Head Position Helps Correct for Light Refraction

photograph of grey heron standing on a rock during the day

Biological Strategy

Western reef heron

AskNature Team

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Sense Light (Visible Spectrum) From the Environment

Living systems constantly receive signals from their environment that help them survive. Light (in the visible spectrum) can come from other living systems (such as fireflies) or from non-living sources (such as the sun). Survival often depends on sensing and responding to challenges like low light conditions or light that has been altered in some way. Because basic survival is at stake, living systems must excel at meeting those challenges. A well-known phenomenon is how water bends light. A stork trying to catch a fish underwater can compensate for this bending effect so that when it strikes at the fish, it has a good chance of catching it.

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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 head of the reef heron corrects for light refraction at the water's surface by adjusting position and keeping a constant relationship between real and apparent prey depth.

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“Storks are highly successful fishermen, and have been for at least 50 million years. Only now are their numbers seriously threatened as their wetland habitats are drained or polluted. The black stork is found across Europe and Asia from Spain to China, and is rather a shy bird, nesting on cliffs or in tall forest trees. The stork seeks its prey while standing in water, and seizes it with the long pointed beak. Its eyes are directed slightly forward, giving it a certain degree of binocular vision, and it seems well able to allow for the effect of light refraction at the water surface when lining up its beak on the fish. It is thought that the relative angle of its eyes and beak assist in this.” (Foy and Oxford Scientific Films 1982:156)

Last Updated August 10, 2017

References

“The ability of a piscivorous bird, the western reef heron,Egretta gularis schistacea, to cope with light refraction at the air/water interface was investigated. The heron’s capture rate of small stationary underwater prey from a variety of angles was high, indicating an ability to correct for refraction. Two distinct phases were described during head movement: i) ‘Pre-strike’ (mean path angle 60° to the vertical, mean velocity 52 cm/s). ii) ‘Strike’ (mean path angle 33°, mean velocity 270 cm/s)…The point of change between phases (STR) was assumed to be the point at which corrections for refraction were performed. Calculated disparities between real and apparent prey positions at STR may reach 10 cm. At STR, highly significant correlations were found between i) the heron’s eye height above the water, and prey depth, ii) the apparent prey depth and real prey depth. A model is presented to explain the heron’s manner of correcting for light refraction.” (Katzir and Intrator 1987:517)

Journal article