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In the Field

Not All Sweetness and Light

article spread
by Jack Connor
 

Every birder knows that hummingbirds have a symbiotic relationship with certain wildflowers. Browse through any textbook about bird biology, and you will almost surely find a section detailing the connection—complete with photographs of the birds nectaring and diagrams of bills, tongues, stamens, pistils, and corollas. Because they are tubular and showy, and often red, the plants gain pollinators with long memories, who return to the same flowers again and again. And because their bills are long and their tongues flexible and grooved, the birds have access to energy sources not readily available to other animals. The birds probe the flowers to collect the nectar at the bottom of the corollas and come away dusted with pollen, which they carry from flower to flower. The birds are fueled; the plants are fertilized.

Hummingbirds and “hummingbird” plants make such an eye-catching ecology lesson that textbook authors repeat it in book after book, and birders often fail to see that it’s not the whole story.

What’s easy to forget is the proviso that most texts generally mention in a sentence or two but rarely illustrate: “Hummingbirds supplement their sugar diets by feeding on small arthropods.” Most of us know this truth, but my sense is that too many of us consider it a trivial point. How many birders have made a determined effort to watch how hummingbirds feed themselves away from flowers? And how important is that supplement, anyway?

One problem may be that we don’t know what to look for. In a fairly lengthy library and Internet search, I found few illustrations of arthropod-hunting by hummingbirds (although it’s easy to find photographs and drawings of the much rarer activity of hummingbirds being killed by praying mantises). The oldest explanatory images I could find dated to 1946 and appear in an article by German ornithologist Helmuth O. Wagner titled, “Food and Feeding Habits of Mexican Hummingbirds,” published in The Wilson Bulletin.

Wagner studied the foraging behaviors of more than a dozen species in Mexico during the 1940s. He illustrates the techniques used by starthroats, sabrewings, violet-ears, and other hummingbirds to capture flying insects from the treetops, over cornfields, and above forest streams. Several hummers are drawn in silhouette flying up from tree branches to snatch insects from the sky, kingbird-style—a foraging technique that today would be called sally-hawking. He also describes hummingbirds gleaning prey from bark, leaves, and even water surfaces.

Based on his field observations, his work with captive hummingbirds, and the stomach contents of collected specimens, Wagner concluded that insects and spiders were far more important in hummingbird diets than is generally understood and that nectar was not as essential as many authorities believed. Hummingbirds adapt their foraging to whichever sources are available, and arthropod prey often replaces nectar. “The food of hummingbirds is determined primarily by habitat and season,” he wrote. “A given species may feed mainly on nectar or mainly on insects, depending on the time of year. The majority of the hummingbirds in Mexico are not dependent on flowers.”

Although few researchers have claimed that hummingbirds can go without nectar indefinitely, a number of them have argued that arthropods are more important in the birds’ diets than is generally supposed. A 1980 study published in The Condor involved just a single individual hummingbird, but it is frequently cited. Robert D. Montgomerie and Catherine A. Redsell tracked the foraging activities of a female Broad-tailed Hummingbird for two weeks in May in and around her nest in Rose Canyon in Arizona’s Santa Catalina Mountains. The researchers surveyed for nectar plants in the nearby landscape (in a one-kilometer-diameter circle around the nest) and found none in bloom that were appropriate for hummingbirds. Over several subsequent days, they recorded time budgets for the female’s activity for 10-minute intervals during each daylight hour (6:00 A.M. to 8:00 P.M.) and followed all her movements away from the nest as best they could. “During the 19 foraging bouts…that we observed,” they reported, “the hummingbird spent all her time either flycatching, gleaning from leaves, probing among lichens on tree trunks, or flying between foraging sites.” Their conclusion: “[T]he bird must have subsisted only on arthropods for at least two weeks.”

A much larger study by J. V. Remsen, F. G. Stiles, and P. E. Scott published in 1986 in The Auk examined the stomach contents of more than 1,600 individuals of 140 species of tropical hummingbirds and found that 79 percent held arthropod remains. “The data indicate that most hummingbirds, at any given moment during the day, are digesting arthropods [and suggest that] most hummingbirds feed on arthropods on a daily basis and probably at regular intervals throughout the day.”

Why don’t we notice hummingbirds hunting more often? Could it be that they spoil us—chasing each other through our yards, preening in the open on leafless perches, or, best of all, zooming right up under our noses to nectar at our feeders and garden plants? In those situations they make themselves hard to miss. Hunting for insects, however, they seem to keep a lower profile, noted often only by those who are most alert.

In a report published in The Condor in 1995, summarizing his 18 years of hummingbird observations in the lowland forest in La Selva, Costa Rica, F. G. Stiles describes and illustrates four primary methods tropical hummingbirds use to capture arthropods. Do temperate-zone hummers use the same?

The first method, hover-gleaning, looks like nectaring: the bird holds itself in the air a few inches from a spider web or a leaf and reaches for the prey with bill and tongue. This may be the most easily observed of the four methods. If you witness this behavior, you might try to determine the bird’s target—sometimes it’s the spider, but hummers also parasitize from the webs, taking insects the spider has already captured. (And, of course, female hummers often steal the threads from webs as adhesive building material for their nests.) A second method, hover-hawking, also involves hovering, but here the bird zigzags through swarms of insects, picking them off one by one, almost as a swallow might. The final two methods are probably the hardest to notice because both involve the bird sitting quietly and mostly motionless, scanning for prey. In sally-hawking, the bird flies up to snatch a single flying insect and then returns to its perch. In sally-gleaning, the hummer flies up to pick an insect off a leaf and returns to perch.

Stiles’ superb and lengthy article cannot be summarized adequately in the space available, but here are a few highlights:

More than half of all foraging efforts Stiles recorded (both nectaring and predation) involved hunting for arthropods. Nectaring is actually the less-frequent activity.

Hawking is more frequent higher in the vegetation; sally-gleaning and hover- gleaning are more common at lower heights. Foraging activity of all types is most intense early in the morning, when the birds are at their hungriest.

Spiders are a favored prey among many species; some tropical hummers feed almost exclusively on them. Stiles even noticed some spiders retreating from their webs when the birds hovered near, and he suggests this might point to a competitive advantage for hummingbirds with longer bills, which can still probe and grab without alerting their prey by the whir of their wings.

During the nesting season (at La Selva at least), females spend three to four times more time searching for arthropods than males do. In fact, the importance of arthropod foraging may be one reason why female hummers in many species—including several North American species— have longer bills than their mates. In all hummingbirds, females are the sole caretakers of their nests and young, obligated to gather all the food for their nestlings, which need protein for growth.

Can observers equipped only with binoculars study these phenomena? The answer is an unqualified yes. Stiles depended extensively on his visual observations of birds in action in compiling his data, as did Wagner in the 1940s, when binoculars were much weaker instruments than they are today.

Searching them out away from our feeders is one key, of course. That involves a little extra legwork. As is the case in many other birding challenges, however, the real challenge is mental: to learn how these birds truly live their lives, we must make the effort to look past their flash and dazzle to the grittier reality just beyond.

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Living Bird Magazine

Autumn 2010

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