Retinal morphology and electrophysiology of two caprimulgiformes birds: the cave-living and nocturnal oilbird (Steatornis caripensis), and the crepuscularly and nocturnally foraging common pauraque (Nyctidromus albicollis).

ABSTRACT

Oilbirds (Steatornis caripensis) breed in the total darkness of caves and forage at night on fruits. Common pauraques (Nyctidromus albicollis) are crepuscular and nocturnal foragers on flying insects. We examined if their retinal structure and function can be correlated with their types and periods of activity. Electroretinograms (ERGs) were obtained from anesthetized birds in photopic and scotopic conditions to a wide range of light intensities, following which the retinas were processed for histological analysis. Retinal sensitivity is higher in oilbirds than in common pauraques. Under scotopic conditions with maximum flash luminance, the average (+/- 95% CI) b-wave amplitude of oilbirds is double that of common pauraques (500.4 +/- 49.8 and 245.4 +/- 40.9 microV, respectively) but, under photopic conditions, the results are the reverse (common pauraque 69.4 +/- 18.1; oilbird 23.0 +/- 4.4 microV). On the other hand, the retina of both species is highly rod-dominated, but rods are highly more numerous in oilbirds than in common pauraques (rodscones ratio 1231 and 51, respectively). In oilbirds, rods are largely thinner and their outer segments are 1.0 microm in diameter and 18.6 microm in length. They are distributed over various levels in the photoreceptor layers, an arrangement known for deep-sea fishes, but so far unknown for birds. In common pauraques, rods are patchily distributed and their outer segments are 4.0 microm in diameter and 53 microm in length. The oilbirds rod thinness allows more rods per area unit, and thus to catch more photons per area unit under darkness, while the low cone number suggests that the species has poor daytime vision, which concurs with the species cavernicolous daytime habits. The lower rod number of common pauraques, compared to oilbirds, appears counterbalanced by their patchiness and longer and thicker outer segments to provide high retinal sensitivity. In addition, common pauraques also have a tapetum. These features, combined with a higher proportion of cones, show that common pauraques are well equipped for crepuscular and nocturnal foraging on flying insects in an open environment.