Some birds, such as kingfishers, can hover transiently, but hummingbirds (of the order Trochiliformes) are the only known avian species able to hover for long periods of time. This unique ability to hover in mid-air for prolonged periods evolved with the hummingbird’s “on-the-wing” feeding strategy.
The hummingbird has a number of morphological and physiological specializations which enable it to maintain its hovering. For example, its wings contain modified muscles and bones, and can be flapped 15-80 times per second (up to 50 times faster than other bird species). The wings generate force during both the up and the down strokes of the wingbeat cycle. These wing movements are unlike those of any other bird; the aerodynamic mechanisms of the movements actually resemble those of insects such as hawk moths. The hummingbird also has an increased metabolic rate, and an enlarged heart, which supplies the wings with the oxygen they need for flapping.
In a forthcoming paper in The Journal of Comparative Neurology, Andrew Iwaniuk and Doug Wong-Wylie, of the University of Alberta’s Bird Brain Laboratory, describe, for the first time, one of the neural specializations that enable the hummingbird to remain stationary in mid-air. They show that a nucleus in the hummingbird’s brain is enlarged in comparison to other bird species.
Crucial to the hummingbird’s ability to hover is the optokinetic response, a visual following reflex which occurs in response to the movements of large visual stimuli. The reflex involves eye, head and body movements in the direction of motion, and minimizes the amount of visual motion across the retina. The reflex minimizes the speed with which large stimuli move across the retina – the speed approaches zero, stabilizing the bird’s gaze so that it can accurately place its beak inside flowers to obtain the nectar.
A visual pathway called the accessory optic system is crucial for the optokinetic reflex. The accessory optic system consists of two distinct nuclei, called the pretectal nucleus lentiformis mesencephali (LM) and the nucleus of the basal optic root (nBOR). Both of these nuclei receive inputs from the retina and project to nuclei in the cerebellum, a part of the brain which involved in the control of movement. Both nuclei are also essential for the optokinetic reflex, as damage to either severely impairs or completely abolishes the reflex.
The LM contains a number of neuronal cell types: large multipolar neurons, which project to the cerebellum, and direction-selective neurons (i.e. cells that are activated in response to visual stimuli moving in a specific direction). These cells are involved in processing information about the apparent visual motion that occurs with movement (optic flow), and appear to be essential for the hummingbird’s sustained hovering. They have large receptive fields and are most responsive to slow-moving stimuli. The cells are thought to maintain the optokinetic reflex by detecting minimal retinal motion.
Iwaniuk and Wong-Wylie compared the sizes of the LM and nBOR in 9 hummingbird species and 28 other bird species. They found that the LM is hypertrophied (enlarged) in the hummingbird brain, relative to brain volume, in comparison to the other bird species. Neither the nBOR, nor any other of the visual nuclei examined, were enlarged. The LM is between 2-5 times larger in the hummingbird than in the other species examined. Furthermore, there seems to be a direct relationship between the size of the LM, and therefore the number of direction-selective neurons, and a bird’s hovering ability. In birds that can hover transiently, such as the eastern spinebill, the belted kingfisher and the American kestrel, the LM was also enlarged, but less so than that that of the hummingbird. Presumably, a larger LM means more direction-selective neurons, and, therefore, an enhanced ability to stabilize in mid-flight.