Diurnal and Nocturnal Visual Function in Two Tactile Foraging Waterbirds: The American White Ibis and the Black Skimmer

Abstract

We compared the diurnal and nocturnal visual function in two tactile foraging waterbird species, the red subspecies of the American White Ibis (Eudocimus ruber ruber, formerly the Scarlet Ibis), which is known to feed exclusively during daytime, and the Black Skimmer (Rynchops niger), which forages primarily during darkness. Live birds were captured in coastal lagoons of northeastern Venezuela. Electroretinograms (ERGs) were obtained at different light intensities from anesthetized birds, and the retinae were subsequently processed for histological observations. The ERGs of the skimmer were of much larger amplitude than those of the ibis in scotopic (rod-mediated) conditions, but, in contrast, under photopic (cone-mediated) conditions, the amplitude of the ERGs always was significantly larger for the ibis than for the skimmer. The 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 for the ibis. The retina of the ibis contained, on the average, 18.8 rods/310 μm and 56.6 cones/310 μm, for a rod:cone ratio of 1:3. The retina of the skimmer contained 90.2 rods/310 μm and 16.8 cones/310 μm, 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 nocturnally active skimmer, the ibis has highly inferior rod function and, consequently, potentially inferior nocturnal visual capabilities. The latter would seem to explain the temporal differences observed in feeding behaviors of the two species.