Flowers that Attract Hummingbirds Confuse Bees

Flowers pollinated mostly by hummingbirds seem to have evolved to confuse bees rather than to attract hummingbirds. So says a recent paper in the journal Ecology, "'Hummingbird' floral traits interact synergistically to discourage visitation by bumble bee foragers,"

Here are some starter facts. Flower preferred by bees ("bee" floral variants) tend to be upright and have blue or purple coloration, since bees have trouble seeing the color red. "Bird" variants, meanwhile, tend to be horizontal with red or orange coloration. Also, bee flowers yield small amounts of concentrated nectar, while bird flowers give pollinators larger amounts of dilute nectar. 

Robert Gegear, assistant professor of biology and biotechnology at Worcester Polytechnic Institute (WPI), wanted to understand how flower characteristics combine to influence the decisions bumblebees make about which flowers to visit. In other words, What kinds of flowers encourage or confuse bees? 

For the first step of the study, Gegear and his team of students trained bees to forage on arrays of paper flowers that all had the same color, orientation, and type of nectar reward. The bees learned that every color and orientation combination yielded the same reward.

The team then gave the bees arrays in which flowers of one color/orientation combination contained nectar and the other combinations contained distilled water. Gegear and his students recorded how long it took the bees to learn which flowers were worth visiting. 

The bees took longer to learn about certain combinations than about other combinations. That is, fake flowers that would favor birds in real life were more confusing for bees than fake flowers that would be better for bees in real life.

Why? Gegear explains, "These data suggest that the reason bee-to-bird evolutionary transitions are often accompanied by a floral shift to classic 'bird' trait complexes is because bees have a particularly difficult time combining red with other sensory traits, including nectar rewards." In other words, bees have a hard time recognizing red flowers, so any trait associated with red flowers is not worth their time to learn, even if learning would mean a greater nectar reward.  

Then where do hummingbirds come in? Well, if bees tend to ignore flowers that are difficult for them, then other pollinators, such as hummingbirds, make their move. Gegear says, "In the case of the two species of Mimulus, the costs associated with bird combinations are much greater than the costs associated with bee combinations, so bees avoid them to increase their foraging efficiency....When you put all this together, you find that 'bird flowers' are really 'anti-bee flowers' that function by exploiting specific sensory and cognitive limitations." That is, hummingbirds forage where bees don't bother to forage. 

Like most pollinators, bees are not genetically programmed to visit only particular flowers; instead, they seek to gather the most nectar in the least time however they can. In other words, they're generalists. From the plant's perspective, however, the best pollinator is a specialist in that plant. (Think of a building toy, like Lego, that clicks only with itself, which forces shoppers to buy only that one brand of building toy.) By combining particular floral characteristics, plants manipulate pollinators to become specialists because generalizing becomes a waste of time. In Gegear's words, "From an ecological perspective, an ideal pollinator is one that always forages on flowers of the same type so pollen is transferred effectively. In reality, pollinators are generalists and they should simply forage randomly. So the big question has been, how do plants get the pollinators to do what they want?"

Gegear suggests that most hummingbird-pollinated flowers once had bee-pollinated ancestors. He says his study shows that at least two floral characteristics had to change for the bird flower Mimulus cardinalis to evolve from the bee flower Mimulus lewisii, and that those changes served to discourage bees.  

Regardless of the flower, we can be kind to pollinators by avoiding pesticides in our gardens and by providing shelter and water for pollinators.