I\u2019ve been looking at a lot of shorebirds recently now that migration has commenced.\u00a0 Have you ever seen a shorebird bob its head up and down rapidly?\u00a0 That behavior is the way that a bird with monocular vision can measure distance.<\/p>\n
Humans have binocular vision.\u00a0 That is, we can see most objects with both of our eyes.\u00a0 The slight angular difference between our left and right eye as we focus on an object allows our brain to do some trigonometry and determine how far away that object is.<\/p>\n
But shorebirds and many other birds suffer the risk of predation have their eyes set on the sides of the head.\u00a0 It is really hard to sneak up on a bird with lateral \u00a0displaced eyes.\u00a0 But except for a narrow field in front of the head (and sometimes behind!), an object can only be soon with one eye.\u00a0 So one would think that a shorebird would not be able to determine the distance with only a single eye fixed on an object.<\/p>\n
That\u2019s where the head bobbing behavior comes in.\u00a0 The bird is changing the angle at which it beholds an object by moving its eye up and down.\u00a0 The bird\u2019s brain that then figure out the distance using the same angular measurements that an animal with binocular vision uses.\u00a0 Birds essentially make one eye do the work of two to allow it to have depth perception.<\/p>\n
Birds have the best eyes of any animal.\u00a0 Bird eyes are huge; the eye of the Ostrich is two inches across, the largest eye of any land animal.\u00a0 Relative to the weight of the head, bird eyes are large.\u00a0 Fully 15% of the weight of a European Starling\u2019s head is contributed by eyes.\u00a0 By contrast, our eyes only contribute about 1% of our head weight.<\/p>\n
One would expect that birds have a well-developed sense of color given the striking variety of colors of the feathers and skin of birds.\u00a0 Like the human eye, the retina of a bird eye is richly endowed with cones, sensory receptors that detect color.\u00a0 The density of cones in birds is higher than in any other vertebrate.\u00a0 The density of cones in bird retinas varies from twice to five times the density of cones in the human eye.<\/p>\n
At the end of each cone in the retina, one can find a colored oil droplet through which light must pass to reach the retina.\u00a0 These droplets are red, orange, yellow and green.\u00a0 The function of these droplets is somewhat controversial but all agree that the droplets must serve to increase visual acuity.<\/p>\n
It\u2019s intriguing to note that the oil droplets in the cones on the lower half of the eye tend to be yellow.\u00a0 Images projected on the lower retina come from above and the yellow pigments filter out the blue of the sky, making objects like Peregrine Falcons easier to pick up.<\/p>\n
The cones in the upper retina tend to have red droplets.\u00a0 Images hitting the upper retina come from below the bird.\u00a0 The red droplets filter out the green of vegetation, making objects of other colors easier to see.<\/p>\n
Recent research has shown that some birds are capable of seeing ultraviolet light, wavelengths that are too short for humans to see.\u00a0 The world must look quite different through birds\u2019 eyes.<\/p>\n
How good is bird vision compared to humans?\u00a0 It\u2019s hard to be quantitative but it is certain that birds have much more acute vision than we do.\u00a0 We have some remarkable anecdotes for Old World birds.\u00a0 The European Bee-eater can see a bee at a distance of 100 yards!\u00a0 The Eurasian Hobby, a falcon that specializes on dragonflies, can see a dragonfly at 200 yards.\u00a0 From North America, Golden Eagles can detect the movement of a rabbit from a distance of a mile.<\/p>\n
[First published on September 2, 2012]<\/p>\n\n
I\u2019ve been looking at a lot of shorebirds recently now that migration has commenced.\u00a0 Have you ever seen a shorebird bob its head up and down rapidly?\u00a0 That behavior is the way that a bird with monocular vision can measure distance. Humans have binocular vision.\u00a0 That is, we can see most objects with both of […]<\/p>\n","protected":false},"author":146,"featured_media":0,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_s2mail":"yes","ngg_post_thumbnail":0,"footnotes":""},"categories":[430],"tags":[],"builder_content":"","_links":{"self":[{"href":"https:\/\/web.colby.edu\/mainebirds\/wp-json\/wp\/v2\/posts\/525"}],"collection":[{"href":"https:\/\/web.colby.edu\/mainebirds\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/web.colby.edu\/mainebirds\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/web.colby.edu\/mainebirds\/wp-json\/wp\/v2\/users\/146"}],"replies":[{"embeddable":true,"href":"https:\/\/web.colby.edu\/mainebirds\/wp-json\/wp\/v2\/comments?post=525"}],"version-history":[{"count":1,"href":"https:\/\/web.colby.edu\/mainebirds\/wp-json\/wp\/v2\/posts\/525\/revisions"}],"predecessor-version":[{"id":526,"href":"https:\/\/web.colby.edu\/mainebirds\/wp-json\/wp\/v2\/posts\/525\/revisions\/526"}],"wp:attachment":[{"href":"https:\/\/web.colby.edu\/mainebirds\/wp-json\/wp\/v2\/media?parent=525"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/web.colby.edu\/mainebirds\/wp-json\/wp\/v2\/categories?post=525"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/web.colby.edu\/mainebirds\/wp-json\/wp\/v2\/tags?post=525"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}