SPATIAL CONTRAST SENSITIVITY OF BIRDS
Contrast sensitivity (CS) is the ability of the observer to discriminate between adjacent stimuli on the basis of their differences in relative luminosity (contrast) rather than their absolute luminances. Prior to this study, birds had been thought to have low contrast detection thresholds relative to mammals and fishes. This was a surprising phenomenon because birds had been traditionally attributed with superior vision. In addition, the low CS of birds could not be explained by retinal or optical factors, or secondary stimulus characteristics. Unfortunately, avian contrast sensitivity functions (CSFs) were sparse in the literature, so it was unknown whether low contrast sensitivity was a general phenomenon in birds. This study measured CS in six species of birds sampled across different taxa and different ecological backgrounds in order to answer this very question. The species chosen for this experiment were
American kestrels (Falco sparverius), Barn owls (Tyto alba), Japanese quail
(Coturnix coturnix japonica), White Carneaux Pigeons (Columba livia), Starlings (Sturnus vulgaris), and Red-bellied woodpeckers (Melanerpes carolinus). CSFs were obtained from these birds using the pattern electroretinogram (PERG), and compared with CSFs from the literature.
The quail and pigeon data obtained in this experiment fit well with existing CS data for these species. The kestrel data were not similar to kestrel data in the literature; however the data in the literature were collected from a single subject. All of the birds studied had contrast sensitivities that were consistent with their retinal or optical morphologies relative to other birds (in species for which such data exists) and seem well suited for their natural environments. In addition, all of these birds exhibited low CS relative to humans and most mammals, which suggests that low CS is a general phenomenon of birds. Explanations for this avian low CS phenomenon include a possible trade-off between contrast mechanisms and UV mechanisms in cone systems, and lateral inhibitory mechanisms that are perhaps categorically different from mammals. Lateral inhibition affects contrast gain, and has been shown to differ according to ganglion cell types, which in turn will differ in vertebrate species.
Survival in the visual environment depends on the ability to use the properties of light to find food, select a mate, avoid predators, and care for young. Among these properties are (1) the wavelength of light, (2) the intensity of light, (3) relative light intensity or contrast, and (4) the spatial and temporal distributions of the first three properties. One of the factors in the evolutionary success of birds in terms of their large numbers of species and the wide range of environments that they inhabit, has been their exploitation of these properties of the visual world. For example, birds have at least three, and often four or more cone photo-pigments, which in combination with colored oil-droplets that act as additional wavelength filters, act to create many more spectral sensitivity maxima (Bowmaker, 1977; Chen and Goldsmith, 1986). These pigments and oil-droplets give birds richer color vision than could ever be experienced by humans with their three cone photopigments and no oil-droplets. Birds are even capable of utilizing the ultra-violet portion of light, as demonstrated in behavioral studies using pigeons (Emmerton, 1983; Remy and Emmerton, 1989; Wright,
1972) and passerines (Bennett and Cuthill, 1994; Bennett et al., 1997; Smith, Greenwood, and Bennett, 2002).
Birds also possess superior acuity, which is the ability to resolve tiny objects or fine detail. Hawks and eagles are especially notorious for having acuities that far outstrip human acuity (Fox, Lehmkuhle, and Westendorf, 1976; Reymond,
1985; Schlaer, 1971). Acuity is only one narrow aspect of spatial vision, however, and it is of interest to obtain a more complete representation of spatial vision from such visually endowed species. Contrast sensitivity testing offers a broader measure of spatial vision, which includes in its measure an indication of visual acuity as well as the ability to see larger objects of varying contrasts within the environment. The research proposed here will investigate the spatial vision of several species of birds using contrast sensitivity.