RETINAL MORPHOLOGY AND ELECTRORETINOGRAPHY IN TWO VISUALLY FORAGING CHARADRIIFORMES BIRDS WITH DIFFERENT FEEDING ACTIVITY RHYTHMS: THE DOUBLE-STRIPED THICK-KNEE (BURHINUS BISTRIATUS WAGLER, 1829) AND THE SOUTHERN LAPWING (VANELLUS CHILENSIS L., 1758)
Our study compares the visual function of the Double-striped Thick-knee (Burhinus bistriatus Wagler,1829), which forages primarily during dusk and at night, and the Southern Lapwing (Vanellus chilensis L., 1758), which is known to forage during daytime and occasionally at night, analyzing morphological and electrophysiological aspects of their retina. The fact that thick-knees have large eyes and are nocturnally actives suggest that, compared with the diurnal lapwing, they should have a very sensitive retina under low light intensity. Electroretinograms (ERGs) were obtained from anesthetized live birds at different light intensities in photopic and scotopic conditions, and the retinae were subsequently processed for histological analysis. The scotopic ERG b-waves of B. bistriatus, at all light intensities, were always of larger amplitude than those of V. chilensis. However, the a-waves of both species were of similar amplitude. Under photopic conditions, V. chilensis yield highest a- and b-wave amplitudes than B. bistriatus. The latter has a larger dialated pupil diameter and a greater axial length/equatorial diameter ratio than V. chilensis. Likewise, the rod density of B. bistriatus significantly exceeds that of V. chilensis. In the latter, cone density tends to be higher than in B. bistriatus while the rods:cones ratio were lower. Rod outer segments of B. bistriatus strongly exceed in length those of any other Charadriiformes species studied so far, but are thinner than those of V. chilensis. In contrast, the latter has thicker cone outer segments and outer and inner plexiform layers than B. bistriatus. Similarly, ganglion cells are more abundant per unit area in V. chilensis. Our combined results reveal a higher retinal sensitivity of B. bistriatus under low light conditions, in accordance with their crepuscular and nocturnal foraging strategies. V. chilensis, although mainly active during daylight, appears to have a moderate retinal sensitivity under low light conditions. Thus, we conclude that Southern Lapwings present visual adaptations that enable them to function in nocturnal as well as in diurnal conditions.