Hummingbirds are bigger than bees, so they need more energy (plant sugar). Moreover, it is known that hummingbirds use a lot of energy in flight and are often times very close to starvation between meals. As a result, Hummingbirds need to feed at high energy source locations such as flowers with lots of nectar, as flying around to different sources and just consuming small amounts of sugar might lead to starvation. WITH RESPECT TO POLLUNATION, if they don't have enough energy to fly and transfer the pollen to a receiving plant of the same species, then they won't reach that plant, and the pollen will go to waste with respect to reproduction ( the pollen may still be used as food ). So if the the receiving plant is, say, 50 miles away, the Hummingbird needs to have enough energy to fly that distance to transfer the pollen, and therefore that is why it eats only flowers with lots of nectar.
Bees on the other hand don't need as much energy, so they don't need to feed on plants with large amounts of nectar. They have enough energy to fly around and transfer the nectar by just feeding on small flowers. Since the bird feeds on multiple flowers, it gets a greater variety of pollen (presuming the flowers are of different species or even the same species but not clones) so it has a greater chance of transferring the to a correct species receipient since there are more choices (since it has pollen from multiple species) or at least a greater chance of increasing genetic diversity if the pollen is from the species but not a clone.
biology please help - SraJMcGin, Sunday, May 24, 2009 at 2:13am
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