Cattle egrets are less able to cope with light refraction than are other herons

The majority of heron species (Aves, Ardeidae) forage on aquatic prey in sh allow water. Prey detection, aiming and the beginning of the capture strike s are performed while the heron's eyes are above water. For most angles, as a result of air/water light refraction, the apparent image available to a heron is vertically displaced from the prey's real position. Herons must th erefore correct for refraction. We tested the hypothesis that species that forage in aquatic habitats should be more able to correct for image dispari ty than those of terrestrial habitats. The ability of hand-reared herons of four species to capture stationary prey (fish) underwater (submerged) or i n air (aerial) was tested. Three species (little egret, Egretta garzetta,, squacco heron, Ardeola ralloides, and night heron, Nycticorax nycticorax) n ormally forage in aquatic habitats while the fourth (cattle egret, Bubulcus ibis) forages in terrestrial habitats. No individuals missed aerial prey. Success rates of little egrets and of squacco herons with submerged prey we re high, while night herons became less successful with increased prey dept h and/or distance. In cattle egrets, success rate was low and negatively co rrelated with prey depth. The observed interspecific differences may thus b e related to (1) differential. ability to correct for air/water light refra ction and (2) the species' foraging behaviour. We suggest that cattle egret s are in the process of losing their ability to cope with submerged prey. ( C) 1999 The Association for the Study of Animal Behaviour.