Rufous hummingbirds use numerical order to figure out which flower in line is the juiciest.
Donald M. Jones/Minden Pictures
Tiny, feisty rufous hummingbirds are known for their long migrations, which take them up and down the length of North America each year. Now, they have a new claim to fame: They can keep track of particularly juicy flowers depending on where they appear—first, second, or even fourth—in a line-up of blooms. Although this understanding of “numerical order” may sound simple, it’s a complex skill that may help hummingbirds remember the easiest routes between nectar-rich flowers. It’s also the first time researchers have seen the ability in a wild vertebrate.
It’s a “really impressive” study says Stuart Watson, an animal cognition researcher at the University of Zürich who was not involved with the work.
Lots of animals can count, and some can understand how things fit together in a sequence. For example, rats, guppies, and monkeys trained in a lab can all use sequences to find food. But this doesn’t tell us whether—or how—wild animals might use that ability in a natural setting.
So Susan Healy, a biologist at the University of St. Andrews, and colleagues turned to rufous hummingbirds (Selasphorus rufus). The rust-colored males of the species, which weigh less than a nickel and are just 8 centimeters long, have well-defined feeding territories and excellent memories of what’s on their turf. “They would never lose the car in the car park,” Healy says.
The birds also use efficient routes to head from one nectar-rich flower to another, much like a shopper carefully planning the best route through a grocery store. Healy’s team wanted to find out how they create these routes: Do they simply move from one visual target to the next that’s in sight? Or do they learn a sequence, knowing which items follow the current one?
To find out, the researchers set up feeders with a nectarlike syrup in a valley in North America’s Rocky Mountains, just in time for the hummingbirds to start arriving in May. Once they saw that a bird was consistently eating from a certain feeder (and defending his territory from other birds), the scientists trapped and marked him for identification. Then they trained nine marked hummingbirds to feed from an artificial “flower”—a yellow foam disc on a wooden stake, with a syrup-containing tube in the center.
To see whether the animals had a sense of numerical order, the researchers lined up 10 identical artificial flowers. They put syrup in the first flower and watched to see where the hummingbirds went to feed. Unsurprisingly, the birds went almost uniformly to the first flower, sometimes giving the others a quick check to see whether they also held a tasty treat.
Then, the team began rearranging the flowers after each visit, mixing them up—and even moving the entire line—so that the position of the flowers couldn’t give the birds information about which flower had the syrup. Even then, the birds chose the first flower in the line, suggesting they had a concept of “first.” And when the team repeated the entire experiment but baited, say, the third flower, the birds usually zoomed straight toward the third flower. This suggests they knew the third flower in line—regardless of where the line actually was—had the treat.
In all, the findings suggest the hummingbirds have a conception of numerical sequence—and that they can use it to efficiently find food, the team reports today in the Proceedings of the Royal Society B.
“It’s an ambitious study,” says Andreas Nieder, a neurobiologist at the University of Tübingen who studies animal number cognition. But, he adds, the results don’t eliminate another possibility: that the birds were using other information to find the flower. It’s also possible that different birds used different strategies, he says. Perhaps some hummingbirds, like humans, have an easier time wrapping their heads around numbers.