The nectar of flowers in the tropical rainforest is the food for small birds known as hummingbirds and sunbirds.
Hummingbirds (family Trochilidae) are distributed throughout North and South America.
Most hummingbirds are only a few inches long.
The smallest bird on Earth, the bee hummingbird, is not much larger than a bumblebee.
Sunbirds (family Nectariniidae) can be found in Africa, Asia and the northern part of Australasia.
When the canopy is unbroken, hummingbirds and sunbirds feed on the flowers of giant trees, but when a clearing has been made, either by human beings or by a falling tree, they feed among the luxuriant understory that quickly develops.
All are highly specialized for removing nectar from flower heads.
Since many flowers are too delicate to support a perching bird, these nectar-feeding birds often hover in front of the individual flower while carefully probing it with their long bills.
This demands high-precision flying, which can be achieved only by small birds with high power-to-weight ratio. (If an average-sized man produced as much power as a hummingbird, relative to his weight, his work output would be equivalent to 40 horsepower.)
Carrying limited reserves of fuel because of their size, hummingbirds and sunbirds have to feed frequently -every 10 to 15 minutes during active periods-to support their high power output. (If an average-sized man were to match the work output of a hummingbird, he would need a daily energy intake of 155,000 calories, which he could obtain from 370 pounds of potatoes or 130 loaves of bread.)
Where temperatures are high, hummingbirds do not have to use much energy to maintain their body heat.
However, in western Amazonia, in the foothills of the Andes, where the nighttime temperature may be very low, these birds do not attempt to keep warm. If they did, they would deplete their meager flight reserves.
Instead, they have evolved a means of allowing their body temperature to drop each night rather like hibernation.
Sunbirds and hummingbirds have brilliant plumages. Because they are so agile and their flight is so rapid, -up to 40 miles per hour (65kmph)-it is difficult for predators to catch them. Therefore, they do not require camouflaged.
Pollination
Nectar feeding is not a one-way process. By producing copious, high-energy nectar, a flower entices a bird to visit it.
Since the nectar is normally located deep within the recesses of a flower, a bird has to brush against the anthers of the flower to reach them.
The bird goes on to another flower and the pollen grains on its bill and feathers are rubbed off on to the stigma, thus ensuring pollination.
Flowers have evolved to attract the particular animals that pollinate them.
Flowers pollinated by birds are usually brightly colored but odorless. Red flowers seem to attract birds more than flowers of other colors