Encounter with a Mixed-Species Flock | Yuko Takeda, Dean of the Faculty of Nursing and Medical Care
2022.09.20
I've been hearing more and more indescribable calls—"kyata-kyata," "gyako-gyako," "gekyo-gekyo"—coming from the hill behind my house. Thinking they might be the calls of a medium-sized bird, I strained my eyes but couldn't catch a glimpse of any such creature. As the calls continued over the weekend, I was watching when I noticed a tree branch moving unnaturally. Looking closer, I saw about three squirrels leaping about. "Could it be squirrels?" I wondered, and a quick internet search revealed their identity: Pallas's squirrels. It seems that in addition to raccoon dogs, Pallas's squirrels are also breeding on the hill. My residential area in Yokohama City has numerous small green spaces, and it is believed that this environment has enabled the rapid increase in population and distribution of Pallas's squirrels. While they are designated as pests, which is understandable given their impact on the ecosystem, it was humans who originally introduced them. It's a rather selfish matter of human convenience.
As I watched, what I thought was a small bird flying in turned out to be an incredible flock of several dozen birds flying around. They darted about dizzyingly, all the while chirping "tsupi-tsupi-tsupi" to each other. I often see flocks of about ten Japanese tits, but I had never seen such a large group before.
Looking closely, I saw that it wasn't just Japanese tits; there were also Japanese white-eyes and Japanese pygmy woodpeckers mixed in. A flock made up of multiple species is called a "mixed-species flock," and such bird flocks are apparently often seen from autumn to winter. It looked as if they were shaking the branches as a group to flush out and prey on insects. By moving in a group that includes fledglings, they seem to efficiently secure both food and safety. Mixed-species flocks have no leader; even though the species are different, they understand the meaning of each other's calls and are said to react quickly, especially to alarm calls.
As I stood there stunned by the sheer size of the flock, wondering if it was a harbinger of some cataclysm, they flew away in an instant with a "tsuu-pipipipi" call.
It seems there is much to learn from the survival strategies of the diverse small animals that communicate with each other while living in the limited green space on the hill behind an apartment building in a residential area.
Reference: "Summer Diet and Habitat Use of Pallas's Squirrels in Urban Small Green Spaces in Yokohama City." Wildlife and Human Society 8: 47-57, 2020