Humming bird feed on the nectar, a sweet liquid inside certain flowers. Their tongues have tubes which help the humming bird to drink the flower nectar. They are able to access the amount of sugar present in the nectar; they prefer only those flowers in which sugar content is higher. Nectar is indeed a poor source of nutrients, so they prey insects and spider for their needs for protein, vitamins, minerals, etc., and also to feed their young.