With more than 300 species, all native to the western hemisphere, hummingbirds are among the most important avian pollinators. The smallest, the bee hummingbird, is roughly the size of a bumblebee, weighing a meager 2.2 grams. Yet it can fly at speeds of 40 miles an hour, beating its wings 100 times a second—a fluttering motion that produces the birds' familiar hum. Working fast, the rubythroated hummingbird can visit more than 200 jewelweed flowers in 15 minutes. Indeed, everything these animals do is at a hectic pace. At rest, their heart beats 500 times a minute, and that rate more than doubles during flight. Owing to their extraordinarily high metabolisms, hummingbirds seek plants that yield copious amounts of nectar. Hovering in front of the flower, they siphon nectar from tubular troughs with their long bills.
The 3.5-inch-long rufous hummingbird, with its bright orange throat and reddish sides and tail, is a champion long-distance pollinator, flying 3,000 to 4,000 miles in its annual trek between Mexico and the Pacific Northwest—easily the greatest distance per body weight of any known bird. But how can this creature, which constantly lives near the brink of starvation, muster the energy needed for such an arduous journey? While preparing for lengthy, nonstop flights, the rufous quickly turns carbohydrates obtained from nectar into fat—the most efficient way to store energy. The bird also conserves energy, and gets fatter, while asleep by entering a lowmetabolism, hibernation-like state called torpor.
The population of rufous hummingbirds is dropping for unknown reasons, though the expansion of agriculture into undeveloped lands has clearly disrupted some of the birds' normal travel routes.
Migratory pollinators like the rufous hummingbird are vulnerable compared to more localized species because the high-energy expenditures involved in long-distance excursions leave little room for error. The loss of a few customary feeding sites, or lower than expected floral resources, could be enough to spell doom for these transcontinental voyagers.
More than 80 percent of the world's 250,000 flowering plants depend on animal pollinators, and roughly 15 to 30 percent of the food eaten throughout the world comes from plants pollinated by these creatures.