Gustave Kramer conducted important research related to the ability of birds to orient and navigate. A. Because caged birds become disoriented when the sky is overcast, Kramer hypothesized that birds orient themselves according to the Sun's position. B. In one set of experiments, Kramer placed the box containing food at the same point of the compass each time he put food boxes in the birds' environment. C. Kramer demonstrated that an internal biological clock allows starlings to compensate for the Sun's movement. D. After several studies, Kramer surmised that an internal biological clock allows some species of birds to navigate at night. E. The role of environmental cues in birds' navigation is clear, for on overcast days, birds use objects besides the Sun to orient themselves. F. Kramer showed that night-migrating birds use the sky to navigate by the stars.