Hummingbirds: their natural history
Extracted from the book “Hummingbirds of Colombia” - Luis A. Mazariegos
Introduction
Its small size and fleeting presence, its loud, iridescent colors—at times accompanied by extravagant ornament—its ability to hover and its association with flowers are just some of the characteristics that allow us to identify a hummingbird and to easily distinguish it from other birds. These are also the characteristics that have awed European explorers and naturalists since the discovery of the New World, and that continue to captivate the attention of scientists and observers.
Classification
Hummingbirds belong to the Trochilidae (or simply “trochili”)—a numerous and varied family exclusive to the New World. There is controversy, however, regarding their nearest winged relatives. Some taxonomists assert that hummingbirds are more closely related to swifts (Apodidae) than to any other family. Consequently, they classify hummingbirds in the same order—Apodiformes¬. This classification is based on their peculiar wing structure, in which the bones of the hand are longer than those of the arm and forearm combined.
Diversity and geographical distribution
With slightly over 320 species, hummingbirds are exceeded in diversity only by the neo-tropical flycatchers, of which there are some 370 species. Hummingbirds are found throughout the Americas, from Alaska to Patagonia, and habitats ranging from the tropical rainforest to the driest desert and from sea level to altitudes of 5,000 meters (15,000 feet)—the edge of perpetual snow in the Andes. Hummingbirds are also to be found in the most remote islands of the American continent, from the sultry islets of the Caribbean to the blustery Juan Fernández Archipelago, over 600 kilometers (375 miles) off the coast of Chile. However, the distribution of hummingbirds throughout this domain is far from even. As with other birds, more than half the species is concentrated along a narrow band extending five degrees of latitude to either side of the equator.
Plumage and coloration
No other bird rivals the extraordinary design characteristics and versatility of the hummingbird’s plumage. Far from a simple body covering, feathers are the source of many specialized functions. Despite their light weight and apparent simplicity, they enable birds to maintain high body temperatures—with far-reaching consequences for their metabolism. In the case of hummingbirds, their feathers account for its complex flight characteristics, coloring and communications system—including the production of sounds. The gaudiest color schemes include bright, iridescent splashes across the crown, throat and chest, whose sheen calls to mind a polished gem or highly reflective metal surface.
The metallic luster of a hummingbird’s plumage is due to a complex interplay of factors as light passes through a series of minute structural elements within the feathers themselves. A sectional view of these elements, as seen through a microscope, reveals a number of small, hollow platelets of melanin, arranged in layers. Depending on the thickness of the plates and the number of layers, some wavelengths are reflected so as to be mutually reinforced—thus intensifying the color—whereas other wavelengths cancel out and the corresponding colors are not seen.
Many hummingbirds also sport specialized, highly ornamental plumage as part of their wardrobe. In the Spatule-tailed (Loddigesia mirabilis), native to the Peruvian Andes, the outside tail feathers—exceptionally long and thin—form two crossed arches, each with a large, black circular marking with purple specks. So thin, in fact, that in faint light or against a dark background the “rackets” do not even seem to be connected to the rest of the body. In flight, it appears—especially to those unfamiliar with the species—that the hummingbird is being followed by two dark spots that it never seems to be able to shake off.
Flight
Marvelous enough in ordinary birds, the flight characteristics of a hummingbird represent the highest state of evolutionary development. Just a few moments of observation of hummingbirds in the vicinity of a flowering bush are enough to convince one of their extraordinary domination of the air. Zooming in at full speed, a hummingbird comes to a dead stop at just the right point to introduce its bill into the corolla—the while remaining absolutely stationary. It then flies to one side, then the other, then up, then down in search of more nectar-laden flowers. If attacked by a rival, the hummingbird quickly shifts into reverse and backs off, only to return to its former spot as soon as the aggressor has continued on his way. Once there, the hummingbird applies its brakes and resumes stationary flight. Having collected the nectar, it is off again at top speed —almost from the instant of departure.
Only by resorting to sophisticated photographic apparatus and wind tunnels has it been possible to study and describe the wing movement of a hummingbird in all its aerodynamic complexity. In forward flight a hummingbird follows the general pattern of other birds. Flapping is initiated with the wings in the extreme upper position, with respect to the longitudinal axis of the body. The wing then moves downward to provide both forward propulsion and lift. Once the wings reach their lowest point they initiate the motion that brings them back to their initial position, ready to begin a second flap cycle.
Metabolism
The intense vitality displayed by hummingbirds is quite genuine. In fact, all the specialized physiological characteristics that define birds in general, and that enable them to fly and achieve such a widespread geographical distribution, find their highest level of development in the hummingbird.
The minute size of the hummingbird’s digestive system does not permit the storage of large quantities of food. Furthermore, the bird’s intense energy requirements does not allow for slow digestion processes, as well as foods containing large amounts of non-digestible residue. Therefore, the hummingbird is quite restricted as to its potential food sources, and must limit itself to a high energy diet. The only food source that satisfies these conditions is nectar. With its high sugar content and easily digestible ingredients, it seems a made-to-order fuel for this diminutive dynamo.
In all diurnal birds, one observes a slight drop in body temperature during sleep, accompanied of a drop in energy consumption when in a state of rest. Hummingbirds, however, go further. During sleep they enter a state of quasi-hibernation in which their body temperatures drop to a cool 65º F. The ambient temperature, of course, remains substantially lower and so hummingbirds must continue to maintain their body heat, though at a somewhat lower level. It is not simply a matter of allowing body temperature to drop to that of the surroundings—as is observed in reptiles and poikilotherms. The ability to adjust body temperature to a substantially lower level, and which hummingbirds share with only a few other species of birds and mammals, is known as “heterothermia”.
Reproduction
As do other birds, hummingbirds exhibit courtship behavior or “gallantry”. This behavior is the series of activities that culminate in mating and fertilization. By means of this behavior the potential mates identify themselves with regard to species, sex and reproductive disposition. In general, it appears that hummingbirds take partners only for the duration of the mating ritual. Unlike other birds, which often exhibit greater commitment on the part of the male, hummingbird homemaking is a decidedly female activity. It is the female that builds the nest, incubates the eggs and feeds the chicks— with no male participation whatsoever.
Hummingbirds generally hatch but two eggs. As might be expected, they are minute. Those of the Steely-vented Humminbird measure but 13.7 millimeters in length and 8.7 millimeters in diameter, one-half and one-third inches, respectively. Their weight is hardly more than half a gram. The eggs are generally laid early in the morning and on consecutive days—although sometimes there is a two-day interval between one and the other.