Ever wonder what goes through a bird’s mind as it flaps across the sky? Scientists are now a step closer to finding out. A study published today in Current Biology used a small device to record the brain activity of homing pigeons as they made training flights; by pairing the device with GPS, the scientists could determine how the birds’ brains reacted to different landmarks along their journey. It is the first time such a technology has been used in free-flying birds, and it opens up a new view into how animals respond to their environment outside the laboratory.
Homing pigeons, which are trained to return to their home loft, can make the journey back even when released from unfamiliar places hundreds of miles away. But how they manage this feat has puzzled scientists for decades. Earlier research relied on simply watching where the birds were headed, but recent studies with GPS have created a more detailed picture. When pigeons are far away, they seem to use a combination of cues to determine the location of home: the position of the sun, the planet’s magnetic field, and even smells in the air. But when closer to home, the birds seem to rely on familiar landmarks and roads to guide their way.
For the new study, 26 birds were anesthetized, and electrodes were placed on the surface of their brains through small holes in the skull. A small electroencephalography (EEG) device was affixed to each bird’s head and attached to the electrodes. The birds were also given backpacks carrying a GPS monitor that recorded their position over time. Alexei Vyssotski, a behavioral neuroscientist at University of Zurich who led the study, says that his team decided to release the birds from the sea, about 30 kilometers from their home loft, so that they had to traverse a relatively featureless environment before passing over familiar land.
EEG measures the electrical activity of neurons in the brain, revealing different patterns depending on the animal’s state of consciousness. When the scientists analyzed the EEG data from a series of flights, they were able to identify at least three bands of brain-wave frequencies that seemed to be important in flying behavior. They could then plot out how these frequency bands changed at different positions along the journey.
Vyssotski says that lower-frequency brain waves seemed strongest when something commanded the birds’ attention: when they passed over landmarks or other sites of interest. These frequencies were weak when the birds began their journey over water but strengthened dramatically as they passed over land. The researchers were even able to correlate brain activity to specific landmarks. For instance, one striking visual feature to the left of the birds–a large open-pit mine–caused some birds to veer briefly off course. As they did, the researchers saw a jump in activity in the right hemisphere of the brain–consistent with the fact that birds process visual information from each eye in the opposite hemisphere. In another instance, the pigeons attended to two seemingly uninteresting spots on land, which puzzled the researchers until they visited the sites and realized that the interest wasn’t navigational: they were sites where flocks of feral pigeons hung out.
The higher-frequency bands are “even more intriguing,” Vyssotski says, “because they may reflect some cognitive process.” These frequencies were most active at the beginning of the trip, when the birds were orienting themselves. He says that the activity may correlate with the process of finding their way, but further study will be needed to bear that out.
The study is “a wholly novel approach,” says Dora Biro, a scientist at University of Oxford who has previously used GPS to understand how homing pigeon use landmarks when flying over familiar places. She says the data confirm a growing body of evidence that pigeons rely on visual cues in the landscape to get around. The use of EEG signatures to pinpoint areas of interest to the birds, she says, “opens up an entirely new observational window onto how birds in flight perceive, memorize, interpret, and utilize their visual environment.” One important question, she says, is the role of familiarity in their brain activity. In this study, the birds were not released far enough away from home for the researchers to see how their brains responded to completely unfamiliar features.
György Buzsáki, a neuroscientist and EEG expert at Rutgers University, says that the EEG data “are quite striking and surprising, especially the large changes between navigation over sea and land.” He adds that very little is known about EEG patterns in birds, so there is still a lot of work to be done in interpreting the signals. Future devices that take higher-resolution measurements–perhaps even detecting changes in single neurons–could help clarify what is happening in the birds’ brains.
The device these researchers used, which they term a “neurologger,” has been used in one other published study, which analyzed sleep patterns in sloths and found that the animals don’t sleep as much as their reputation suggests. Niels Rattenborg, a scientist at the Max Planck Institute for Ornithology who led the sloth study, is currently using the device to study sleep patterns of birds in the wild. “The vast majority of what we know about how the brain functions is derived from animals confined to the laboratory setting,” Rattenborg explains. The neurologger, he says, makes it possible to bring the laboratory into the field, giving researchers a chance to investigate how animals use their brains on their own turf.