After naming my blog The Humming Dinosaur, I obviously needed to write about humming dinosaurs as soon as possible. I’m not talking about hypothetical musical sauropods, but actual living, breathing dinosaurs. Today, we call them birds. And the birds that hum are, well, hummingbirds.
Tiny, jewel-like and acrobatic, hummingbirds might be among the most intriguing birds. The Aztecs gave such credit to their fiery nature and liveliness that they carried pictures and body parts of hummingbirds as talismans (photo: Michael Woodruff / Flickr).
If you ever get the chance to see a hummingbird (which we Europeans rarely do, though for American readers it might be an everyday occurrence), the first thing you’ll probably notice is how insect-like the bird is. In fact, I have more than once heard someone claim they saw a hummingbird in Southern Europe, having mistaken a hummingmoth (Macroglossum stellatarum) or one of the other diurnal hawkmoths for one.
Like the hummingmoth, hummingbirds are masterly fliers. They can fly forwards and backwards, hover at one spot, even fly upside down. These skills are partly enabled by their small size, but also by a unique wingbeat resembling the number 8 instead of the up-and-down stroke of most birds. Small species of hummingbird can beat their wings more than a hundred times per second when hovering, and even those of larger species are far too fast for the bare eye (photo: Dan Pancamo / Flickr).
Hummingbirds are on the dream diet of any toddler: they live almost excusively on sugar, even though they snatch a tiny insect or spider once in a while as a supplement. In their long, narrow beaks they have an even longer, weirdly shaped tongue that laps up nectar from flowers. The lenght and curvature of the beak of some species exactly matches the flowers which they visit. Thus, some species of hummingbird are confined to feed on a few or just one species of plant. Many plants pollinated by hummingbirds have red, pink or orange flowers, probably because the bees that compete with hummingbirds cannot see those colours.
So the bird pollinates the flower, and as a payment, receives some nectar. Everyone benefits. However, mutualism in nature is never completely free of conflict.
In this case, the plants are blackmailers. Their nectar is a lot more diluted than that of plants pollinated by insects. This forces the hummingbird to visit many flowers fast to get as much energy as it spends. The plant gets two birds with one stone: the diluted nectar is cheaper to produce, and pollination more effective (photo: a sword-tailed hummingbird by Paul Bartram / Flickr).
Energy would be critical for hummingbirds even without mingy plant partners. Their average metabolic rate is the fastest of all known vertebrates: the heart of a hummingbird can beat more than a thousand times per minute. Oxygen consumption (a relatively easy way to estimate metabolic rate) of an active hummingbird is very close to the suggested theoretical maximum value for vertebrates: 40 ml of oxygen / gram / hour. During bursts of extreme activity, such as hovering, oxygen consumption values of more than 80 ml/g/hour have been measured. As a comparison, that’s about twenty times more than humans are capable of.
Vertebrates, you say? So are there invertebrates that are capable of even more insane metabolic rates? Even though invertebrates are generally regarded as ectothermic – or cold-blooded – and ectotherms as sluggish and slow, neither assumption is necessarily true. In fact, many active flying insects, such as dragonflies, butterflies and hawkmoths, are endothermic when flying, with body temperatures on the same range as in birds or mammals, or even higher (45 °C in the hummingmoth). For them, there are oxygen consumption measurements of 100 to 200 ml/g/hour! In this regard, insects are incredibly adaptable: at rest, they consume less oxygen (and, I suppose, energy) than mammals or birds do in torpor.
Insane moths aside, hummingbirds are the metabolic rate record holders among vertebrates. They hold other records too: they have the largest lungs and hearts relative to body size, the highest density of mitochondrias and capillaries, smallest red cells, and smallest genomes.
Genome size correlates with cell size, actually so closely that we can estimate genome sizes of extinct vertebrates by looking at the little holes in their bones that used to contain cells. Because smaller cells have a higher mass-to-surface ratio, they also have a more efficient metabolism. Since most of the genome of animals is non-coding (though not necessarily junk), there is plenty of stuff that can be gotten rid of if a more efficient metabolism is needed.
Birds, in general, have genome sizes of less than half of those of mammals. In birds, the DNA of one cell weights on average 1.42 picograms. In hummingbirds, the value varies from 0.9 to 1.3.
The best known world record of hummingbirds is obviously that of the smallest living bird. Many species of hummingbird are extremely small, but the record holder is the bee hummingbird (Mellisuga helenae) from Cuba. With a body lenght of 5 cm and weight of less than two grams, it is in the size class of a largish bumblebee.
The bee hummingbird shares the prize for the smallest living endothermic vertebrate with the Etruscan shrew (Suncus etruscus) and the bumblebee bat (Craseonycteris thonglongyai). The picture above also depicts some of the smallest ectothermic vertebrates.
Among mammals, shrews are known for their extremely high metabolic rates that almost rival those of hummingbirds. They are also tiny, and similarly in a constant battle to get more energy than they consume. Unlike hummingbirds, however, shrews and other small mammals are very short-lived. Small size and high metabolic rates are generally associated with short life spans. For reasons unknown, both the maximum and average lifespan of hummingbirds is about five times that of shrews, the average being 3 to 5 against 1 year, and the maximum 10 against two years.
Since bats also live longer than land-bound mammals of the same size, maybe it’s the ability to fly away from danger that makes you live longer: if you can expect getting eaten any minute, it doesn’t make much sense to save strength for a long life. Or perhaps it’s the efficient metabolism associated with a small genome, as bats also have smaller genomes than land-bound mammals.
Despite seeming extremely specialized, hummingbirds are a pretty successful group. There are more than three hundred known species distributed all the way from Canada to Tierra del Fuego. As hummingbirds don’t handle cold climates or long starving well, they have not gotten out of the Americas: on other continents, they are unknown (picture: Ernst Haeckel’s Trochilidae, 1904).
However, the niche of a nectar-feeding bird does exist elsewhere, and different birds have adopted it on other continents: the sunbirds (Nectariniidae) of Africa and Asia, the honeyeaters (Meliphagidae) of Australia and the now extinct ‘O’os and kioeas (Mohoidae) of Hawaii. None of them, however, are nearly as extreme as hummingbirds.
Curiously, all the “hummingbird copycats” are passerines, belonging to the great branch of bird family tree that also includes finches, tits, crows and trushes. The hummingbird themselves have fully different origins: they are cousinds of the swifts and tree-swifts, which all share exceptional flying skills and tiny, weak legs. Maybe their different origins explain why none of the other groups of nectar eaters have became as extremely specialized.
References and further reading:
Gregory et al. 2009: The smallest avian genomes are found in hummingbirds. Proceeding of the Royal Society B.
Opazo et al. 2005: Cell size and basal metabolic rate in hummingbirds. Revista Chilena de Historia Natural. (pdf)
Morgan-Richards et al. 2008: Bird evolution: testing the Metaves clade with six new mitochondrial genomes. BMC Evolutionary Biology.
Mayr 2003: Phylogeny of early Tertiary swifts and hummingbirds (Aves: Apodiformes).
Niven & Scharlemann 2005: Do insect metabolic rates at rest and during flight scale with body mass? Biology Letters.
Oliver P. Pearson 1950: The metabolism of hummingbirds. The Condor.