Eye size in birds and the timing of song at dawn

Phylogenetic and ecological determinants of the neotropical dawn chorus

The concentration of avian song at first light (i.e. the dawn chorus) is widely appreciated, but has an enigmatic functional significance. One widely accepted explanation is that birds are active at dawn, but light levels are not yet adequate for foraging. In forest communities, the onset to singing should thus be predictable from the species' foraging strata, which is ultimately related to ambient light level. To test this, we collected data from a tropical forest of Ecuador involving 57 species from 27 families of birds. Time of first song was a repeatable, species-specific trait, and the majority of resident birds, including non-passerines, sang in the dawn chorus. For passerine birds, foraging height was the best predictor of time of first song, with canopy birds singing earlier than birds foraging closer to the forest floor. A weak and opposite result was observed for non-passerines. For passerine birds, eye size also predicted time of first song, with larger eyed birds singing earlier, after controlling for body mass, taxonomic group and foraging height. This is the first comparative study of the dawn chorus in the Neotropics, and it provides the first evidence for foraging strata as the primary determinant of scheduling participation in the dawn chorus of birds.

Comparison of the Retinal Structure and Function in Four Bird Species as a Function of the Time They Start Singing in the Morning

We postulated that the retinas of bird species that are the earlier singers are more sensitive to low light conditions than species that sing closer to sunrise. The selected species were the American Robin (Turdus migratorius) and the Hermit Thrush (Catharus guttatus) as early singers, the Common Grackle (Quiscalus quiscula) and the Mourning Dove (Zenaida macroura) which join the dawn chorus near sunrise. Scotopic electroretinogram (ERGs) intensity-response functions were obtained from anesthetized birds, following which the animals were euthanized and their retinas processed for histological analysis. Based on k values, generally considered an adequate measurement of the scotopic (rod) retinal sensitivity, all species yielded comparable night vision capabilities. However, based on the maximal (mixed rod-cone response) amplitude of the scotopic b-wave, our results indicate that robins and thrushes yield larger scotopic ERGs compared with doves and grackles, but unexpectedly do not have higher rod:cone ratios. Increased thickness of the retinal inner nuclear layer and higher numbers of ganglion cells in robins and thrushes compared with doves and grackles suggest a greater number of synaptic connections maximizing vision under low light conditions, and might support their higher retinal responses under scotopic conditions. The higher ERG V(max) of robins and thrushes might also be explained, at least in part, by the optics (i.e., their lower minimum F-numbers), resulting in brighter retinal images, rather than from higher sensitivity of their retina. Our results suggest that an early onset of dawn singing might be correlated with the retinal ability to detect the first dim crepuscular lights.

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)

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 (rods:cones ratio: 123:1 and 5:1, 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.

Video-oculography in the gerbil

Normative vestibulo-ocular and optokinetic reflexes (VOR and OKR) and pupil diameter were measured in young adult gerbils using infrared video-oculography with 60 Hz sampling during head-fixed binocular recordings. The pupillary light-sink technique was preferred over a single-beam retinal reflection method because its measurements were less affected by pupil size. Eye movements were generally conjugate with occasional independent saccadic movements, and independent drifting movements in the dark. The horizontal optokinetic response to sinusoidal motion of a randomly spaced white dot pattern was maximal at low velocities (5 degrees/s), stronger temporonasally, and dropped off quickly at approximately 20 degrees/s. Constant velocity gain was near unity through 60-80 degrees/s with a sharp drop-off. Monocular viewing revealed almost no nasotemporal optokinetic response. Pupil diameter was found to vary as a saddle function with optokinetic gain from cycle to cycle, but also have a circadian rhythm (smaller at dusk) that related inversely to mean horizontal VOR gain. Gerbils with eyes open sometimes had no optokinetic response during long stimulus periods, which then resumed after a brief vestibular stimulus. The horizontal angular VOR gain was relatively flat across 0.1-1.0 Hz and 30-120 d/s sines (phase near zero), with a mean gain of approximately 0.78 in the dark, and 1.0 with the fixed pattern surround (n=15, for both raw calibrated and normalized data). Most animals also revealed a strong slow phase eye velocity asymmetry (dominant during ipsilateral rotation) in the half-cycle gain of their horizontal angular VOR response in the dark. A constant velocity horizontal optokinetic bias velocity did not change the gain or symmetry of the sinusoidal VOR response, but shifted the VOR response velocity in an additive (linear) fashion. Both cross-coupling (pitch or roll while rotating) and pseudo-OVAR (off-axis counter-rotation) stimuli generated horizontal nystagmus. The findings suggest that the gerbil, like other lateral-eyed rodents, relies on otolith cues to interpret angular motion.