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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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
Striking of underwater prey by a reef heron, Egretta gularis schistaceaJournal of Comparative Physiology AJuly 1, 1987
Katzir G; Intrator N

“…a heron will attempt to reach spatial positions at which prey’s real depth and apparent depth are linearly correlated.” (Katzir et al. 1989:573)

Journal article
Stationary underwater prey missed by reef herons, Egretta gularis: head position and light refraction at the moment of strikeJournal of Comparative Physiology AJuly 1, 1989
Katzir G; Lotem A; Intrator N

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Organism
Western reef heronEgretta gularisSpecies


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