A keen sense of touch helps hummingbirds hover near flowers without hitting them

Hummingbirds seem to be a marvel of nature and engineering: a living creature capable of hovering near a flower with surgical precision. How do they do that?

Although hummingbird flight mechanics have been well studied, much less is known about how their sense of touch helps these energetic little birds sip nectar from a flower without bumping into it. Most of what scientists know about how touch is processed in the brain comes from studies of mammals, but bird brains are very different from mammal brains.

Research led by UCLA and published in Current Biology shows that hummingbirds create a 3D map of their body when neurons in two specific locations in the forebrain fire – when gusts of air hit the feathers on the edge of their body. attack their wings and the skin of their legs. The receptors on their beak, face and head also work in this direction. The intensity of air pressure, influenced by factors such as proximity to an object, is sensed by nerve cells at the base of feathers and in the skin of the legs and transmitted to the brain, which measures the orientation of the body in relation to an object.

Zebra finches, also studied by the researchers, have the same general organization with slightly lower sensitivity in some areas than hummingbirds, suggesting that these areas contribute to the hummingbirds' highly specialized flight dynamics. This work adds to knowledge about how animals perceive and navigate their world and can help identify ways to treat them more humanely.

Humans produce a tactile map of the body that progresses from the toes in the center of the brain to the legs, back, and a much larger area that represents touch of the face and hands. These areas, used for touch and tactile tasks, are enlarged in the human brain.

“In mammals, we know that touch is processed on the outer surface of the forebrain in the cortex,” said Duncan Leitch, corresponding author and professor of integrative biology at UCLA. “But birds have brains without a layered cortex structure, so the question of how touch is represented in their brains was wide open. We showed exactly where different types of touch activate specific neurons in these regions and how the touch is organized in their forebrain.

Previous studies in which birds were injected with dye showed that their brains had a region in the forebrain for processing touch of the face and head, and another for touch anywhere else on the body . In owls, for example, the tactile centers that generally correspond to facial touch are devoted solely to the claws. But since hummingbirds lead very different lives than owls, it seemed unlikely that this would be true for them.

Leitch and his co-authors from the Royal Veterinary College and the University of British Columbia were able to watch neurons fire in real time by placing electrodes on hummingbirds and finches and gently touching them with cotton swabs or puffs of air. A computer amplified the signals from the electrodes and converted them to sound for easier analysis.

The experiments confirmed that head and body touch map to different regions of the forebrain and showed for the first time that air pressure activates specific groups of neurons in these regions. Examination of the wings showed a network of nerve cells that likely sent a signal to the brain when activated by puffs of air on the feathers.

The researchers found particularly large clusters of brain cells that responded to stimulation of the wing edges, which they believe helped the birds adjust their flight in nuanced ways. They also discovered that feet are extremely sensitive to touch and that touch is widely represented in the brain, presumably to aid perching. Researchers think these areas might be even larger in parrots and other birds that use their legs to grasp and move objects.

In their study, the researchers identified receptive fields on birds, in which contact would trigger a neuron to fire. In hummingbirds, some of these fields – particularly on the beak, face and head – were very small, meaning they could feel even the slightest touch. Zebra finches had the same receptive fields, but larger, suggesting that these regions in finches are not as sensitive and probably more relevant to hummingbirds that rely on constant, steady precision flight.

“Hummingbirds often responded to the smallest thresholds we could give them,” Leitch said.

Learning more about how various animals map touch onto their bodies could lead to advances in technologies that use sensors to move or perform a task, such as prosthetic limbs or autonomous devices. But perhaps a more immediate outcome of the research is improved animal welfare.

“If we can understand how animals perceive their sense of touch, we can develop practices that disturb them less,” Leitch said.