Visual control of hovering flight in hummingbirds

Little is known about the role of vision in controlling flight in birds. The avian brain has large regions dedicated to processing visual and spatial information, suggesting that visual information is important for bird behavior. We studied free flight behavior of Anna’s hummingbirds in a virtual reality arena with back-projected stationary and moving patterns to investigate how visual motion is used in hovering position control. Hummingbirds lost positional stability when hovering in the presence of background motion along three axes: backward-forward, right-left, and up-down, and their flight response matched the direction of the motion stimulus. The presence of prominent stationary features did not abolish the response to moving patterns, suggesting a high sensitivity to visual motion in hovering birds.

HFSP Program Grant holder Douglas Altshuler and colleagues
authored on Mon, 08 December 2014

Optic flow - the motion of images across the retina – is an important source of information for visual animals moving through their environment. There are two types of visual motion termed (1) self or ego-motion: where the observer moves relative to the environment, and (2) object motion: where visual features move relative to the observer. Self-motion could provide information used to steer or regulate speed, while object motion may indicate the approach of a predator or conspecific. In a stationary behavior like hovering, the flying observer produces no self-motion, so we hypothesized that hummingbirds could use visual motions to control hovering position. We therefore introduced optic flow during hummingbird hovering and filmed the position of a single point on the bird’s head. The 3-D position of the head marker over time could then be used to quantify changes in hovering position along three axes: backward-forward (x), right-left (y), and up-down (z). We show that hummingbirds are stable in the absence of a moving stimulus, but respond to moving visual patterns by drifting to match the motion direction (Fig. 1).

Figure 1. Moving visual patterns caused hummingbirds to drift in space but stationary patterns did not affect hovering. Two opposing motion directions and a no-motion treatment were tested for each of three patterns (a): spiral (red), vertical bars (blue), and horizontal bars (green). The distance a bird drifts along x-, y-, and z-axes during non-feeding hovering events is shown in rows b-d respectively. The plots show all events we recorded for eight hummingbirds. The mean of all the drifts for a given treatment is then shown from top (e) and side (f) views. Drifts were minimal during no-motion treatments. Figure reproduced with permission from Proceedings of the National Academy of Sciences USA.

Behavioral responses to immersive visual motion are common in animals, and perhaps this is partly explained by the unnatural strength of a large, moving black-and-white pattern dominating a visual scene. We predicted that hummingbirds would be able to stabilize their hovering flight if presented with a more heterogeneous visual scene composed of stationary and moving patterns. To test this prediction we combined looming motion with stationary checkerboard patterns in different relative amounts and orientations. The hummingbird’s response to looming motion was apparent even with a large proportion of stationary background (Fig. 2). Our results suggest that hovering hummingbirds are highly sensitive to background motion and change body position to match the direction of background visual motion.

Figure 2. Hummingbirds respond to looming motion even when a prominent checkerboard pattern is visible, and the response increases with an increased proportion of looming spiral. The birds fly backward in response to looming motion when it is centered in a checkerboard frame (a, green). Similarly, they respond to a looming spiral frame with embedded checkerboard patterns (b, blue). The lines are a non-linear model for the response data and are used to predict the response at >80% spiral (dashed lines). The actual 100% response data from the previous experiment with single motion types are shown for reference. Two treatments without motion (but visible stationary spiral and checkerboard patterns) are shown as 0% in a and 100% in b. Figure reproduced with permission from Proceedings of the National Academy of Sciences USA.

Text by Benjamin Goller and Douglas L. Altshuler

Reference

Hummingbirds control hovering flight by stabilizing visual motion. Benjamin Goller and Douglas L. Altshuler. PNAS (2014) published ahead of print December 8, 2014, doi:10.1073/pnas.1415975111.

Link to abstract