Sensory ecology sharpens ideas about the worlds that birds inhabit
LINKED PUBLICATIONS
White-headed Vulture Trigonoceps occipitalis shows visual field characteristics of a hunting raptor. Portugal, S.J., Murn, C.P. and Martin, G.R. 2017. IBIS. DOI: 10.1111/ibi.12448 VIEW
The Sensory Ecology of Birds. Martin, G.R. 2017. Oxford University Press, Oxford. VIEW
“Bird’s eye views” are everywhere. A finger on a touch pad can glide us across the globe; we can casually sweep from the view that an albatross apparently gets as it flies to its nest in South Georgia, to what a vulture apparently sees when looking for carrion in Tanzania’s Ngorongoro Crater. The notion that these really are birds’ eye views is deeply engrained. When we use the term “bird’s eye view”, we actually think that this is how the world looks to a bird.
A recent advertising stunt for a miniature video camera used a tame White-tailed Eagle. From a camera strapped to its back, the scene below was filmed as the bird was released from the Eiffel Tower. The strap line read, “The 100% authentic eagle eye view”.
But what did this eagle actually see? Does that video really give a glimpse of what the eagle saw? Would a dove, a vulture, or an albatross see the same thing as the eagle? Would any of them see the same thing as the digitised images in the advert?
The key question is do birds, whether on the ground or in flight, see the world through the same eyes as us? Do any other animals see the world as we do?
This is a rather old and fundamental question. First raised by the Greek philosopher Epicurus, it cuts straight to that troublesome idea of “reality”. Pondering how the world appears to other animals asks the most uncomfortable of questions, “What can we actually know about the world?”
Reality: Partial and Particular
The idea that culture and learning poses limits on ways of seeing is an important and much debated idea. However, there is a much broader perspective on reality: one which sees each animal embedded in its evolutionary history. This perspective sees us as just one type of animal that, like all others, has evolved to gather only certain information from the world about them, information that natural selection has honed to become crucial for the survival and reproduction of each particular species. Recent investigations show us that that information is always partial with respect to all that the world offers; it is never comprehensive.
Thus, each animal is trapped in a particular reality that is the product of its evolutionary history. This history has produced a unique suite of senses that extract, or filter, particular information from the world about. At base it is information which ensures sufficient control of bodies for their survival and reproductive success.
Sensory Ecology
Explorations of the senses of birds now shows us just how specialised each animal’s view of the world is. Reality becomes a very flexible concept when we start to compare the information that we have available through our senses, with what is available to different bird species. In The Sensory Ecology of Birds, it is shown that the idea of a bird’s eye view soon evaporates into just a lazy metaphor. A camera strapped to an eagle’s back shows us just a view of the world as seen from a camera strapped to an eagle’s back; it does not give us an eagle’s eye view.
The sensory worlds of birds are particularly intriguing because it is so easy to think that what we see, they also see. It is easy to assume that birds see the world as we do; the only difference is they see it from a different location. We are seduced into believing birds apparently share the same world with us. After all they live alongside us and have the same sensible habit of completing activities in day light. When birds are active at night this is seen as something strange or quirky, needing special explanations.
Figure 1 Nocturnal birds. Both North Island Brown Kiwi Apteryx mantelli (left) and Oilbirds Steatornis caripensis (right) are highly nocturnal, but they differ in the suite of senses used to guide their behaviour in the very low light levels of their preferred habitats. Kiwi have almost given up on vision and rely upon information primarily from olfaction and touch. On the other hand, Oilbirds have eyes that are very close to the absolute limits of sensitivity but they also rely upon information from hearing (both active and passive SONAR) and olfaction to conduct their lives © Graham Martin
Partial information, collisions and entrapment
Each bird species relies upon a different suite of information from its different senses, and all birds have available to them very diverse information from hearing, olfaction, touch, taste and magnetoreception, as well as vision. The information gained from any one sense may only partially solve a particular perceptual challenge that is thrown up by the execution of specific tasks under particular environmental conditions. But this information is often used with partial information from other senses in a complementary way to guide complex behaviours.
This does, however, provide severe limits on what a bird can detect and knowledge of these limits can take us to the heart of some challenging issues that face birds, such as collisions with power lines, wind turbines, motor vehicles or glass panes, and also to the vexed question of why some diving birds are so prone to ending up as the by-catch of fisheries. Understanding the sensory world of birds can explain why human structures, which are so often obvious to us, pose very real problems for some birds. Crucially, this understanding can also indicate ways to mitigate the problems.
Figure 2 An extreme case of flying blind. Griffon Vultures Gyps fulvus are prone to collisions with wind turbines. It has been shown that their vision is highly adapted to the detection of information from below or to the side of them, not ahead or above them. This means that when foraging they may simply not see what lies ahead. This vulture is taking a keen interest in what is below and clearly not looking where it is going! This may seem an extreme case of flying blind, but it is not atypical of this species
© Olivier Duriez
Fine tuning and foraging
Knowledge about bird senses can also give us hints about the basic ecology and evolutionary history of a species. Our recent work on the sensory ecology of White-Headed Vultures Trigonoceps occipitalis has provided evidence that supports the idea that these birds behave as active predators, they are not just scavengers like other vultures. This is seen in subtle difference between the vision of White-Headed and Gyps Vultures. These differences indicate that the vision of White-Headed Vultures is more like that of other raptors than vultures. Fine tuning of these Vultures’ senses to the tasks of foraging seems to be evident. Furthermore, this fine tuning of vision to foraging tasks may be the key to what drives the vision of all birds.
The aphorism that “a bird is a wing guided by and eye” was once thought to capture the essence of a bird. However, I now argue that this should give way to an alternative, “a bird is a bill guided by an eye”. This is because all evidence on bird senses points to them being moulded and honed primarily by the sensory challenges of foraging, not flight.
It is now clear that every eye, every sense, tells a different story about the challenges that have honed its capacities. Unravelling these stories and how vision is used alongside other senses is providing rich and rewarding insights into what birds are and, by contrast, it also poses the question of just what we are.
References and further reading
Portugal, S.J., Murn, C.P. and Martin, G.R. 2017. White-headed Vulture Trigonoceps occipitalis shows visual field characteristics of a hunting raptor. IBIS. DOI: 10.1111/ibi.12448 VIEW
Martin, G.R. 2017. The Sensory Ecology of Birds. Oxford University Press, Oxford. VIEW
Image credit
Featured image: White-headed Vulture Trigonoceps occipitalis © Frank Wouters
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