A bird's flight weight may vary due to which of the following factors?

The flight weight of a bird can indeed vary due to temperature variations. Birds are warm-blooded animals that rely on maintaining their body temperature within a specific range to function optimally. When temperatures drop, birds may need to add more weight in the form of fat reserves or adjust their body weight for insulation and energy efficiency during flight. Conversely, in warmer conditions, a bird might shed weight, particularly if it is utilizing energy reserves or if food becomes less plentiful. Thus, temperature changes can have a direct impact on a bird's flight weight as they adapt to maintain their physiological functions and performance.

While the other factors listed, like diet composition, time of day, and altitude changes can influence a bird's overall weight or condition, they do not have the same direct impact on the immediate adjustments in flight weight that temperature variations can cause.