Since they can see only during the daylight, some birds must return to roost as darkness approaches. This is because their eyes contain?

The correct answer indicates that the birds have cones but no rods in their eyes. Cones are the photoreceptor cells in the retina that are responsible for color vision and functioning well in bright light conditions. They help with the perception of fine detail and are essential for diurnal (daytime) activity.

If the birds can only see during the daylight, it suggests that they rely primarily on cones for their vision, which aligns with their need to return to roost as darkness approaches. The absence of rods, which are more sensitive to light and responsible for vision in low-light conditions, would result in an inability to see in dark environments. This explains the behavior of these birds needing to find safety before nightfall.

In contrast, the other choices suggest the presence of rods or inefficient forms of photoreceptors that do not align with the scenario of being active only during the day. Therefore, the emphasis on the presence of only cones logically supports the behavior described in the question.