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

As an undergraduate, I had read about another group of birds whose bills were associated with differences in diet: Darwin's finches of the Galapagos Islands. If feeding ecology could give rise to differences in the shapes of bills from species to species, couldn't it do the same for the sexes? But since the huia was extinct, I had to search for years for another species that would enable me to investigate that hypothesis. Then I discovered Larry L. Wolf's work with the purple-throated carib. It was Wolf, a biologist at Syracuse University, who noted how easily male and female could be distinguished in the field by their bills. Here at last was an avian candidate for a study of feeding and sexual dimorphism. So in May 1999 I embarked with my students on our first field expedition to Saint Lucia.

Wolf and another investigator, Karl Schuchmann, an ornithologist at the Alexander Koenig research institute in Bonn, Germany, had focused mainly on the purple-throated carib's courtship and nesting behaviors. Little was known, however, about the bird's food plants. I decided that our first step would be to hike through Saint Lucia's four rainforest reserves and record all the plants that hummingbirds did, or could, feed on. The task was not without its hazards. In addition to being deceptively steep, slippery, and often exceedingly narrow, the trails through the rainforests are also home to the fer-de-lance, one of the Western Hemisphere's deadliest snakes.

On our first trip, after several snake-free days of hiking, my students and I determined that the only food plants available to purple-throated caribs near the ground during the months of our initial observations (May and June) were two species of Heliconia. We have since confirmed that those two Heliconia are the birds' primary food plants from January through July. (Purple-throats also feed from flowers of morning glory and tree hibiscus in the treetops.)

Commonly known as lobster claws, Heliconia are close relatives of ginger, banana, and the bird-of-paradise plants familiar from florists' shops. In the wild, Heliconia often reach heights of thirty feet and have broad leaves from nine to twelve feet long. But it is the colorful inflorescence, or flower stalk, that defines the plant. Each inflorescence includes between one and twenty-four bracts (the parts that resemble lobster claws). The bracts are actually modified leaves that may carry from one to as many as fifty flowers, depending on the species. What makes Heliconia a delight for flower fanciers is the tremendous variation in the colors of the bracts. A single species may have several color varieties, or morphs, ranging from brown to red, orange, yellow, and even chartreuse. We found purple-throated caribs feeding from the flowers of a red-bracted morph of Heliconia caribaea and from a green-bracted morph--unique to Saint Lucia--of H. bihai.

To determine how male and female purple-throats were using the two Heliconia species, my students and I spent three weeks watching patches of the plants from dawn until midafternoon, recording all avian visitors. We found that purple-throated caribs are the sole pollinators of both H. caribaea and H. bihai, and that, as we had hypothesized, the sexes differ markedly in their use of the two plant species. Of the birds we observed, all of the fifteen males but only seven out of the eighteen females fed at H. caribaea. The males were more sedentary than the females and tended to monopolize the largest, densest stands of H. caribaea, which bear two to three times as many flowers as H. bihai.