Figuring out what visual stimuli hummingbirds focus on when flying
As we walk, run or drive around each day, we never really notice how we keep track of the world around us. Our brains are actually piecing together a variety of visual stimuli to help us figure out how much space is around us and how we’re moving through it. Just because we’re used to these methods doesn’t mean they’re universal in the animal kingdom. Animals that fly, for instance, may need to focus on different spacial information than walkers like us. This is probably why hummingbirds and bees seem to have a few differences in how they navigate the world around them.
As notably agile fliers, hummingbirds were tasked with flying from one end of a box to another in order to retrieve a sugary snack. As the birds fly through the short, simple course, different shadows were projected on the walls of the box around them to see how they’d react. Without other major cues to react to, the birds seemed to treat the moving shadows as an indication of either their movement, or movement around them, and would adjust their flight accordingly.
Big, small, near, far
The first thing researchers noted was that hummingbirds ignore parallax displacement as a way to gauge distance. An example of this would be the way a sign on the side of the road zips by quickly when you drive by it, but distant mountains don’t appear to change position much at all. While your brain then interprets these relative speeds as markers of distance, hummingbirds apparently ignore them. Instead, they use things like relative sizing (which we do to) to figure what’s near and what’s far. In the box experiment, birds would fly closer to the side of the box with many smaller shadows, because they looked farther away, than fewer large shadows of the same shape, because they were “closer.”
Looks like a landing
Hummingbirds were also found to use relative, vertical motion as a reference point as well. If horizontal shadows were moving up or down, the birds seemed to treat this as an indication of their own altitude, and would try to compensate by flying higher or lower. This reaction has been observed in other hovering, flying animals, like flies and bees. In that case, the insects can’t refocus or even move their eyes without moving their heads, and so relying on movement in their local horizon is likely the easiest way to touch down for a landing. With the right visual stimuli, flying insects are likely to even move their legs to a landing position when they think they’re lowering down, which is probably a safe and efficient strategy that doesn’t require giving up their view of the rest of the world around them.
All of these animals need to see the world around them, but probably not to take in the view. It seems that their visual perception is specialized to navigate their local environments with enough accuracy to avoid collisions and get to food, but without as much concern over what’s much farther than the next few flowers. A predator like an eagle presumably needs to rely on different cues to interpret larger distances, but it will be hard to know for sure until we can get some flying in a suitably large box.
My second grader asked: Can the bees fly backward if they think they need to adjust?
Probably not? Bees have been seen pushing themselves backwards when taking flight to avoid turning around, but they don’t seem to be able to hover well enough to just reverse like say, a hummingbird. Dragonflies, damselflies and Mayflies can hover and reverse though. Wasps, which are closely related to bees, can sort of back up in a zig-zagging motion. If you really want to see a bee backing up, watching the bee-mimicking hoverfly may be your best option.
Source: Earth Science Hummingbird vision wired to avoid high-speed collisions, Scienmag
