Diurnal and nocturnal visual capabilities in shorebirds as a function of their feeding strategies

Some shorebird species forage with the same feeding strategy at night and d uring daytime, e.g, visual pecking in the Wilson's Plover (Charadrius wilso nia) or tactile probing in the Short-billed Dowitcher (Limnodromus griseus) . The American Woodcock (Scolopax minor! uses tactile probing, by day and b y night, but sometimes pecks for insects during daytime. The Black-winged S tilt (Himantopus himantopus) is a visual pecker, both by day and by night, and sometimes forages tactilely on windy (agitated water surface) moonless nights. Territorial Willets (Catoptrophorus semipalmatus) are visual pecker s during daylight and on moonlight conditions but switch to tactile feeding under lower light conditions. It could be postulated that some shorebird s pecies would switch from visual feeding during daytime to tactile foraging at night because they have poor night vision compared to species that are a lways sight foragers irrespective of the time of the day. This issue was ex amined by comparing retinal structure and function in the above species. El ectroretinograms (ERGs) were obtained at different light intensities from a nesthetized birds, and the retinae were processed for histological observat ions. Based on ERGs, retinal sensitivity, and rod:cone ratios, both plovers and stilts are well adapted for nocturnal vision. Although they have low r od density compared to that of stilts and plovers, Willets and woodcocks ha ve a scotopic retinal sensitivity similar to that of stilts and plovers but rank midway between plovers and dowitchers for the b-wave amplitude. Dowit chers have the lowest scotopic b-wave amplitude and retinal sensitivity and appear the least well adapted for night vision. Based on photopic ERGs and cone densities, although stilts, Willets and dowitchers appear as well ada pted for daytime vision, plovers occupy the last rank of all species examin ed. Compared to the nighttime tactile feeders and those that switch from da ytime visual pecking to tactile feeding at night, nighttime sight feeders h ave a superior rod function and, consequently, potentially superior nocturn al visual capabilities.