When a flock of pine grosbeaks unexpectedly spent the winter in a Bend, Oregon, neighborhood, Santa Fe Institute External Professor Stefani Crabtree (Utah State University) saw a rare opportunity.
The northern finches typically inhabit remote boreal forests, making close observation difficult. But during a holiday visit to her father's home in the winter of 2023–24, Crabtree found herself watching the colorful birds as they fed in his backyard.
Then she began hearing something she couldn't explain.
“The birds were making this very quiet sound while they were feeding,” she says. “At first, we thought it was a squeaky gate.”
That observation ultimately led Crabtree and her father, longtime birder Tom Crabtree, to document what may be a previously undescribed pine grosbeak vocalization in the journal Birds. After comparing recordings from the flock against more than 2,600 archived pine grosbeak recordings, the researchers found no clear match. The findings suggest the birds may have been engaging in a little-known form of group communication, with multiple flock members joining in the quiet calls while feeding. The vocalizations spread gradually through the flock until all birds present were participating, a pattern that led the researchers to view the sounds as a collective behavior rather than an individual call.
“There is a lot of information contained within bird vocalizations that we still don't fully understand,” Stefani Crabtree says. “These quiet calls may be one way birds share information with one another without broadcasting it more widely.”
The flock's extended stay attracted birders from across Oregon and Northern California, eager for a glimpse of the red-and-gold finches in the Crabtrees' backyard and nearby trees throughout the neighborhood.
As the father–daughter pair watched the grosbeaks feed on crabapples, they noticed that the faint chirps occurred only when multiple birds were feeding together and stopped when construction noise or other disturbances interrupted them. The calls resumed only after the environment quieted down.
Crabtree says her interest in the birds’ behavior was shaped in part by conversations with her friend and former Santa Fe Institute colleague Liz Hobson, now an associate professor at the University of Cincinnati, whose research examines collective animal behavior. An archaeologist and complexity scientist, Crabtree was intrigued by the possibility that information was moving through the flock, and she taught herself how to analyze bird vocalizations and compare them against thousands of bird recordings collected by researchers and birders. The study suggests the calls may help the pine grosbeaks maintain social cohesion or exchange information without broadcasting it widely.
The project also became a meaningful collaboration between father and daughter. A retired attorney and lifelong birder, Tom Crabtree spent weeks observing the flock and helping document its behavior.
“I've spent a lifetime watching birds,” he says. “To be able to contribute something new and do it alongside my daughter made the experience especially rewarding.”
For Stefani Crabtree, the study underscores a simple lesson.
“There is still so much about the natural world that we don't know,” she says. “Sometimes all it takes is slowing down and paying attention.”
Read the study "Pine Grosbeak (Pinicola enucleator) Exhibit Collective Vocal Behavior During Foraging Bouts" in Birds (June 18, 2026). DOI: 10.3390/birds7020035
Pine Grosbeak feeding sounds
Recording courtesy of Stefani Crabtree
Male Pine Grosbeak eating crabapples in Bend, OR. (image: Tom Crabtree)