Not necessary, because the argument is confined to a comparison of two bird species with the same overall energy requirements. So whether birds typically have the same energy requirements is irrelevant.

Not necessary, because even if the nectar-eating bird does sometimes eat seeds, the comparison relates to the overall time spent eating. We don’t have any reason to think that sometimes eating seeds would have a significant impact on the overall eating time of a nectar-eating bird.

Necessary, because if it were not true — if the time it takes for a nectar-eating bird to eat a given amount of nectar IS longer than the time it takes the seed-eating bird to eat the same amount of seeds — then the seed-eating bird might NOT spend more time eating than does the nectar-eating bird. Although the seed-eating bird has to eat more seeds to get the same amount of energy, the lower time required to eat seeds might offset the extra time spent eating more seeds. The author must assume this isn’t true in order to reach the conclusion.

Not necessary, because the conclusion concerns a comparison between birds with the same overall energy requirement. So whatever impact body temperature might have on energy requirements is already controlled for.

Not necessary, because the conclusion concerns a comparison between birds with the same overall energy requirement. So whatever impact other factors may have on energy requirements are already controlled for.