For birds, as for people, youth is a voyage of self-discovery—a truism that may be more pronounced for brood parasites like the Brown-headed Cowbird. Most young birds learn who they are from their parents, and this sets the foundations for crucial life lessons: how to sing, how to find food, and how to find a mate of their own species.
But cowbird chicks are different. They don’t receive this sort of guidance because they aren’t raised by their biological parents. Brown-headed Cowbirds are brood parasites, which means that mothers lay their eggs in the nests of other bird species, such as sparrows. There, the cowbird chicks are fed by their foster parents until they fledge, often at the expense of the host chicks. This may save some childcare for cowbird parents, but it creates a whole new set of problems.
“This is something like taking a young toddler away from other humans during the time that it’s just learning the sensory and motor skills associated with talking,” says Kathleen Lynch, a biologist who studies the evolution of maternal care in birds at New York’s Hofstra University. If a bird doesn’t learn how to behave from its own species, it won’t be able to function socially later in life and, crucially, won’t be able to find a mate.
Last month, Lynch published a study in the Journal of Experimental Biology that for the first time identifies the brain area that unlocks social learning for young cowbirds. The work builds on previous work by Mark Hauber, a Hunter College psychologist based in Manhattan who studies how birds recognize themselves and others. In 2001, he proposed a mechanism for how cowbirds recognize other cowbirds: they use a password that only other cowbirds can produce and understand.
At first, Hauber wasn’t sure what this password might be. It could be a cowbird-specific vocalization, visual cue, or behavior that, when heard or seen by a young cowbird, allows them to identify their fellows and learn from them. “We think this password opens up the learning mechanism,” Hauber says, “the plasticity, the ability to become cowbirds.”
In his hunt for the password, Hauber first looked for possible visual signals in their plumage, but soon he realized that cowbirds as young as five days old responded to a vocalization known as 'chatter'. This sound is produced primarily by female cowbirds, both as a territorial display and in response to male songs that they think are sexy. Chatter is common, and a reliable indicator of where a cowbird flock will be. Hauber had found his password.
He knew this chatter password couldn’t be a learned behavioral cue; by its very nature it had to be programmed into cowbirds at birth. "There has to be a brain area that encodes for this password,” Hauber says. “It can’t work without that.” He wanted to investigate this idea further, but it was only when Lynch, a colleague with expertise in avian neuroscience, moved to Hofstra in a nearby New York City borough that he spotted the opportunity for a collaboration. After a productive brainstorming lunch, the pair decided to continue the work Hauber had started two decades earlier and pinpoint the area of the cowbird brain involved in password recognition.
The resulting experiments, published in the new paper, revealed that, in cowbirds, the auditory system generally associated with recognizing birdsong is divided into two components. One part helps them to identity other cowbirds using the chatter password, and the other enables them to learn songs from those cowbirds once they have joined a flock.
Hauber thinks that the password serves two functions. First, when heard, it switches the brain into a sensitive “learning mode” in which it can learn to forage, roost, and even sing in a dialect specific to that group’s location—in essence, how to be a cowbird. The password is also an attractant that ensures cowbirds find their own kind in the first place. “It is key that they remove themselves from the foreign environment nearly as soon as they fledge the nest,” Lynch says, “in order to locate other cowbirds and begin learning appropriate social cues and song from adults of their own species.”
This aligns with Hauber’s previous research, which showed that fledglings sneak out of the nest at night and head to habitats preferred by cowbirds. “[This] places the juvenile cowbird in the proximity of the adult cowbird, where chatter would be heard for the first time,” Hauber says. From there, fledglings will be attracted to the sound of nearby chatter and drawn into flocks of adult cowbirds, ensuring that they don’t pick up any bad, non-cowbird habits. Indeed, the new experiments also revealed that prolonged exposure to non-cowbird songs can trigger a brain response, emphasizing the importance of getting out of that host nest fast.
The consequences of getting this wrong are very real. When cowbirds have been kept with canaries under experimental conditions, they have learned canary songs. In the wild, confused cowbirds have been found flocking with starlings.
The new study makes for a tidy hypothesis, but it may be incomplete. Gregory Kohn, who studies bird behavior at New Mexico State University, says that while these experiments do a great job identifying a potential cue, species recognition may not rely on a single password. “Species recognition is a developmental process,” he wrote in an email. “[It] likely relies on a diverse range of multisensory cues from other cowbirds as young individuals move from being nestlings to integrating into cowbird flocks."
In other words, it’s complicated, and Lynch and Hauber’s story may not be the full one. Still, it’s a big step forward in understanding how cowbirds, and birds generally, figure out who they are.