Hummingbirds & Cardinal Flowers: A Pollination Tale

Photos by Laurie Dirkx.

The natural world holds countless examples of things intricately connected and seemingly perfectly designed. One spectacular example is the relationship between the ruby-throated hummingbird (Archilochus colubris) and the cardinal flower (Lobelia cardinalis). A member of the lobelia family, cardinal flower grows throughout the eastern half of North America, often in moist soils and at the edges of wetlands. Its brilliant red flowers bloom on long racemes from mid- to late summer. Some bees “rob” the nectar, which sits deep within the flower, by biting into the base of a bloom. Other pollinators – including some species of swallowtail butterflies – use their long proboscises to reach the nectar. In the Northeast, however, only the ruby-throated hummingbird is an effective pollinator of the cardinal flower.

Photographer Laurie Dirkx captured these images, which illustrate the hummingbird-cardinal flower relationship, over the course of two summers as hummingbirds visited the self-seeded cardinal flowers in her perennial garden. Always conscious of how her presence may affect wildlife, Dirkx used her longest lens (500 mm) and spent minimal time in the yard, camera pointed toward the shade. “The cardinal flowers have been in my garden for six years now,” she said. “But it wasn’t until a couple of years after they appeared – when I saw pollen on the forehead of one of the birds I’d photographed – that I began to understand this incredible process.”

A ruby-throated hummingbird uses its long, thin beak to sip nectar, which sits about ¾ inch from the tube-shaped flower’s opening. Notice the bird’s head is touching the flower’s anther.

The raceme of a cardinal flower plant may stretch to 2 feet in length, with flowers near the bottom blooming first, while those near the top remain closed, as in this photo. The plants are protandrous, meaning the male parts (anthers) mature before the female parts (stigma). Cardinal flowers use “secondary pollen presentation,” meaning the pollen residing deep within the flower tube is pushed to the top of the flower by the expanding style. The style continues to grow, eventually extending above the flower tube whereupon the stigmatic surface becomes receptive to pollen. “Given the phased flower maturity from bottom to top of the inflorescence, there likely will be flowers at the base that have receptive stigmas while those at the top are still shedding pollen,” said Bob Popp, a botanist with the Vermont Fish & Wildlife Department. “But the mechanism of secondary pollen presentation prevents self-pollination of the same flower.”

Grains of pollen released by the anther fall onto the head of this ruby-throated hummingbird as it reaches its beak into the flower. While other pollinators, including some large butterflies, may reach the nectar, they don’t make contact with the flower’s anther or stigma, and are unlikely to transfer pollen.

As this hummingbird pushes its beak into the flower to reach the nectar deep within, the anther releases pollen onto its head. “The adaptation and co-evolution are just mindboggling – how the hummingbird perfectly fits,” said Popp. “The head hits the anthers and pollinates the stigma while its beak is probing for the nectar. You couldn’t have designed it any better. It’s just amazing.”

Left: During the male phase, each flower comprises five anthers fused together, which is the curved, gray structure shown here. The white hair-like filaments, sometimes called a “beard,” may help make the pollen more available to pollinators.

Right: During the female phase of the flower, the two-lobed stigma, shown here, pushes through the end of the style and is receptive to pollen.

Yellow grains of pollen are clearly visible on the head of this male ruby-throated hummingbird.

Grains of pollen – collected during an earlier visit to a flower in the male stage – are visible on this hummingbird’s head. As it reaches into the flower, some of the pollen transfers to the stigma. (Look closely to see the fused anthers, through which the stigma has emerged.) The pollen will germinate on the stigma, stimulating the growth of a pollen tube, which will extend down to the flower’s ovary. Each grain of pollen may fertilize one ovule to create a seed.