Ruby-throated Hummingbirds love red flowers. In my garden, they particularly like the Cardinal Flower, which sends up 4 foot tall stalks with a bright red plume of flowers that develop sequentially from bottom to top of the stalk over a 4-6 week period. Apparently, it produces some really good nectar too, because it is the first flower they visit in the morning.
Here I come — bring out the nectar. This little female Ruby-throated Hummingbird was extremely cooperative, foraging for several minutes on a clump of cardinal flowers, while I sat in a very hot blind about 15 feet away.
Sometimes the probing is just a test to see what might be there, as the bird tastes with the tip of its tongue.
In this shot, her bill is inserted only part way into the flower, and her head is completely clear of the flower structures. This foraging position won’t accomplish what the plant intended the bird to do in return for a nectar reward.
When there is sufficient nectar at the base of the flower, hummingbirds hover for several seconds in place while they mop up every drop. In this case, the bird’s head almost disappears into the flower as it inserts the bill all the way in.
It looks like the hummingbird is getting a pat on the head from the flower. In this case, the bird is indeed performing a vital service for the plant — moving pollen from one flower to another.
The bill of the Ruby-throated Hummingbird is the perfect size and shape for this flower. When it dips its bill deep into the base of the flower, the top of its head brushes a fringe of anthers which deposit pollen on the surface of its head feathers. Later when it visits a different flower, perhaps on a different plant, the pollen will be rubbed against that flower’s stigma, adhering to the sticky surface. If it is the right type of pollen, it will germinate and a pollen tube will grow down the stalk supporting the stigma (the style) into the ovary. Sperm can then leave the pollen grain, swim down the tube, and fertilize the ova to begin the process of seed production.
The contact between the stigma of the flower and the hummingbird’s head is more obvious in this close-up.
Hummingbirds don’t care which flowers deposit pollen on their head, but the plants want to ensure that a particular flower receives pollen from a different individual (i.e., avoid self-pollination). To maximize the potential for out-crossing, Cardinal flowers develop the male parts first — promoting pollen donation, and after a few days, the style and stigma emerge to protrude beyond the anthers — promoting pollen reception.
The anthers look like a white fuzzy fringe on the end of a gray stalk that sits above the petals and the opening of the flower. The flower on the left shows the pink stigma just beginning to grow beyond the level of the anthers. The flower on the right is newer and does not yet have female reproductive parts showing.
A mature Cardinal flower, with the stigma (and style) protruding beyond the anthers. This stigma could easily pick up pollen from the back of a hummingbird’s head.
Coevolution of cardinal flower structure and Ruby-throated Hummingbird sugar appetite has made this system a perfect fit for both.