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A bumblebee flying towards foxgloves.

In Europe, bumblebees pollinate the flowers called foxgloves, but foxgloves that spread to the Americas are also pollinated by hummingbirds and have evolved as a result. Credit: Getty

Ecology

Flowers adapt to welcome the birds — but not the bees

Once in the Americas, foxgloves swiftly evolved under pressure by pollinating hummingbirds.

Evolution can forge new relationships between plants and pollinators in fewer than 85 generations.

The showy purple flowers called common foxgloves (Digitalis purpurea) are native to Europe, where they are pollinated by bumblebees. When admiring humans took the foxglove to the Americas, it was enthusiastically embraced by a new guild of nectar-drinkers — the hummingbirds.

Maria Clara Castellanos at the University of Sussex in Brighton, UK, and her colleagues tallied visitors to foxgloves in the United Kingdom, Colombia and Costa Rica during more than 2,000 3-minute study periods. They found that hummingbirds pollinate up to 27% of foxgloves in Colombia and Costa Rica, where the flowers’ corollas — the long purple tubes that gardeners love so much — are 13% and 26% longer, respectively, than those of UK foxgloves.

So why would foxgloves with longer corollas do better? Plants with corollas too long for bumblebees to reach their nectar are guaranteed to be pollinated by hummingbirds, which are more effective than bees at depositing pollen on the next flower. The longer corolla also creates a more comfortable fit for a hovering hummingbird, perhaps improving pollination rates.

Hummingbirds can travel further between flowers than can bees, which might reduce plant inbreeding.

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