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2. Hummingbird Biology for Gardeners: Lifestyles of the Nectar Sippers
by Stephen W. Kress
Hummingbirds are unique to the Americas. The vast majority of the 340 species of hummingbirds occur in the tropics. Twenty-two species are found in the United States, most in the Southwest. Only one species, the Ruby-throated Hummingbird, nests in the eastern states and Canadian provinces.
Hummingbird Migration
While most hummingbirds are non-migratory or short-distance migrants, there are two notable exceptions. The Rufous Hummingbird migrates from its winter home in Mexico as far north as southern Alaska. Remarkably, many Ruby-throated Hummingbirds migrate more than 600 miles across the Gulf of Mexico, making landfall in the southeastern states. After crossing the r, some continue north, while others move eastward, reoccupying nesting habitat in all the eastern states and Canadian provinces.
As hummers, such as these Anna's, sip nectar, they pick up pollen on their crowns. The pollen is transferred to the next flower on which they feed. (Illustration: Steve Buchanan)
Nectar Sipping
The appearance of migrating hummingbirds in spring may be timed to the appearance of nectar-producing wildflowers—or perhaps the flowers time their blooming to the arrival of the hummingbirds! For example, arrival of the Ruby-throated Hummingbird in the northern part of its range, where it nests, coincides with the flowering of certain nectar plants such as wild columbine (Aquilegia canadensis), as well as with the arrival of migrating Yellow-bellied Sapsuckers, which provide "sap wells" from which hummingbirds readily feed. Hummingbirds of the western mountains also appear to time their migration to coincide with the flowering of favorite nectar plants. (Hummingbirds' southern migration in autumn is triggered by changes in day length, not by the scarcity of nectar plants, which are still abundant.)
While many birds, including orioles and at least 50 other species, have an appetite for sugar-water and tree sap, hummingbirds are the ultimate nectar specialists. This high-energy diet provides ample fuel for flight speeds of 66 miles per hour—and up to 200 wing strokes per second—during courtship. Ruby-throated Hummingbirds typically weigh less than 4 grams (females are slightly heavier), but their body weight can double in a week as they fatten on nectar, as well as insects and spiders, before beginning their fall migration.
Flexible Flying
Hummingbirds are the only birds that can fly up, down, sideways, and even backward, a talent that enables them to easily sip nectar from flowers and pluck invertebrates out of thin air. Such extraordinary flight is due in part to their relatively huge breast muscles, which comprise up to 30 percent of their total weight—the highest proportion of any bird.
Their fast-paced foraging style permits hummingbirds to hover where there are no convenient perches—for instance, at the entrance to swaying flower stems, where they feed in place. This unique hovering ability permits them to feed on as many as 1,500 flowers each day; they may drink twice their weight in sugar-water daily. Their skill at backward flight allows them to back off to look over the banquet scene before charging forward for refueling.
Body Heat
Relative to their body size, hummingbirds have the largest heart of all warm-blooded animals, with the fastest heartbeat—1,260 beats each minute. Their daytime body temperature reaches 105 degrees F. Hummers are the only birds that regularly become torpid at night, when their temperature plummets from daytime levels.
Hummingbird IQ
Although hummingbirds are curious and are often attracted to bright red colors on objects that have little resemblance to flowers (such as colorful clothing and red stripes on flags), they quickly learn which red objects provide productive treats. They also have remarkable memories and can recall from one year to the next the location of a particularly rich nectar patch or reliable hummingbird feeder. Observations of partially albino birds (which have unique color patterns) have demonstrated that individual hummingbirds often return to the same backyard over several consecutive years, which may be most of their relatively short lives.
Fitting the Bill
Hummingbirds and their favorite flowers have clearly coevolved for mutual benefit. Most hummingbird flowers have a recognizable shape, regardless of their family affinities. Look for tubular-shaped flowers with stamens and pistil often dangling from the flower's entrance. The typical flower shape protects a nectar bait that encourages these long-billed birds to probe deep for their sweet meal. In the process, birds pick up pollen on their crown, and the pollen is then transferred to the next flower from which the birds feed. Some hummingbirds have learned to "steal" nectar by piercing the side of the flower without picking up pollen, but plants such as bird-of-paradise and other heliconias have evolved to counter this behavior with a thickened calyx that discourages nectar thieves.
Many hummingbird-pollinated plants have red or orange flowers, colors that are readily seen by hummingbirds. Unlike insect-pollinated flowers, hummingbird-pollinated blooms are usually not fragrantly perfumed, because hummingbirds (like most birds) have a poorly developed sense of smell.
Busier than Bees
Compared to insect pollinators such as bees, hummingbirds offer distinct advantages as cross-pollinators. Bees are inactive in cool, wet weather and visit flowers only during the warmer part of the day when their wing muscles have adequately warmed for flight. Hummingbirds are generally better pollinators than bees because they must feed continuously from dawn to dusk. In addition, bees' activities are more confined to fields and forest edges, and they seldom visit forest interiors, as hummingbirds do. One study that compared the pollinating efficiency of Ruby-throated Hummingbirds and bumblebees found that hummingbirds deposited ten times as much pollen (per stigma per visit) onto the flowers of the trumpet flower, or trumpet creeper, Campsis radicans. Another study found that closely related species such as bee balm, Monarda didyma and M. clinopodia, may avoid hybridization because particular hummingbird species show preferences for certain nectar flavors and pollen is placed on the hummers in slightly different locations.
Courtship Rituals
The hummingbird nesting season begins when the male establishes a courtship territory. When a female enters the territory, the male performs a courtship flight unique to his species. In Ruby-throated Hummingbirds, this is a looping, U-shaped flight performed as high as 36 to 45 feet above the female. If impressed by the skill of his flight, the female lands near the male, encouraging him to shift the performance to a series of very fast, close, side-to-side horizontal arcs, which he performs (with throat gorget extended) within one to two feet of the female. Mating soon follows the courtship performance.
Nesting
Females build their nests by themselves on a shrub or tree branch, using mainly thistle and dandelion down. The nest is secured to the branch with spider web and pine resin; its exterior is sometimes camouflaged with lichens, held in place by sticky spider web. Construction takes six to ten days, and during this time, the male continues courting additional mates. Females sometimes refurbish old nests for a second brood, or they may build another nest for their second brood while still feeding the first brood.
A female usually lays two tiny white eggs and incubates them for 12 to 14 days. She broods the young almost constantly until they are nine days old, leaving her nest and young only to obtain food for herself and her brood. Nestlings are fully feathered and capable of flight when they are 18 to 20 days old.
Predators
Hummingbirds have few predators, but shrikes, American Kestrels, and Sharp-shinned Hawks sometimes take adults, and Blue Jays will take eggs. Attacks by praying mantises, dragonflies, and bullfrogs have also been documented, and hummingbirds are sometimes accidentally entangled in spider webs. House cats are probably the most common predators, and collisions with windows, cars, and radio towers are believed to take a huge toll on hummingbird populations.
Stephen W. Kress is Vice-President for Bird Conservation of the National Audubon Society and Director of the Society's Seabird Restoration Program. He teaches ornithology classes at the Audubon Camp in Maine and for the Cornell Laboratory of Ornithology. Stephen guest-edited the Brooklyn Botanic Garden handbook Bird Gardens (1998). He is author of The Audubon Society Bird Garden, The Audubon Society Birder's Handbook, the Golden Guide Bird Life, and other publications on birds and their management.