Flower lips as nectar guides: bee balm (Monarda didyma) and wild bergamot (M. fistulosa).

Ever since the 1700's, pollination biologists have known that flowers pollinated by bees have guides to direct them to the flower's pollen and nectar sources. When it comes to hummingbird flowers, however, the prevailing view has been that birds are too intelligent to need nectar guides.

During my studies at the National Zoological Park of the Smithsonian Institution, I discovered that ruby-throated hummingbirds occasionally missed the openings of narrow, artificial flowers with their bills and tongues, resulting in feeding errors (1996 Oecologia 105:517-523). I've subsequently determined that birds also miss the openings of real flowers. Such misses might reduce pollination. If so, natural selection might favor flowers that enhance pollination by reducing feeding errors.

More than 50% of North American hummingbird flowers (e.g., bee balm, Monarda didyma) have prominent lower lips around the flower opening, and those that don't tend to have flower petals (e.g., fire pink, Silene virginica). The lips and petals of hummingbird flowers have been viewed as holdovers reflecting evolution from lipped or petalled bee pollinated-ancestors, or alternatively, as landing platforms for secondary pollinators such as bees.

I hypothesized that the lower lips of hummingbird flowers act as guides to direct hummingbirds to the flowers' nectar and to place the birds in the appropriate position for pollination. I tested this hypothesis in collaboration with my students, Catherine Smith, Jodi Stevens, and Rachel Bonkovsky, and my colleagues, Paul Ewald and Ron Hebert, by experimentally removing the lower lips of bee balm (Monarda didyma) and wild bergamot (M. fistulosa).(see 1996 Oecologia 106:482-492).

We found that birds made errors twice as frequently at lipless flowers than at lipped flowers (see film clip for a "good" feeding visit by a male ruby-throated hummingbird at an intact, lipped bee balm flower, and a feeding error by the same bird at a lipless flower of wild bergamot). As a consequence of these feeding errors, birds spent more time attempting to insert their bills into flowers. In contrast, the duration of anther contact between a hummingbird and a flower's anthers was significantly longer at lipped than at lipless flowers. Subsequent work with my student Adam Rankin indicates that the duration of anther contact is positively associated with the amount of pollen deposited onto the bird. (2000. Canadian Journal of Botany 78:1164-1168)

So, like the guides of bee flowers, lips of hummingbird flowers act as nectar guides and benefit hummingbirds by reducing errors. Likewise, lips benefit hummingbird flowers by enhancing the amount of pollen deposited onto birds. But it turns out that there is a manipulative side to this story. Once a bird's bill enters a Monarda flower, the lower lip greatly increases the amount of time the bird spends feeding. This increase in feeding time at lipped flowers once the bill enters the flower is approximately five times greater than the time savings due to making less errors at lipped flowers. The reason for this increase is that the lip acts like a crash barrier that causes the bird to hold back from the flower. Because its bill tip is farther from the nectar, it takes longer for the bird to feed. Moreover, by causing the bird to hold back, the lower lip keeps the bird positioned under the flower's anthers for a longer period of time, thereby doubling the amount of pollen deposited onto the bird. So, even though both the flower and bird benefit, the flower benefits even more at the bird's expense.

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