Flight has been an important innovation for animals and has led to adaptive radiations in both birds and insects. For birds, what additional fitness benefits have been gained through adaptations for flight? A. Reductions in reproductive organs has led to the production of eggs with a larger allantois, increasing nutrients for their offspring. B. Birds rely on metabolic heat to stay warm and feathers help birds regulate their body temperature. C. The lack of teeth means that birds have become dietary specialists, optimized for eating exclusively grains (land-dwellers) or fish (shore-dweller). D. The relative metabolic rate (per kg) is lower in lighter vertebrates, which means that birds need loss food than other reptiles of a similar size.