Doshisha University and Nagoya University discover differences in direction and magnetic sensing activity according to head orientation in migratory birds: Compass cells that prefer north found in the brain

Susumu Takahashi, Professor at the Graduate School of Brain Science, Doshisha University, and Ken Yoda, Professor at the Graduate School of Environmental Studies, Nagoya University, jointly announced that they had discovered "head direction cells" that control the sense of direction in the brains of the chicks of the streaked shearwater, a migratory bird. They also revealed that the cells prefer a north orientation. They observed the behavior of chicks walking freely in the experimental space while measuring their brain activity. Activity of the head direction cells was high when the chicks were facing north. The findings may help in clarifying the long-distance movement mechanism in migratory birds and determining methods for the conservation of these animals. These results were published in the February 4 issue of the international scientific journal Science Advances.

Head direction cells that activate when the head faces a specific orientation are present in the brain of mammals, birds, fish, and insects. These cells were thought to be unrelated to geomagnetism because they are distributed evenly in a specific direction; however, none of the organisms in which these cells had been studied were long-distance migrators. Pigeons, salmon, and sea turtles that move long distances use geomagnetism to move; among them, pigeons have cells that sense magnetism in the vestibular nuclei of their brain. The vestibular nuclei are deeply involved in the activity of head direction cells, indicating that directional and magnetic sensations may share the same nervous system. However, these functions in the brain remained unclear.

So far, Professor Yoda and his group have tracked streaked shearwater with GPS loggers, such as in a 2017 study. Through this they have revealed that the migratory route from the breeding ground at Awashima Island, Niigata Prefecture, to the south, in regions such as Indonesia, differ between adult and young birds. The streaked shearwater has adapted to sea flight and enables long-distance travel by efficiently flying using maritime winds. During migration, adult birds migrate together in a flock, whereas the young birds that have fledged approximately a month prior migrate in a flock consisting only of young birds. Adult birds migrated on a maritime route over the Pacific Ocean or the Sea of Japan, bypassing Honshu, whereas young birds migrated on a straight route across the mountains of Honshu. The route taken by the young birds is extremely risky and requires constant flapping. More than half of the individuals die within one month after fledging. Mortality rates could be as high as 70% in certain cases. These findings show that the young birds have no knowledge of the topography of the Japanese archipelago and may rely on magnetism to detect 'south' when migrating.

Therefore, the research group conducted an experiment to investigate the relationship between nerve cell activity and head orientation. First, a small wireless device (neuro logger) was attached to the chicks (about a month before they left the nest), and their behavior of walking in a circular experimental space with walls was analyzed using a camera installed at the top. The position and orientation of the head were estimated using the camera recordings, and their correspondence with the measured brain activities was investigated. This experiment was conducted indoors in Awashima Island.

The activity of the head direction cells was detected in the medial mantle, which is thought to be closely related to space and orientation recognition. Approximately 23% of the medial pallium was occupied by head direction cells. Furthermore, the head direction cells are frequently active (17 times per second) when the chicks turn their heads to the north. The cells are so active that the direction of the head of the birds is biased towards the north, and the cells are completely inactive in the south. Generally, in animals, head direction cells are distributed uniformly, controlling only the sense of direction, but in streaked shearwater chicks, they are distributed in the northward direction like a compass. This was thought to be responsible for both directional and magnetic sensations. This tendency to prefer the north was also observed in experiments conducted outdoors 3 kilometers away from the indoor experiment site.

The results showed that the head direction cells of streaked shearwater may utilize geomagnetism regardless of the animal's position as a clue to know orientation. The fact that head direction cells prefer north but the birds head south may be because they do not react in the south when the birds migrate. Professor Takahashi said, "We would like to verify how the head direction cells of the parent bird behave or whether they are really related to geomagnetism by conducting experiments in which geomagnetism is changed."