The diversification of hummingbirds (Trochilidae) has shaped the pollination strategies and floral trait evolution in at least 68 families of flowering plants in the Western Hemisphere. The trumpet creeper (Bignoniaceae) is the quintessential example of ornithophily in eastern North America. The mutualistic relationship between this flamboyantly-flowered liana and the ruby-throated hummingbird (Archilochus colubris) was illustrated as early as 1731. However, neither historical nor modern accounts accurately describe the feeding behavior of ruby-throats at trumpet creeper flowers or the floral adaptations for ornithophily. This paper explores their surprisingly immersive mode of foraging at trumpet creeper flowers and quantitatively assesses floral traits in two populations in the Ozark Mountains. The ruby-throat's bill is approximately one-third the length of the trumpet-shaped flowers, which counters the tendency for the corolla length of ornithophilous plants to match the bill length of their principal hummingbird pollinator. To reach the nectary, ruby-throats cling to the ventral petal lobe of the corolla with their claws and thrust their head and neck deeply into the flower. This immersive “floral-diving” has not been previously described among the 356 species of hummingbirds. The didynamous anthers and stigma are strategically positioned inside the corolla to brush the crown feathers when the ruby-throat inserts its head. A narrow stricture in the corolla, about a third of the way up, allows the bill and tongue of hummingbirds to pass, but blocks bees from reaching the nectary. None of the bee species native to eastern North American have tongues long enough to access floral nectar. Consequently, the abundant sucrose-rich floral nectar appears to be reserved for hummingbird pollinators.

This dataset presents measurements for 238 trumpet creeper flowers from two populations in the Ozark Mountains in Baxter County, Arkansas: (1) White River (36° 11.7ʹ N, 92° 16.9ʹ W; elevation 110 m) and (2) Norfork Lake (36° 22.5ʹ N, 92° 13.9ʹ W; elevation 177 m).

Overview of trumpet creeper floral traits

The sympetalous, zygomorphic flowers (60-90 mm in length) are arranged in terminal inflorescences composed of 12–35 flowers organized in 3-flowered dichasial cymes. The campanulate corolla widens from a narrow elongated cylindrical base. The floral tube terminates in five rounded petal lobes that unfold at anthesis. The ventral petal lobe functions as a landing platform for hummingbirds and bees. The pentagonal corolla opening is vertically compressed, wider horizontally than it is tall. The base of the corolla, which encloses the basal nectary and superior ovary, is protected by a leathery cup-shaped calyx. Most corollas curve subtly downward above the calyx. The corollary stricture marks the point where the trumpet-shaped corolla narrows abruptly, forming a narrow basal tube. The four staminal filaments and a short curled staminode emerge from the interior wall at the corollary stricture and curve along the inner walls toward the opening. The anthers are arranged in a didynamous configuration, with a proximal and distal pair. The dorsifixed anthers are positioned along the dorsal wall inside the corolla opening. The style protrudes through and partially blocks the aperture of the stricture. The flattened, approximately diamond-shaped stigma usually lies between the proximal and distal pairs of anthers along the dorsal wall of the floral tube.

Floral data

I stored newly opened flowers in plastic bags and transported them in a refrigerated cooler for later measurement and dissection. Insect-damaged flowers were culled in the field, while undersized flowers were retained in the population samples. Most floral measurements were made with digital calipers to the nearest 0.1 mm. The width of the corolla opening (horizontal plane) was measured first. The corolla was then split distally along the ventral midline far enough to measure the position of the stigma in relation to the proximal and distal pairs of anthers. The calyx, nectary, and stigma were then removed as a single unit. Corolla length (minus calyx) was measured from the base, which approximates the position of the nectary, to the notches flanking the ventral petal lobe at the mouth of the floral tube. The corolla was then split from end to end along the ventral midline, pressed flat under a thin sheet of glass, and photographed alongside a scale bar. The distance from the corolla base to the corollary stricture (stricture-nectary distance), approximated by the base of the distal anther filaments, was measured from scaled photographs (to the nearest mm). I also measured the distance from the corolla base to the filament tips of the longest proximal (proximal anther-nectary distance) and distal pairs of anthers