, who confirmed his suspicions.
Brian Peer, a biology professor and ornithologist at Stephen F. Austin State University, told WBEZ Tuesday morning that gynandromorphism is believed to result from an error during meiosis, which is the process that produces egg cells.
Within this mutation, the resulting bird has male and female genetic characteristics that’s shown on separate parts of its body.
Peer told WBEZ the phenomenon is extremely rare, with only about 30 to 40 observations of gynandromorphic wild birds, but the number could be higher as it is difficult to detect and is most noticeable in species where males and females have distinctly different appearances.
Peer previously co-authored a study documenting a bilateral gynandromorphic northern cardinal observed in northwestern Illinois from 2008 through 2010. The bird was monitored over three separate winters, providing researchers with detailed information about its behavior.
According to Dahlberg, Black and Tan had established a territory at Hill ’N Dale, allowing other volunteers to observe the bird throughout the summer.
Black and Tan behaved largely like a male bobolink, singing a song often associated with male members of the species.
Observers also saw the bird interacting with a female and carrying insects to a location where nesting activity appeared to be taking place.
These observations could potentially help researchers better understand how gynandromorphic birds behave and whether their behavior more closely resembles that of males, females or a combination of both.
“Every time that we see one of these birds, it really contributes to our knowledge of that kind of phenomenon because it is so exceedingly rare,” Peer said.
The bird is no longer at the preserve, but Dahlberg hopes it returns next spring.
Bobolinks migrate thousands of miles between their breeding grounds in the northern United States and southern Canada and their wintering grounds in southern Brazil and northern Argentina.