It was the final day of safari, and our Land Cruisers were slowly making their way to the bottom of Ngorongoro Crater, in northern Tanzania. The road was like a dark ribbon between patches of colorful flowers. Dominant in the patches was the mint Leonotis nepetifolia (lion’s ear), a magnet for sunbirds.
I was with a group of students and Greg Murray and Virginia McDonough, colleagues from the Biology Department of Hope College, in Holland, Michigan.
The guides turned off the engines, and we coasted silently. Suddenly, a student whispered, “Golden-winged Sunbird, left side.” The male Golden-winged is a large sunbird, jet black with gold wings and tail feathers. Variable and Malachite Sunbirds also appeared.
Variable is small, with a short tail. The male has a purple chest and yellow belly. Malachite is large, long-tailed, and green.
The three sunbirds foraged in the orange flowers of the lion’s ear. It’s a sunbird favorite — tall with two to four round whorls, or clusters of flowers, spaced 8-12 inches apart on strong stems pointing skyward. The individual flowers are tubular, curved, and up to an inch and a half long. From a distance, the flower clusters look like Christmas-tree ornaments.
A sunbird typically perches on the vertical stem below a flower cluster and reaches up with its bill, extending it through a flower tube into its basal nectar chamber. Then the bird moves on to another flower. In the process, it collects pollen from one flower and deposits it on the anthers of another one.
Sunbird bills come in a variety of sizes and shapes, and some are better suited for particular flowers than others. In our case, the Golden-wing’s curved medium-length bill was the best fit for the lion’s ear. The Malachite’s bill was too long and too straight, forcing the bird to probe more to find the nectar chamber, and the Variable’s tongue was too short to reach the nectar chamber, so it drilled into it from the outside. For both species, the efforts were too inefficient.
The similarity of sunbird bills and flower tubes is thought to result from co-evolution, a process whereby two species undergo reciprocal genetic change that develops traits that help each other. In simple terms, a plant says, I’ll give you nectar if you bring me pollen. Natural selection should maximize these similarities.
Nectar is a high-energy food; foraging for it is a high-energy task. Nectarivores, birds that feed primarily on nectar, are often limited by the availability of food. They seek strategies to feed more efficiently — that is, to boost the ratio of calories taken in versus calories expended or, more simply, to increase their net caloric intake per unit of time.
In the late 1970s, Frank Gill, of Philadelphia’s Academy of Natural Sciences, and Larry Wolf, of Syracuse University, studied Golden-winged Sunbirds and lion’s ear in Kenya. They found that the birds went on foraging bouts about every 15 minutes and visited several hundred flowers. The sunbirds typically begin foraging by feeding on three or four flowers of a cluster. If the nectar volume is good, the birds stay. If not, they move to other plants or other flower patches. By rejecting clusters whose nectar volume is low, sunbirds can increase their nectar uptake about 15 percent over random feeding.
Because nectar production is continuous, sunbirds that feed early in the morning can visit the same flowers before noon. In between, the birds try to feed on flowers not yet visited, such as those higher on the stems. When flower clusters are three or four deep on a stem, a sunbird might be able to survive on one large flower patch. If the patch is shared by several sunbirds, however, everything changes, and all of the sunbirds will have to expand into additional flower patches. Competition can significantly reduce feeding efficiency for all competing individuals.
To counteract competition, sunbirds often become territorial and defend a flower patch as their own. Only patches with above-average nectar volumes qualify as sites to be defended. There must be enough nectar so the territory holder can chase away competitors and still have enough food reserves to live on.
Sunbirds resemble hummingbirds in that they are small, often colorful, and very active, and they feed primarily on nectar. A major difference is that sunbirds feed while perching and hummingbirds usually feed while hovering. The birds are not closely related. The similarities are a product of convergent evolution. Sunbirds are songbirds, while hummingbirds are akin to swifts. Sunbirds and hummingbirds are ecological equivalents — unrelated but occupying similar niches in different ecosystems. Sunbirds are found in Africa and Asia, while hummingbirds are restricted to the Americas.
Sunbirds are powerful little dynamos fueled by high-octane nectar and favored for their beautiful iridescent plumages. Their complex biology is yet another example of those amazing birds.
This article from Eldon Greij’s column “Amazing Birds” appeared in the September/October 2016 issue of BirdWatching.