All the right curves: in an island hummingbird, the shape of the female's bill enables her to feast on local flora inaccessible to the male

Natural History, Nov, 2002 by Ethan J. Temeles

It turns out that, compared with the flowers of most hummingbird-pollinated plants in North America, Heliconia produce copious nectar. By covering flowers with bags to exclude hummingbird visitors, my students and I determined that a single H. caribaea or H. bihai flower can produce a hundred microliters of nectar in twenty-four hours. That is more than fifty times the amount produced by a single jewelweed or bee balm flower visited by ruby-throated hummingbirds during the summertime in eastern North America. Because a single inflorescence of H. caribaea can have as many as twenty-four bracts, each supporting twelve flowers that bloom sequentially over several weeks or months, a patch of forty or more inflorescences is the hummingbird equivalent of Fort Knox.

Such riches are not without robbers, and we observed male purple-throats defending their patches, repelling other purple-throats as well as Saint Lucia orioles and Lesser Antillean bullfinches. The latter two species intruded on the purple-throats' territories not to steal nectar but to consume flowers--petals, sepals, nectar, and all. Male purple-throats were so protective of their flowers that they even attacked us; one particularly aggressive male struck me in the chest as I tried to measure one of the flowers in his territory.

Female purple-throated caribs behaved very differently. They ventured into male territories to feed and to mate. They also fed at small patches of H. caribaea and at both large and small patches of H. bihai. Those differences between male and female behavior arise from differing reproductive and social roles. Female purple-throats, like the females of all other hummingbird species, incubate eggs and rear offspring without any male assistance. While on the nest, a female cannot guard a flower patch, and so she must feed from undefended patches or steal nectar from territories of males. All the nests I have identified have been situated near small patches of Heliconia or near richer, male territories. Of course, once their young have left the nest, females can defend Heliconia territories. But their smaller size precludes them from competing successfully against males for the richest territories, which usually contain flowers of H. caribaea. In four field seasons, we have never observed a female in possession of an H. caribaea territory.

To find out whether the differences in flower use were directly related to the shapes of bills, we needed to measure the length and curvature of Heliconia flowers. Because so many animals (birds, ants, and beetles, to mention a few) eat Heliconia flowers, species such as H. caribaea and H. bihai have responded by burying their flowers deep within the plants' bracts. There the flowers are surrounded by thick tissue and, in some cases, even a protective moat filled with rainwater or the plants' own secretions. The only way to determine the size and shape of Heliconia flowers is to cut them from the bracts.

When we did so, we found that H. caribaea flowers are significantly shorter and straighter than the flowers of H. bihai. The lengths of the flowers differed, on average, by six millimeters. Remarkably, the bills of the male purple-throats were also just six millimeterss shorter than those of the females. Similarly, the curvature of H. caribaea flowers, which measures about 20 degrees of arc, fitted the bills of males quite well, whereas the curvature of H. bihai flowers, which measures 30 degrees of arc, matched the bills of the females.