Why hummingbirds like red flowers (hint: actually, they don't.)

I'd been meaning to write this post for a while, and then I realized: It is National Pollinator Week!  So I didn't really write this specifically for the week, but hey, it is all about pollinators, so I'll go with it.

The hummingbirds are back, zipping around my garden, sipping nectar from their preferred plants. Imagine for a moment the flowers the hummingbirds are visiting. You are envisioning a big bright red trumpet shaped flower, right? Because everyone knows that hummingbirds like red flowers.

Except they don't.

In fact, the flowers hummingbirds like are only red because of bees.

Two researchers, Bradshaw and Schemske, did a super cool study which explains why -- as I will summarize here (Images of mimulus are also from this paper). Sadly, you need a subscription to get the full text. I'll do a super job summarizing it for you, but if you there is a lot more to it than the bit I describe here, so if you have a chance (especially if you are into evolutionary biology) read the whole thing.
So these researchers took these two very closely related species of Mimulus from California:

Mimulus lewisii, on the left, is bee pollinated, and Mimulus cardinalis, on the right, is hummingbird pollinated, and they show all the classic differences in color and flower shape of these two types of flowers. You obviously can't tell this from the picture, but they also produce different amounts of nectar, M. cardinalis producing much more for the benefit of the hummingbirds.

Because these two species are so closely related, they were able to make a fertile hybrid between them, and grew out a massive F2 population (as I explain here, F2 just means the second generation, and is the generation where you see all different crazy combinations of the genes of the parents.) This image shows a bit of the variation they saw:

As you can see, all the traits of the two parents are thoroughly scrambled, some with the color of one parent, but the shape of the other, just as you have may your father's nose but your mother's eyes. This includes the traits you can't see, like nectar production. For example, though the flower in the lower left corner looks much like M. lewisii, it may very well have inherited the gene for producing lots of nectar from M. cardinalis.

They took literally hundreds of these different F2 plants, put them outside, and watched how often bees and hummingbirds visited each plant. Which sounds like loads of fun. Sitting there, trying to watch 200 some different plants and keep track of every single bee and hummingbird that visits each one. Better them than me! But they did it, and then they crunched the numbers to find out what traits actually caused bees and hummingbirds to prefer different flowers.
For bees, the answer is much as you would expect. They visited lighter colored flowers that looked like M. lewisii more than the darker flowers. Hummingbirds, on the other hand, only really cared about one thing: nectar. The more nectar a plant produced, the more they visited it. They didn't care if it was pink or red or big or small -- they just wanted nectar.
How did they know which had more nectar? Turns out hummers are smart, smart enough to visit each plant once, then remember which plants produce the most nectar so they can then only come back to the ones they like. Which is kind of amazing. Makes me glad I'm not a hummingbird. Too much to remember.

This just brings up another question. If all the hummers care about is nectar, why are virtually all hummingbird pollinated flowers red? Why is this pattern of shape and color repeated over and over in different species? (as seen again here in bee and hummingbird pollinated species of wild petunias)

Well, it turns out hummingbird flowers aren't red to attract humming birds, but rather to hide them from bees! Birds have color vision very much like ours (which, as a random aside, is part of the reason there are so many colorful birds. Virtually all mammals (except apes like ourselves) are color blind, which is why mammals are so uniformly boring colored). Insects, on the other hand, see the world very differently. They can see ultraviolet light, and more relevantly, they can't really see red. So those bright red flowers that stick out so much to us and the birds are almost invisible to bees. Unnoticed by bees, the red flowers can keep all their nectar waiting for the hummingbirds, who then come everyday to drink nectar and, in the process, carry pollen from flower to flower.

So next time you see a red flower, don't think, "Oh! The hummingbirds will like that!" Instead think, "Aha! Hiding from the bees with that red camouflage!"

(Bonus animal color vision explanation: Lots of plants from New Zealand -- and almost no plants NOT from New Zealand -- have brown leaves (like this and this). Why? Because New Zealand has no native mammals (except bats), so all the major plant eaters were birds. To a color blind cow, a brown grass looks just the same is a green one, and both get eaten. But to a bird with color vision, a brown plant looks dead and doesn't get eaten. All of which goes to show this world would be a lot cooler without mammals.)