On Feb. 4, 2023, in Phil Hardberger Park in San Antonio, a large bird glided into a tree. Another bird, already perched there, began to preen the newcomer’s head and neck.
This caught the watchful camera and eye of Lora Reynolds, a volunteer with the Bexar Audubon Society. The bird receiving the attention was a crested caracara, a raptor in the falcon family. The one doing the preening was a black vulture.
“It was just very weird to see that,” Ms. Reynolds said. She had been birding since 2009, but seeing a bird from one species preen one from another was new to her.
Later that month, the park shared photographs of the curious interaction on its website and on social media. Lori Boies, a local cancer biologist with a passion for nature photography, noticed them. She said she had seen the same behavior just around a mile away the year before.
First, Dr. Boies had noticed two crested caracaras preening each other on the roof of her house. Then two black vultures joined them and started preening the caracaras’ heads, necks and chests. The interaction lasted about 20 minutes.
It looked like “a rooftop party,” Dr. Boies joked. Initially she thought it was “just a really neat, great picture to take and put on Instagram.” But a look at the scientific literature revealed that preening between members of different bird species, what ornithologists call interspecific allopreening, has rarely been documented.
Together with her colleague Terry Shackleford and an undergraduate student, Dr. Boies wrote up both sightings in a report published last month in the journal Ecology and Evolution. The paper adds to only a handful of previous records, all from rural areas in Texas and in Central and South America. The researchers say it is the first such report from an urban environment, and the first to describe two mixed-species preening pairs in close proximity.
“People think that nature is always about violence and conflict, red in tooth and claw,” said Neil Buckley, a professor of biological sciences at SUNY Plattsburgh, who was not involved in the paper. “But actually there’s a lot of collaboration and cooperation that goes on.”
Dr. Buckley documented one of those earlier observations in a rural area while conducting his doctoral research. On four occasions from 1988 to 1992, he saw crested caracaras bow their heads and be preened by black vultures. Twice, he saw the caracaras “reciprocate by preening a vulture in return.”
Similar behavior has also been observed in China, where a spot-necked babbler preened a Nonggang babbler at a bird-feeding station in the Guangxi region.
“I got excited because this is something I had never heard of, and I hadn’t even imagined to be possible,” said Wenyi Zhou, an ornithologist who witnessed the interaction while bird-watching with his wife.
Between members of the same species, allopreening serves to maintain pair bonds between mating partners. It also serves hygienic purposes by removing parasites from hard-to-reach areas.
Dr. Zhou speculates that cross-species allopreening might help different species coexist peacefully, much like social grooming in primates. But he emphasizes that more evidence is needed to support that.
“Even within species, allopreening is really poorly studied,” he said, “let alone interspecific allopreening, which is very hard to observe.”
Dr. Buckley says the behavior may be a simple stimulus-response, an instinctive behavior in which a specific cue leads to a specific action. The most common example is when a bird sits on an egg that does not belong to it. In this case, when a bird sees another bird with its head lowered, it responds by preening it, regardless of the species.
Dr. Boies and her colleagues propose that urban green spaces might encourage cross-species interactions by concentrating bird populations at higher densities than in rural areas. Dr. Zhou finds the idea plausible.
But Dr. Buckley is skeptical of that explanation, arguing instead that the concentration of birds and people in one place may simply heighten the chances of observing the behavior. In rural areas, “unless you’re spending time sitting in a blind watching a carcass,” he said, “you’re not going to see it very often.”
Dr. Zhou suspects that interspecific allopreening may be more common than records suggest. After he published his observation, several birders shared their own experiences of the behavior with him. “Maybe we haven’t paid so much attention to these birds,” he said.
Dr. Boies and Dr. Shackleford, both researchers at St. Mary’s University in San Antonio, see their paper as a testament to citizen science. Scientists cannot have “boots on the ground everywhere at all times,” Dr. Shackleford said. Photographs posted to platforms like iNaturalist, once verified, can become valuable data points, revealing moments of cooperation that might otherwise go under the radar.
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