Many of the 349 glistening species of hummingbirds have gemological labels, with amethyst, emerald, garnet, jewel, gem, ruby, sapphire and topaz mounted in their English names. These gems are alive, the names reflecting their iridescent colors, and they provide illustration of biological principles.
One simple approach to ecology is to consider pairs of species. “Mutualism” is a relationship in which two species each benefit from their interaction. When two species both suffer from the presence of each other, we call the relationship “competition.”
If one species benefits at the expense of the other, we use different terms depending on details of the interaction: “parasitism” if the benefiting individuals are small relative to the others and have small deleterious effects in any single encounter; “predation” if the benefiting individuals are relatively large and typically kill the others at once.
In “commensalism,” one species benefits from an association with little or no effect on the other.
Our New England hummingbird, the ruby-throated hummingbird, shows each of these relationships with other species in its environment.
Hummingbirds are not the only local living precious stones — in late summer wet edge habitat is bedecked with the orange flowers of spotted jewelweed, also called touch-me-not for its explosive seed pods. Ruby-throated hummingbirds have a mutualist relationship with spotted jewelweed and many other plants.
The plants provide nectar that fuels the hummingbirds who, in turn, aid the plants’ reproduction by carrying pollen from flower to flower.
A century and a half ago, Amherst poet Emily Dickinson observed that “every Blossom on the Bush/Adjusts its tumbled Head” as a hummingbird feeds, raising the question of the function of such flower motion. Much more recently, ecologist Ethan Temeles and his Amherst College students systematically addressed this question. They found that the curved spur of a jewelweed flower allows a hummingbird to pull the flower toward itself, causing the flower to nod down on its flexible stem, or, as Dickinson said “adjust its tumbled head.” The flower’s nodding increases pollen deposition on the bird’s bill and the value to the flower of the bird’s visit.
Other animals also consume nectar. A wide variety of insects drink at flowers, and every bit of floral sugar taken by a fly, moth, wasp, bee, butterfly or moth is a bit of sugar unavailable to a hummingbird. These species are competitors for food. In some cases the competition involves direct interference, as when a wasp uses the threat of a sting to chase a hummingbird from a food source.
As pancake eaters know, tree sap is another source of sugar in the wild, but it requires a tap for access. Hummingbirds can’t drill into trees themselves, but the yellow-bellied sapsucker, a species of woodpecker, regularly drills sap wells.
The sapsuckers are parasites on the trees, with a relationship to the trees similar to that of mosquitoes to us. Ruby-throated hummingbirds follow sapsuckers to learn the locations of wells, which they then visit for sap. Although hummingbirds remove much less sap than do sapsuckers, any sap that goes into one bird is not available to the other.
The hummingbirds are thus parasites on the parasites. Those parasites that take a resource such as food from another species are called kleptoparasites, an anthropomorphic term based on a Greek word for thief.
Insects are abundant, and most bird species feed on insects. Ruby-throated hummingbirds, like many other species specialized for feeding on non-insect foods, supplement their diet with insects, especially in the breeding season when protein-rich food is particularly important.
From the perspective of the small insects caught near flowers and sapsucker wells or grabbed from the air, the hummingbirds are predators. Conversely, many larger birds and even large invertebrate predators such as preying mantises and large spiders view hummingbirds as prey.
What about hummingbirds and humans? We have an odd sort of mutualism. People provide flower gardens and sugar water feeders, which offer benefits that probably outweigh the damage done to hummingbirds by our cats.
In turn, we get aesthetic benefits, both directly, in our yards, and culturally, filtered through our artists and writers. Unlike the birds, though, we probably get no benefit in evolutionary currency — increased reproduction (unless people with more hummingbirds in their gardens have more or healthier children).
In a biological sense, the relationship might be slightly parasitic (the hummingbirds gain and we have a slight loss in resources) or commensal (the hummingbirds benefit with little biological effect on us).
Subjectively, we benefit from the pleasures of poetry, painting, photography and understanding biological principles. I find these gains worth spending a few greenbacks to enjoy what Emily Dickinson called a “Resonance of Emerald,” the scintillating green backs (and ruby throats) of the flying gems.
David Spector is a retired biologist and former board president of the Hitchcock Center for the Environment.
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