I. Introduction

We have been studying motion perception in birds because we believe that understanding motion is critically important to vision in general and to avian vision in particular.  The chapter starts by explaining why. A different kind of motion studies seem to have captured people's imagination since early times and Frans Snyders' amazing painting depicting birds in all types of motion from the beginning of the 17th century (Museo del Prado, Madrid) seems a nice example of people's growing fascination by nature and the exotic at the time. As obvious in this painting as well, unfortunately, the simple description of positional change as "motion" seems very ambiguous in living animals - what is moving relative to what - eye, head, body, or objects in the outside world? Section II deals with this question.  Once we have solved this question, we can ask how well birds do at recognizing that objects in the outside world are moving, and that question is taken up in Section III.  Then, in Section IV, we consider how well birds can discriminate between different moving objects, or different kinds of motion of the same object.  Section V becomes more theoretical: we argue that discriminating motion or moving objects involves a special kind of concept discrimination, an idea that has been discussed in other chapters of this book.  Finally in Section VI we draw some tentative conclusions - chiefly that much more research needs to be done, but that the way - in terms of conceptual  and technological conditions - is now open to do it.

An ecological perspective 

Moving stimuli are among the most common types of stimuli encountered by birds. Not only are most environmental stimuli embedded in a moving flow field due to the bird's own movements, but also the other objects or animals themselves are more or less constantly in motion. Of course there are times, for example during foraging for seeds, when the fixation of static objects becomes more prominent than that of moving stimuli. But these are exceptions. 

This chapter discusses birds' perception of objects that are moving relative to their own eyes. While we will include some empirical material from a variety of species of bird, as in many other chapters of this volume, much of the experimental work has been done on relatively few species - the great majority of it on pigeons, with a significant additional contribution from experiments on chickens, especially experiments using imprinting techniques on newly-hatched chicks. 

It can be assumed that each species of bird's reactions to moving stimuli have been strongly affected by evolutionary pressures to perceive motion and recognize moving stimuli. Some of those pressures will be the same for all or most species of birds, but some will be specific to particular species. The modern approach to avian cognition can be seen as an integrative approach linking together such disciplines as animal learning, cognitive science and behavioral ecology (see e.g. Cook 1993, Lea & Dittrich 1999). Obviously, birds have evolved so that their motion processing abilities seem to match the circumstances of the restricted world (Umwelt) in which they dwell (von Uexkull 1921). Therefore, a discussion of motion discrimination and recognition, based on the current literature, has to take into account the way in which these processes have been shaped by the ecological demands and evolutionary requirements on birds in general, and pigeons and chickens in particular. We open this chapter with a brief discussion of some of the most important of the general requirements on birds to perceive visual motion. 

1. Predicting the future position of stimuli from information about the current motion of stimuli clearly seems a most useful ability. It is required by any bird that catches moving prey, whether insects in the air, mammals on the ground, or fish under water; by any flying bird, which has to judge its approach to a landing site (static absolutely, but moving relative to the bird) so as to neither crash into it nor fall short of it; and by any bird that has to avoid moving predators. 

2. A most remarkable example of the role of motion discrimination and recognition can be seen in some cases of insect camouflage. A moth resting on the bark of a tree may be indistinguishable from other parts of the tree to the search of a possible insect-eating bird as a matter of camouflage. However, should the insect move the situation can take a dramatic change. Camouflage is immediately broken by the moth's movement and the insect can readily be detected by the bird. In formal perceptual terms, what is happening is that the relative motion of the insect compared to the static bark patterns is used to segment the bird's visual world into figure and ground relationships. 

You can see that this principle works for humans, too. You can see this by clicking on this link, and then clicking the "Fly" button on the page you reach (this is part of an extensive website of visual phenomena provided by Illusion Works). 

In humans we would describe the visual task for the organism as the determination of a figure-ground relationship. Whether birds are faced with a visual problem of the same nature remains to be seen. Nevertheless, it seems very likely (and more formal evidence will be given later in the chapter) that some birds have the ability to define and distinguish patterns and objects using only motion information. Although most birds seem to have high visual acuity, hawks, penguins and insectivorous birds are strictly dependent on motion cues for detecting prey at more or less great distances. Hawks can spot their small prey from distances of more than a few hundred meters. Similarly, one is always surprised seeing a bird that sits on a branch of a tree, suddenly flying to a particular point over a lake or pond, pick up an insect, and return. In neither case would a motionless object at such distances have been discriminated from the background. And the fact that a lot of species that are hunted as potential prey, have developed the behavior of 'freezing' as a very successful anti-predator strategy, seems evidence in itself for the importance of motion information in figure-ground discrimination. Neurophysiological evidence tends in the same direction: it has long been known that there are cells in the pigeon visual system that respond to relative motion between objects and background (e.g., Frost & Nakayama, 1983). 

3. The majority of birds have relatively little binocular overlap in their visual fields. Yet they have an acute need to determine distances to seen objects. Under monocular condition, motion (whether through flow cues or parallax) is one of the best potential sources of distance information.

Next section: Retinal motion