Hi, the author is trying to prove that seed birds spend more time eating than do nectar birds, and to support this conclusion they tell us that nectar provides more energy than the same volume of seed.
There is a big flaw here. Time spent eating is not the same thing as calorie intensity. It might take me an hour to eat a 500 calorie salad, but only 5 minutes to eat a 1000 calorie burrito. Similarly, it might take a bird much longer or shorter to eat nectar or seeds.
This is exactly what C describes.
D goes on a bit of a tangent. We know that all birds need to eat to maintain their body temperature, but we know nothing else about that. While there are surely variations in body temperature, these fluctuations don't impact our argument about Time spent eating,
