Cornelia J. Ketz-Riley, Carlos R. Sanchez Traditionally, hummingbirds were classified in the order Apodiformes.27,28 According to the new SAM (Sibley/Ahlquist/ Monroe) classification using molecular techniques, hummingbirds are placed in their own order: Trochiliformes,29 with the family Trochilidae, divided into two subfamilies, the Phaethornithinae (hermits) and the Trochilinae (typical hummingbirds). The Trochilidae family contains more than 335 species.27,35 In this chapter, we are focusing on the true hummingbirds. Most of them weigh between 6 and 12 grams (g) with the smallest, the bee hummingbird (Mellisuga helenae), weighing at 2 g and the giant hummingbird (Patagona gigas) at 20 g.5 Hummingbirds are important pollinators of a number of plants, even being the only pollinators for some plants.11,28 Free-ranging hummingbirds live between 6 to 12 years, but in captivity, they may live up to 17 years.27,28,35 Hummingbirds, particularly males, are very colorful, with bright iridescent feathers on the tail, crest, and throat patches, the so-called gorgets.27,29,35 Male hummingbirds are generally highly territorial and show impressive courtship displays.27,28,35 Hummingbirds are found only in the Western Hemisphere. Although their range extends far north to Alaska and Labrador in Canada and to the Strait of Magellan in the south, they are predominately tropical and subtropical, with most of the species found in Brazil and Ecuador.27,28 The status and population trends for most of the hummingbirds are listed as unknown by the International Union for the Conservation of Nature (IUCN). Hummingbirds have the highest metabolic rates relative to body size of any animal. Average temperature ranges from 36.5° C to 43.3° C, with around 39° C at resting. Resting respiratory rate is about 250 breaths per minute, up to 400 breaths per minute at flight. Normal heart rate is 500 to 600 beats per minute (beats/min) with up to 1260 beats/min during flight.11,27,28 The heart is proportionally the largest among all birds, representing greater than 2% of the total body weight.11,27,28 Hummingbirds present a number of unique anatomic and physiologic adaptations. Long wings with long carpal and metacarpal bones and a very short, stout humerus bone, as well as a unique shallow cup-and-ball joint that attaches the coracoid bones to the sternum,28,33,35 enable the hummingbird to hover in the air and to fly forward and backward.5,28,29 They may reach speeds up to 15 meters per second (m/sec; 54 kilometers per hour [km/hr]) during flight and flap their wings 12 to 80 times per second while hovering. Hummingbirds have an extendable tongue that forms two parallel C-shaped grooves of keratinized membranes around a rigid supporting rod with a bifurcated end (Figure 26-1).13 The grooves function like rods, drawing nectar via capillary action, but also like a fluid trap for the nectar.12,25,28,29 The digestive tract of hummingbirds includes a small crop and a short intestinal tract and lacks a cecum and a gallbladder. The crop emptying time of about 4 minutes is the rate-limiting factor in hummingbird feeding. During an intestinal transit time of about 15 minutes, 99% of ingested glucose is absorbed.12,16 The flight muscles are composed mostly of fast oxidative-glycolytic fibers, allowing the bird to sustain high aerobic power.33 Mitochondria in hummingbird muscles are able to oxidize carbohydrates equally well as fat.34 The carbohydrate oxidation of newly consumed nectar supports the high adenosine triphosphate (ATP) demands during short-term hovering flight.34 Fat oxidation is selected during long migratory flights.34 To prepare for long-distance migration, hummingbirds may rapidly gain as much as 72% of their body weight in fat. Their liver is one of the most metabolically active known, with the highest levels of enzymes for lipid synthesis.22,32 Most hummingbirds seem to have the ability to use arthropods as an alternative energy source when access to floral nectar is scarce.23 To conserve energy, hummingbirds spend the majority of their day sitting or perching and only an estimated 10% of their day-time flying and hovering while feeding in short meals. Another effective way to save energy during cold nights or prior to migration is going into a torpor.9,16,28,29 The metabolic rate may drop to one fifteenth of its normal rate,29 body temperature as low as 8° C, the heartbeat reduces to 30 to 50 beats/min and the respiratory rate lowers to 50 breaths per minute with apnea episodes of up to 5 minutes.11,21,28 The birds are cold to the touch; they perch with fluffed feathers and closed eyes and their bill pointed straight up. When in torpor, the birds barely respond to stimuli or seem uncoordinated.21,31 Hummingbirds depend almost entirely on a liquid diet of floral nectar. To meet their metabolic demands, they may consume more than three times their body mass in fluid per day.2 The high metabolism depends on an efficient way to extract energy and nutrients from this liquid diet. Processing a large quantity of water for energy coverage requires highly specialized kidneys and gastrointestinal tract.2,9,18 In hummingbirds, more than 99% of nephrons do not possess a Henle loop and cannot concentrate urine.4,15 Hummingbird kidneys are structurally similar to those of reptilians and their waterflux rate close to that of amphibians, and these birds are still able to maintain a high metabolic level of an endotherm animal, which makes them very unique animals.2,9,10 ,15,16 Since males of many species are highly territorial around their food sources, multiple feeding stations should be available on exhibit. One station for every two birds has been reported to be adequate.21 Feeding stations should be in open areas to allow free flight and aerial displays. Optimal ambient temperatures depend on the species. Generally, species from temperate zones are more cold tolerant compared with tropical and subtropical species. Ideally, hummingbird aviaries should be supplied with extensive planting for perching and hiding places. Water should be available in the form of streams, waterfalls, or a pond to provide ample water access. Otherwise, bathing can be encouraged by providing shallow water bowls or daily misting or hosing of the foliage.21,31 Shelter from extreme weather conditions as well as from aggressive conspecifics needs to be available. If mixed with other bird species, the other birds should be of similar size and not very territorial. Any windows of the enclosure should be positioned on a slight downward angle and preferably covered with branches. Feeding stations, water features or bird-attractive plants or flowers should be placed either more than 30 feet from any window or within 3 feet because of reduced velocity within this short distance. Small enclosures of about 3 to 5 feet long by 1.25 to 1.75 feet deep and 2 feet high are possible for individual housing or introduction.
Trochiliformes (Hummingbirds)
Biology
Anatomy and Physiology
Gastrointestinal Tract and Energy Metabolism
Energy Conservation
Osmoregulation
Special Housing Requirements
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