In the minuscule realm of hummingbirds, rufous hummingbirds are pretty average.
At 3.5 grams, the rufous hummingbirds are heavyweights compared to the 2-gram bee hummingbird of Cuba. The bee hummingbird is the smallest-known vertebrate in the world, said hummingbird expert Bill Calder. The largest hummingbird weighs about 20 grams, about the size of a sparrow.
Being so tiny, hummingbirds lose some of the advantages larger birds have. Their wings are too small to get any lift from air flowing over them, the way larger birds and planes do.
"It's all brute force," Calder said.
Rufous hummingbird wings move at a rate of 35 to 40 beats per second, the same rpm as a floatplane, Calder said. The speed of the birds' wings' is really just an outgrowth of their length. The longer the wing, the longer it takes to go through the range of motion. The smallest hummingbirds flutter their wings at 75 beats a second.
For all the effort hummingbirds put into flight, they're actually fairly slow, Calder said. The rufous hummingbirds fly about 25 mph. What they're famous for is being able to hover like a helicopter, and even fly backwards
Working so hard takes its toll. Hummingbird bodies are so dense with mitochondria, which break down sugar into useable energy to fuel cells, that it crowds out muscle.
"It's like putting out fuel tanks that are so out of proportion that the engines won't work," Calder said.
The proportion of mitochondria to muscle may be one of the limits on a hummingbirds' size. Evolved from birds the size of pigeons, hummingbirds seemed to hit the bottom limit of the evolutionary scale.
"You look at the hummingbirds and wonder why didn't the process keep on going?" he said.