Striking of underwater prey by a reef heron, Egretta gularis schistacea

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). When prey was unsubmerged the two phases were less distinct ('pre-strike' 47°, 62 cm/s; 'strike' 42°, 154 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.