Huitzilin — a “ray of the sun” — was the name given by the Aztecs to the hummingbird. Natives of the American continent, the iridescent colours of the 350 species of this bird has often fired the imagination of poets and jewellery designers.
They are small: the bee hummingbird is barely 5 cm long and weighs 2 grams. They can beat their wings up to 50 times per second, creating a hum that defines them. They can hover majestically as they sip nectar from a flower, and even fly backward. Tubular flowers that are bright red or orange (such as lantana and rhododendron) are preferred.
An examination of their wings reveals very long hand bones but very short arm bones that are connected to the body through exceptionally flexible ball-and-socket joints. These joints allow the wings to rotate after each half-stroke, permitting manoeuvrability and backward flight.
Similarities
India has its sunbirds which, though unrelated to hummingbirds, share many common features through convergent evolution. The family that sunbirds are placed in is appropriately named Nectariniidae. Though slightly larger, the sunbirds can hover briefly, and go for bright, tubular flowers. They are critical pollinators of the Flame of the Forest.
However, they need to perch while feeding. Like hummingbirds, they may catch insects, especially to feed their young. The purple sunbird is a common sight in India — the larger and brighter male is at his shiny best in March, the mating season.
The energy demands of hovering are very high. Relative to their body mass, hummingbirds have the highest metabolic rate (calories burnt per minute) among vertebrates. Most of this energy comes from nectar. Rapid sugar uptake by their digestive system ensures that they utilise energy from nectar ingested just a few minutes ago.
Also, their lungs are 10 times better at absorbing oxygen from air than mammals of similar size.
Intense exercise in humans rather paradoxically leads to a spike in blood glucose levels. This is because in its immediate need for energy, your body resorts to gluconeogenesis — converting resources such as muscle protein into glucose. A negative consequence is that you may neither gain muscle nor lose fat after so much activity.
What about the high-intensity activities of hummingbirds? Recent genome studies have shown that during evolution, the gene for a key enzyme involved in gluconeogenesis was lost around the time when hovering appeared.
Removing this gene from bird cell lines grown in the laboratory leads to an increase in energy efficiency of these cells.
Mimicry and dance
Like parrots and some songbirds, hummingbirds are capable of vocal mimicry. When hummingbird pairs are reared in isolation, the two birds produce a song that is subtly different from the standard song of their species.
Remarkably, they are also able to align their muscular movements with auditory sensations that come to their ears — they can dance.
Anirudh Patel, a neuroscientist at the Tufts University, U.S., has theorised that the ability to control throat muscles to mimic a sound precedes the ability to move to the rhythm of sounds. However, hummingbirds cannot dance in pairs, or in groups, as we humans can.
(The article was written in collaboration with Sushil Chandani, who works in molecular modelling. sushilchandani@gmail.com)
