Spatial and Temporal Resolution of the Visual System of the Anna's Hummingbird (Calypte anna) Relative to Other Birds.

Hummingbirds are an emerging model for studies of the visual guidance of flight. However, basic properties of their visual systems, such as spatial and temporal visual resolution, have not been characterized. We measured both the spatial and temporal visual resolution of Anna's hummingbirds using behavioral experiments and anatomical estimates. Spatial visual resolution was determined behaviorally using the optocollic reflex and anatomically using peak retinal ganglion cell densities from retinal whole mounts and eye size. Anna's hummingbirds have a spatial visual resolution of 5-6 cycles per degree when measured behaviorally, which matches anatomical estimates (fovea: 6.26±0.12 cycles per degree; area temporalis: 5.59±0.15 cycles per degree; and whole eye average: 4.64±0.08 ). To determine temporal visual resolution, we used an operant conditioning paradigm wherein hummingbirds were trained to use a flickering light to find a food reward. The limits of temporal visual resolution were estimated as 70-80 Hz. To compare Anna's hummingbirds with other bird species, we used a phylogenetically controlled analysis of previously published data on avian visual resolutions and body size. Our measurements for Anna's hummingbird vision fall close to and below predictions based on body size for spatial visual resolution and temporal visual resolution, respectively. These results indicate that the enhanced flight performance and foraging behaviors of hummingbirds do not require enhanced spatial or temporal visual resolution. This finding is important for interpreting flight control studies and contributes to a growing understanding of avian vision.

Visual guidance of forward flight in hummingbirds reveals control based on image features instead of pattern velocity.

Information about self-motion and obstacles in the environment is encoded by optic flow, the movement of images on the eye. Decades of research have revealed that flying insects control speed, altitude, and trajectory by a simple strategy of maintaining or balancing the translational velocity of images on the eyes, known as pattern velocity. It has been proposed that birds may use a similar algorithm but this hypothesis has not been tested directly. We examined the influence of pattern velocity on avian flight by manipulating the motion of patterns on the walls of a tunnel traversed by Anna's hummingbirds. Contrary to prediction, we found that lateral course control is not based on regulating nasal-to-temporal pattern velocity. Instead, birds closely monitored feature height in the vertical axis, and steered away from taller features even in the absence of nasal-to-temporal pattern velocity cues. For vertical course control, we observed that birds adjusted their flight altitude in response to upward motion of the horizontal plane, which simulates vertical descent. Collectively, our results suggest that birds avoid collisions using visual cues in the vertical axis. Specifically, we propose that birds monitor the vertical extent of features in the lateral visual field to assess distances to the side, and vertical pattern velocity to avoid collisions with the ground. These distinct strategies may derive from greater need to avoid collisions in birds, compared with small insects.