DIURNAL AND NOCTURNAL VISUAL FUNCTION IN 2 TACTILE FORAGING WATERBIRDS - THE AMERICAN-WHITE IBIS AND THE BLACK SKIMMER

We compared the diurnal and nocturnal visual function in two tactile f oraging waterbird species, the red subspecies of the American White Ib is (Eudocimus ruber ruber, formerly the Scarlet Ibis), which is known to feed exclusively during daytime, and the Black Skimmer (Rynchops ni ger), which forages primarily during darkness. Live birds were capture d in coastal lagoons of northeastern Venezuela. Electroretinograms (ER Gs) were obtained at different light intensities from anesthetized bir ds, and the retinae were subsequently processed for histological obser vations. The ERGs of the skimmer were of much larger amplitude than th ose of the ibis in scotopic (rod-mediated) conditions, but, in contras t, under photopic (cone-mediated) conditions, the amplitude of the ERG s always was significantly larger for the ibis than for the skimmer. T he scotopic:photopic b-wave ratio, calculated with b-waves obtained at the highest flash luminance, was 6.82:1 for the skimmer and 0.89:1 fo r the ibis. The retina of the ibis contained, on the average, 18.8 rod s/310 mu m and 56.6 cones/310 mu m, for a rod:cone ratio of 1:3. The r etina of the skimmer contained 90.2 rods/310 mu m and 16.8 cones/310 m u m, for a ratio of 5:1. The higher density of rods in the skimmer is in some way counterbalanced by their thinness. Compared to the nocturn ally active skimmer, the ibis has highly inferior rod function and, co nsequently, potentially inferior nocturnal visual capabilities. The la tter would seem to explain the temporal differences observed in feedin g behaviors of the two species.