The intensity threshold of colour vision in two species of parrot.

We have used behavioural tests to determine the intensity thresholds of colour vision in Bourke's parrots (Neopsephotus bourkii) and budgerigars (Melopsittacus undulatus). We have also examined the relationship between these thresholds and the optical sensitivities of single photoreceptors using morphological methods. Bourke's parrots lose colour vision in brighter light (0.4 cd m(-2)) than budgerigars (0.1 cd m(-2)) and both birds lose colour vision in brighter light (;end of civil twilight') than humans (0.02 cd m(-2), ;moonlight'). The optical sensitivities of single cones are similar in both birds (budgerigar 0.27 microm(2) sr, Bourke's parrot 0.25 microm(2) sr) but Bourke's parrots have more (cone to rod ratio, 1.2:1.0), thinner (2.8 microm) and longer rods (18.5 microm) than budgerigars (2.1:1.0, 3.4 microm, 13.3 microm). Bourke's parrots thus have an eye type that, with a flexible pooling mechanism, allows for high resolution or high absolute sensitivity depending on the light conditions. The results nicely agree with the activity patterns of the birds, Bourke's parrots being active during the day and in twilight while budgerigars are not normally active before sunrise and after sunset. However, Bourke's parrots have fewer cones than budgerigars, which implies that a smaller number of cones are pooled within each retinal integration area. That could explain why Bourke's parrots have a higher intensity threshold of colour vision than budgerigars. Furthermore, the study emphasises the need to expand the sensitivity measure so that photoreceptor integration units are used rather than single receptors.