These trees don’t mind getting robbed

Purple sunbirds rarely consume the nectar of a desert teak flower through its opening (right), preferring to rob the flowers by piercing through the base (left). 

V.K. Singh et al/PLOS ONE 2014

Plants don’t make flowers just to make me happy, and they don’t make nectar just to feed bees, hummingbirds and other pollinators. Those nectar-filled flowers are there to lure species in where they’ll pick up some of the plant’s pollen and transport it to another flower so the plant can reproduce.

But there are some individuals that horn in on this deal — nectar robbers. These are creatures, usually birds or insects, that take nectar without picking up any pollen, often by drilling into the flower from its base. Nectar robbing is typically bad for a plant because the robber often damages the flower. But for the desert teak tree of western India, being robbed is actually helpful.

The desert teak tree Tecomella undulata needs birds to pollinate its flowers so it can make fruits and seeds and reproduce. And a single tree can’t reproduce with its own pollen, so it needs those birds to flit from flower to flower between trees. So anything that promotes the birds to fly farther is helpful for the teak trees.

That’s where nectar robbing comes in, Vineet Kumar Singh and colleagues at the University of Delhi in India report July 18 in PLOS ONE. The researchers studied desert teak trees in Barmer in Western India, recording visits by three species of birds — two pollinators (red-vented bulbuls and white-cheeked bulbuls) and the nectar-robbing purple sunbird.

The two bulbuls have short beaks and have to push deep into the teak’s flower to get its nectar, covering their heads in pollen. The sunbird, though, has a long, sharp beak. It usually pierces through the base of the flower to feed. It also sometimes sips nectar through the front of the flower without picking up any pollen.

But by consuming nectar, the sunbird helps the teak tree. In the morning, these birds show up about 40 to 60 minutes before the bulbuls and snap up about 60 percent of the nectar from the just-opening flowers. By the time the bulbuls arrive, they find they have to visit more flowers and more trees to get enough food, thereby spreading around more pollen, Singh and colleagues found. And that has a positive effect for the tree: Those that get robbed make more fruits.

This system wouldn’t work if the bulbuls avoided the robbed flowers or if the cost of making that extra nectar were too high for the tree. But in this case, the researchers note, “the robber plays a constructive and crucial role in the reproductive performance of [a] threatened tree species.” It’s certainly unusual, since most of us would rather not be robbed.

Sarah Zielinski is the Editor, Print at Science News Explores. She has a B.A. in biology from Cornell University and an M.A. in journalism from New York University. She writes about ecology, plants and animals.

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