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TRUMPET CREEPERS &
HUMMINGBIRD BILLS
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home: www.rubythroat.org |
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TRUMPET CREEPERS &
HUMMINGBIRD BILLS
Judging from the number of readers who ask me about Ruby-throated Hummingbirds, I'd say these little creatures are a part of nature that interests nearly everyone. There's something innately exciting about a bird that hovers, migrates to Mexico, and weighs less than a five-cent piece.
Hummingbirds also are fascinating because they aren't like other birds that pull worms, prey on rats, or eat sunflower seeds. We fancy them to be more like bees or butterflies that survive on nectar from delicate flower blossoms, but this supposition is a little misleading. Although hummingbirds do drink plant nectars, they have demanding metabolisms that require other foods.
A male hummingbird expends considerable energy defending his feeding territory and mating with several females. He never assists with nest building, nor has he anything to do with offspring, which are fed only by his mates. For the otherwise energetic males, flower nectar is certainly useful; like the fluid we mix for artificial feeders, it is loaded with sugar. Female hummers use this quick energy to maintain body temperatures during incubation, but it would be impossible for them to make eggs and raise nestlings on sugar alone.
Close observations of hummingbirds reveal they don't consume just nectar--they pick off tiny insects attracted to flowers, and they eat pollen. Both foods are excellent protein sources, and a nectar/pollen/insect slurry is just right for building baby hummers.
Like this column's readers, I'm intrigued by hummingbirds, and I maintain several feeders for them at my York residence. I even have a banding permit extension allowing me to capture these feathered dynamos. I was amazed to band 114 different Ruby-throated Hummingbirds at Hilton Pond in 1984, and there were 144 more in 1985. Feeding, watching, and banding the little hummers is great fun, but my real interest in them centers around their role in Piedmont ecology.
It's obvious that a hummingbird's long narrow beak is an adaptation for feeding on long narrow flowers, but the corollary is also true: There are species of tube-shaped flowers that are adapted for being pollinated by organisms with skinny, tapered mouthparts. The coiled but relatively short proboscis of a butterfly or bee is useful for sipping nectar from many tubular flowers, and both these insects transfer pollen, but some plant species wouldn't propagate very well without hummingbirds as principal pollinators.
One of these plants is a native Piedmont vine called Trumpet Creeper (Campsis radicans), which produces large numbers of bright orange tubular blossoms. Also named "Cow-Itch," this plant causes a contact skin rash in some people, but I cultivate it at Hilton Pond because it's guaranteed to bring in hummingbirds when artificial feeders won't. (Unfortunately, it deters my mother, who is allergic to it.)
Trumpet Creeper nicely demonstrates the co-evolved relationship between a plant and its pollinator. This showy flower is orange (hummingbirds are particularly attracted to oranges and reds), it is a long tube (its four-inch funnel excludes most butterflies and bees), and it contains a rich nectar source (hummingbirds wouldn't spend time at small flowers with slight nectar rewards sufficient only for tiny insects).
If bees were pollinators for Trumpet Creeper, we would expect its pollen-laden stamens (male parts) and the pistil (female part) to be around the perimeter of the flower opening or at the lower lip; this would increase the likelihood of pollen sticking to special hairs on a bee's legs. However, in a Trumpet Creeper flower the stamens and pistil are located at the top of the tube opening. This coincides exactly with where the forehead of the hummingbird must be placed if it is to reach the nectar at the far end of the flower tube.
When Trumpet Creepers blossom, I can identify quickly which hummers have been visiting them by looking for yellow foreheads. Females in particular acquire a heavy dusting of pollen, which means they probably are getting sufficient protein to feed their offspring. Greater pollen deposits also occur on recently-fledged young of the year who are still growing and need protein to build more muscle and feathers. Diminutive adult males (which are about 25 per cent smaller than females) seem more content with visiting the sugar water feeders and other food sources, and they seldom have yellow foreheads.
This whole topic brings up several interesting questions. How important is the hummingbird in pollinating Trumpet Creepers? If there were no hummingbirds, would Trumpet Creepers survive in the Piedmont? If there were no Trumpet Creepers during hummingbird breeding season, would female hummingbirds get enough protein to produce fledglings? Which evolved first, the long tubular flower or the bird with elongated bill?
We can only speculate on all of these, of course, but it seems certain that a total loss of Trumpet Creepers would have a deleterious effect on Ruby-throated Hummingbirds, and vice versa. The Trumpet Creeper actually has a rough time of it anyway, since several organisms that are lousy pollinators "steal" its nectar without helping out the plant. I've watched bumblebees slit the outside base of the flower tube and lap up nectar, never getting anywhere near the stamens or pistil, and I've seen nibble-mark evidence that mice do the same.
As for which came first, the tubular flower or the hummingbird, the answer is probably neither. Very likely there was a plant with flowers just a bit more tubular than its relatives' flowers, and there was a bird whose bill was slightly longer than those of its siblings. This bird managed to reach the nectar of those long flowers, giving it a slight edge over all those other birds who had to battle for shorter flowers. At the same time, the longer-tubed flowers were "guaranteed" a special pollinator, which also gave these flowers a slight edge.
When you have even a slight edge in nature, you pass on your genes slightly more often than your competitor does, and that is really the main goal of any organism. Biologically speaking, you measure your success not by your own strength or survival, but by whether you have grandchildren who prove your genes have survived into future generations.
Over long periods of time--perhaps hundreds of thousands of years--some long-tubed flowers got longer, some hummingbirds got longer bills, and both these "new" organisms survived to pass on their genes. On the other hand, shorter-billed birds still fed upon and pollinated shorter-tubed flowers; these "original" organisms survived like their "new" relatives. Eventually the "originals" became so genetically different from the "news" that they could no longer interbreed, and this resulted in two distinct species of flowers and two of birds.
Keeping all this in mind, there's another interesting question that needs answering: How is it that we have only the Ruby-throated Hummingbird in eastern North America, while there are 19 hummingbird species in southern Arizona alone?
You can probably figure out an answer by glancing at a field guide that compares eastern wildflowers to those found out west.
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