ULTRAVIOLET VISION IN BIRDS - WHAT IS ITS FUNCTION

Although UV vision was first demonstrated in birds in the early 1970s, its function is still unknown. Here we review the evidence for UV vis ion in birds, discuss the special properties of UV light, lay out in d etail hypotheses for the function of UV vision in birds and discuss th eir plausibility. The main hypotheses are that UV vision functions: (i ) in orientation, (ii) in foraging and (iii) in signalling. The first receives support from studies of homing pigeons, but it would be unwis e to conclude that orientation is UV's primary function in all birds. It is especially important to test the signalling hypothesis because b ird plumage often reflects UV and tests of theories of sexual selectio n have virtually always assumed that birds perceive plumage ''colours' ' as humans do. A priori this assumption is unlikely to be correct, fo r unlike humans, birds see in the UV, have at least four types of cone s and have a system of oil droplets which filters light entering indiv idual cones.