Behavioral, Morphological and Physiological Correlates of Diurnal and Nocturnal Vision in Selected Wading Bird Species

Abstract

We examined in selected wading bird species if diurnal or nocturnal foraging and the use of visual or tactile feeding strategies could be correlated with retinal structure and function. The selected species were the Yellow-crowned Night Heron (Nycticorax violaceus), a crepuscular and nocturnal forager, the Great Blue Heron (Ardea herodias), a mainly crepuscular, but also diurnal and nocturnal feeder, the Roseate Spoonbill (Ajaia ajaja), a mainly crepuscular feeder which forages more at night than during the day, the Cattle (Bubulcus ibis) and Tricolored (Egretta tricolor) egrets and the American White Ibis (Eudocimus ruber) which forage only during daytime. Herons and egrets are visual foragers; ibises and spoonbills are tactile feeders. Electroretinograms were obtained from anesthetized birds in photopic and scotopic conditions to a wide range of light intensities, following which the retinae were processed for histological analysis. Based on rod densities and rods:cones ratios, nocturnal vision capability is greater in the Yellow-crowned Night Heron, followed by the Great Blue Heron and the spoonbill, then by the egrets and the ibis. Visual feeders that forage near dawn or dusk or at night have a higher rods:cones ratio, and consequently a greater night vision capability, than visual feeding species which forage only during daytime. Visual nocturnal feeders have a night vision capability greater than tactile diurnal as well as tactile nocturnal feeders. However, based on maximum scotopic b-wave amplitudes, all species studied have roughly comparable night vision capability. The factor that best discriminates between wading bird species appears to be the daytime visual capabilities. Indeed, the diurnal ibis and egrets have similar cone densities, cones:rods ratios, and photopic a-wave amplitudes, values which are greater than those measured in the two nocturnally active heron species.

The intensity threshold of colour vision in two species of parrot

“…Instead, we conclude that the retinal organization and the pooling of photoreceptors are of more influence. Bourke's parrots have a retina with a cone to rod ratio of 1.2:1 (Table1) that is comparable to values found in earlier studies of birds that are active both during twilight and in daylight (Rojas et al, 1999). The cone to rod ratio of budgerigars is higher, 2.1:1 (Table1), which appears to be typical for diurnal birds (Rojas et al, 1999;McNeil et al, 2005).…”

Ultraviolet sensitivity and colour vision in raptor foraging

“…The change in our predicted chromatic contrasts when including self-screening was less than 5% even for unusually long cone outer segments of 30μm (cf. Rojas et al, 1999;McNeil et al, 2005;Emond et al, 2006) using an absorption coefficient of 0.035μm −1 (cf. Warrant and Nilsson, 1998).…”

Microspectrophotometry of visual pigments and oil droplets in a marine bird, the wedge-tailed shearwaterPuffinus pacificus: topographic variations in photoreceptor spectral characteristics

“…The length of the cone outer segments was assumed to be identical between central and peripheral regions, as appears to be the case for other bird species (e.g. Rojas et al, 1999). Nevertheless, the total photon catch of all single cone types in the centre of the visual streak was less than that at the periphery because of the reduced cross sectional area of the oil droplets (mean diameters 1.4, 2, 2 and 2.1·µm for t, c, y and r oil droplets, respectively), despite the reduction in oil droplet pigmentation (which at the periphery reduces the overall photon catch of the outer segment by 30, 47 and 39%, respectively, for the SWS, MWS and LWS single cones).…”