Coordinated Senses Keep Eyes Fixed in Flight

an apex bird turns its head down in flight while hunting

Biological Strategy

Kestrel

Mary Hoff

Image: TheOtherKev / Pixabay / Free non-commercial use

Adapt Behaviors

The environment is constantly changing, and if living systems can’t adapt to these changes, they don’t survive. Environmental changes can be cyclic such as seasons, sudden such as floods or forest fires, or gradual but long-term, such as ecosystems shifting from early to mature stages. These changes require some flexibility in behavioral responses to match the specific conditions. For example, as a fish called the lamprey swims, it constantly faces changes in currents. Skin sensors help it detect those changes and adjust its motion accordingly.

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Modify Position

Many resources that living systems require for survival and reproduction constantly change in quantity, quality, and location. The same is true of the threats that face living systems. As a result, living systems have strategies to maintain access to shifting resources and to avoid changing threats by adjusting their location or orientation. Some living systems modify their position by moving from one location to another. For those that can’t change location, such as trees, they modify position by shifting in place. An example of an organism that does both is the chameleon. This creature can move from place to place to find food or escape predators. But it also can stay in one place and rotate its eyes to provide a 360-degree view so that it can hunt without frightening its prey.

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Modify Speed

Modifying speed or magnitude of velocity is important for some living systems because it enables them to control their movement to access resources, escape predators, and more. Modifying speed requires not only overcoming inertia, but also minimizing the energy needed to make the change. Therefore, living systems have strategies to safely shift from fast to slow or slow to fast. An example is a bird called the kingfisher, which streamlines its body and feathers to quickly move from hovering over water to diving through the air and into the water. Once in the water, the kingfisher slows down by spreading its wings to avoid diving too deep.

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Manage Turbulence

A turbulent force occurs when air or water creates a chaotic or irregular motion. The source can be such things as wind, waves, and eddies caused by obstructions to air or water flow (such as that created by a rock in a stream). Because the force is irregular, it acts in unpredictable ways on multiple parts of a living system at any given time, decreasing the living system’s efficiency. Strategies used to manage turbulence include dampening the amount of turbulence, having flexibility to handle sudden changes, and making quick adjustments. An example is the mucus on aquatic organisms, such as barracuda sharks, that can reduce turbulent friction of seawater by 66%. In doing so, it decreases drag and increases the sharks’ swimming efficiency.

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Birds

Class Aves (“bird”): Eagles, hawks, sparrows, parrots

Birds are evolutionary engineering marvels. They are descended from dinosaurs, but are far from our idea of heavy, scaly reptiles. Of the specific adaptions that set them apart, most notable is flight—although some mammals can fly, birds take the prize for abundance in the skies. Many birds have hollow, lightweight skeletons and specially-designed wings to help them stay aloft. They also have feathers made of keratin that help them stay warm, attract mates, and improve navigation and aerodynamics in flight. In contrast to their dinosaur ancestors, they lack true teeth and have replaced them with specialized beaks and bills.

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The eyes of a kestrel remain fixed on an object due to the bird’s ability to use reflexes to rapidly reposition its eyes and head with respect to its body.

Introduction

Flying high above a grassland, an American kestrel (Falco sparverius) spies a tiny vole on the ground below. Even though it’s being buffeted by winds, this sharp-eyed member of the falcon family is able to keep its head virtually motionless and its eyes fixed on the creature. After hovering for a moment to collect itself, it plummets to the earth and snatches the meal in its sharp talons.

Image: Kevin Cole / CC BY - Creative Commons Attribution alone

Four reflexes allow the American kestrel to fix its eye on a prey animal even as its body is buffeted by the wind.

Image: USFWS/Southeast / Public Domain - No restrictions

The American kestrel’s keen vision and ability to stay focused allows it to find and capture prey from the air.

The Strategy

Kestrels are small birds of prey that are found throughout much of the Americas. They are among a few birds, including hummingbirds, that have a special talent: They are able to keep their eyes trained on objects even as their bodies move. This helps a kestrel to keep sight of prey it has already located, and also allows it to locate new targets, since it can see clearly whether an object it is observing is moving

A kestrel achieves this feat through a combination of four reflexes, movements that can happen extremely rapidly because they don’t require involvement of the brain. Two of these relate to the vestibular system, which controls balance, and two of which relate to the interaction between nerves and muscles that regulates eye movement.

The first of the four reflexes is called the vestibule-ocular reflex. This uses information from the eye and from the vestibular system to control eye movement relative to movement of the head and neck. The second, the vestibule-collic reflex, mobilizes neck muscles when it senses head movement. The third, the optolinetic nystagmus reflex, responds to the apparent movement of an image of an object on the retina by moving the eye in a way that keeps the image in the same spot on the retina. The fourth, the optocollic reflex, keeps the head motionless while the rest of the body moves. Together these reflexes help the kestrel to keep its eyes on the prize—and ultimately survive to pass the trait along to the next generation.

Image: USFWS/Southeast / Public Domain - No restrictions

The American kestrel’s keen vision and ability to stay focused allows it to find and capture prey from the air.

The Potential

The kestrel’s ability to have its body buffeted by wind or repositioned by other forces while its eyes stay fixed on an object could inform a diverse array of human activities. It could inspire efforts to design aerial structures such as wind turbines or TV towers in a way that allows them to maintain a fixed position regardless of wind or other forces acting on them. It might be useful in the development of augmented and virtual reality imagery, or for enhancing the quality of video or photographic data collected from drones. It might even contribute to developing better motion sickness therapies, since a common strategy for preventing this common ailment is to keep eyes fixed on a distant object.

The ability to maintain a stable visual image is of extreme importance while birds, such as raptors, are attempting to capture prey in flight. Jones et al. 2007