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

We compared the diurnal and nocturnal visual function in two tactile foragingwaterbird species, the red subspecies of the American White Ibis (Eudocimus ruberruber, formerly the Scarlet Ibis), which is known to feed exclusively during daytime, andthe Black Skimmer (Rynchops niger), which forages primarily during darkness. Live birdswere captured in coastal lagoons of northeastern Venezuela. Electroretinograms (ERGs)were obtained at different light intensities from anesthetized birds, and the retinae weresubsequently processed for histological observations. The ERGs of the skimmer were ofmuch 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 alwayswas significantly larger for the ibis than for the skimmer. The scotopic:photopicb-wave ratio, calculated with b-waves obtained at the highest flash luminance, was 6.82: 1for the skimmer and 0.89:1 for the ibis. The retina of the ibis contained, on the average,18.8 rods/310 [mu]m and 56.6 cones/310 [mu]m, for a rod:cone ratio of 1:3. The retina of theskimmer contained 90.2 rods/310 [mu]m and 16.8 cones/310 [mu]m, for a ratio of 5:1. Thehigher 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 functionand, consequently, potentially inferior nocturnal visual capabilities. The latter would seemto explain the temporal differences observed in feeding behaviors of the two species.Key words: waterbirds, Rynchops niger, Eudocimus ruber, Black Skimmer, AmericanWhite Ibis, Scarlet Ibis, vision, retina, rod, cone, electroretinogram, nocturnal foraging,nocturnal activity.

Luz Marina Rojas, Raymond Mcneil, Therese Cabana, and Pierre Lachappelle, Condor: Vol. 99, No. 1, January-February, 1997