Beason R and Loew E. (2008) Visual pigment and oil droplet characteristics of the bobolink (Dolichonyx oryivorous), a new world of migratory bird. Vision Research. 48:1-8.

The type of avian visual pigment and oil droplet has only been studied in 30 species of birds. This study investigates these combinations in relation to the visual communication of the bobolink (Dolichonyx oryivorous), a migratory bird of the New World. Avian photoreceptors, like humans come in rod and cone shape. They have a carotenoid pigment at the outer segment paired with an oil droplet that acts as a “low-pass filter” by limiting the type of light that reaches the photopigment to those which are lower than its cut-off wavelength. This reduces the overlap between cones that are close in the absorbance spectrum and allows birds to discriminate between more colors. Previous studies have found that avian vision is composed of four single cones in various combinations of different types of pigments including: long wavelength sensitive (LWS), short wavelength sensitive (SWS), violet sensitive (VS), and ultraviolet sensitive (UVS) pigments.

Seventeen adult bobolink birds were captured, reared, and given appropriate nutrients up until experimental procedures began. The coloring of their plumage proved that the nutrients were sufficient and accurately mimicked their natural diet. Before being euthanized, the birds were “dark adapted” to create optimal light sensitivity in the visual pigments. The eye was enucleated, hemisected, and the retina was removed from the bird. Before being mounted on a sealed slide, the pigment epithelium was removed from the retina. These slides were examined using a microspectrophotometer (MSP) under infrared (IR) lighting. The pigments and oil droplets were scanned at two ranges of wavelengths. Finally, to prove that pigment was present, the receptors were bleached and scanned, comparing the results to the pre-bleach scanning. Averages for each class of pigment and oil droplet were analyzed by normalizing averages to find peak absorbencies. The cut-off wavelength for the oil droplet was defined as an intercept of a line tangent to 50% absorbance to the maximum absorbance value.

Results found peak absorptions for 4 cone classes and 1 rod class. The UVS pigment was only measured to 350, and a peak at a shorter wavelength is possible, but not enough data was taken to prove this. Also, no blue-sensitive pigments (SWS) were detected among the cones. Six classes of oil droplets were identified and found paired with single cones (4), double cones (1), and both single and double cones (1). The combinations of single cone types and oil droplets is as follows: LWS and R-type oil, LWS and C-type oil, MWS and O-type oil, MWS and Y-type oil, UVS and T-type oil). The double cone had an LWS pigment and C-type oil.

Seven distinct classes (6 cone, 1 rod) of photoreceptors exist on the bobolink species retina, the largest number ever found in an avian species. Though it would be relevant to apply this to some scheme of evolutionary history of photoreceptors, it is not possible due to a lack of species that have been studied. The relevance to this class exists most obviously in the “spectral sensitivity” of receptors due to the combinations of pigment and oil droplets, allowing narrow tuning. This tuning can provide this species with a greater ability to perceive color and light. The purpose of these extra sensitive receptors is not known but can be speculated to involve habitat evaluation, food foraging, predator detection, or some other activity important to the species. Asking these types of questions here, despite the lack of answers, is important for stimulating additional research on this topic to better understand this form of communication which seems to be increasingly important in certain species.