Hummingbirds are unique in many ways. Some of the the smallest birds in the world, they are the fastest fliers relative to their body length, and they are also the only true hoverers in the avian class. To enable this special form of flight, hummingbirds have evolved several distinct adaptations, from a specialized wing shape to breast muscles that take up about 30 percent of their entire body weight (most birds’ breasts weigh in at around 15-18 percent).
Scientists have always suspected that this complex movement requires a more complex brain, but hummingbird brains are very small and hard to study. So much so that only recently did a team of Canadian scientists manage to crack how hummingbird brains handle hovering. Their research, published in this month’s edition of Current Biology, shows that hummers have brains unlike any other bird (or four-limbed vertebrate, for that matter), which let them manage multidirectional flight.
To understand these differences, it's first important to consider how and why we move. Every creature on Earth is either predator or prey, and many are both depending on where they fall in the food chain. For almost all animals, this means moving forward—i.e., whichever way they are facing—either toward food or away from becoming food. Humans are a rare exception among vertebrates in that we have the capability to move relatively efficiently in many directions. (Have you ever seen a dog try to trot backwards or climb a ladder?)
Despite this difference, our first instinct when faced with danger is still to turn around and run away from the threat, not waltz sideways to safety. This instinct can be traced to the way our brains perceive motion. The visual centers in the brains of all studied four-limbed creatures respond most strongly to motion on the back-to-front axis (think of us chasing something or something chasing us).
Except for hummingbirds, according to the new research. Hummingbirds spend a lot of their time hovering, which means they have more to consider than just the front back-to-front axis. When hovering, a gust of wind might push them from the side. Or a predator might strike from below. So they have to be able to move not just in the forward direction to feed on a danglng flower, but in all directions.
Because of this, it would make sense that hummingbirds' brains don't put the same emphasis on back-to-front movement that ours do—and that’s exactly what the scientists found. The research team discovered that in an area of the brain called the lentiformis mesencephali, which is the part that responds to visual stimuli, hummingbirds didn’t have a strong back-to-front preference like all other animals tested thus far. Instead, they seemed to have no preference, responding to motion in every direction equally.
The researchers also found that hummingbird brains are tuned to respond more strongly to fast movements than slow ones. This was a surprise because scientists had assumed that their brains would be tuned for a low-speed hover. But if you think about it, being optimized for high speed makes sense, too. An Anna’s Hummingbird can move at 385 body lengths per second during mating flights, which is screaming fast (in comparison, an F15 Eagle fighter jet has a top speed of Mach 2.5, which translates to around 45 body lengths per second). At that speed, the ability to make near-instantaneous course corrections is the difference between life and death—or mating and not mating, which is basically the same thing in evolutionary terms.
That hummingbird brains perceive the world differently than other vertebrates is a fascinating ornithological find, but the researchers have other motivations for their study—understanding flight in nature to design better robots. The discovery of an animal brain that can move efficiently in three dimensions could be very valuable for artificial intelligence in flying drones, for instance, or computerized autopilot systems for helicopters. But potential commercial applications aside, isn’t it interesting to think that the tiniest bird brains might also be the most complicated?