Welcome to the American Ornithological Society 2018 Annual Conference. We are pleased to have you join us at the lovely Hilton El Conquistador Resort in Tucson, AZ.
Countershading (plumage that is dark above and light below) is pervasive among seabirds. Previous study has shown it to be weakly correlated with multiple potential selective agents, as is true of many adaptations. It is hypothesized to be an adaptation for crypsis and feather durability. White underparts of birds in flight are less visible from underwater than dark underparts, and dark upperparts might be more difficult to spot from above by predators or kleptoparasites. Dorsal eumelaninization may resist feather deterioration due to mechanical abrasion and UV irradiation. It is also correlated with feeding behavior and foraging group size, as well as by phylogeny. We present yet another potential adaptive function for countershading in seabirds: improved flight efficiency by drag reduction. We obtained wing measurements and migratory routes and times of wandering albatross, manx shearwater, and sooty tern from literature. We modeled thermal analysis of all combinations of solid black and white wing color dorsally and ventrally considering ambient temperature, sky temperature, sea surface temperature, wind speed, solar irradiation, humidity, and cloud factor specified hourly for each migratory route. Applying the Blasius solution for heated boundary layers, we found black over white countershading minimizes daily net skin drag, even though dark upperwing drag exceeds that of a light underwing at night. It also outperformed all other color combinations, day and night, although the specific outcomes were temperature dependent. This drag reduction may contribute to the energetic efficiency and unequaled endurance of low altitude dynamically soaring seabirds.
