Finding the perfect place to raise your precious little ‘uns is no easy feat. It's a lot harder if you happen to be a hummingbird. Now, instead of school districts and playgrounds, you’ve got to worry about pesky jays snatching your young straight from the nest. Fortunately, nature has the solution: Just add hawks!
That’s what researchers from the University of Arizona found out after following the fates of more than 300 black-chinned hummingbird nests in the Chiricahua Mountains over three years. These hummers, along with Cooper’s hawks and Mexican jays, coexisted in what ecologists call a “trophic cascade,” a food web in which the behavior of top-level predators directly impacts the survival of those below it. They published their findings today in the journal Science Advances.
Researchers knew that hummingbirds tended to cluster their nests close to hawks. But at first, they couldn’t figure out why. It turned out that, while hawks didn’t pose much of a danger to tiny, agile hummingbirds—“They’d burn more calories chasing them than they would get from catching them,” says Harold Greeney, who led the study—they sure made the jays nervous. So whenever there were hawks roosting nearby, the jays set up shop in the trees far above them, out of the way of hummer territory.
When the researchers plotted the success of hummingbird nests and compared it to the vicinity of hawks, they found a “cone of protection”: an oasis in which hummingbirds could raise their families in peace, enjoying a better chance of survival. Hawks, it turned out, were hummingbirds’ winged protectors.
A stroke of luck aided researchers in discovering the profound impact that these taloned angels had on hummingbirds’s livelihoods. The summer after their first year of observations, a host of raccoonlike mammals called coati climbed up into the trees in the research site and ate as many hawk nests as they could lay their claws on. Within two weeks, researchers saw the cone of protection effect disappear—revealing how much the hummers relied on hawks for their protection. “It was so fortuitous,” Greeney says, adding, “Well, for us. Not for the hawks.”
Once the hawks were gone, the jays moved in, targeting both eggs and hummingbird babies. “Every single hummingbird got eaten,” says Greeney, whose extensive hummingbird expertise has been featured in PBS documentaries.
These new findings provide conservationists with a clear example of how ecosystems can be interconnected on a deep level. The hummingbird-hawk system is not unique: A similar system exists in the Arctic, where snow geese prosper when they cluster around snowy owls who viciously attack their enemy, the arctic fox. In freshwater lakes, algae blooms best in places where predatory crayfish keep away smaller fish and snails that feed on algae.
When humans mess with these kinds of systems, or even try to preserve them by protecting one species, they risk disrupting a delicate balance in which each animal species affects the others. As Greeney puts it: “For conservation, no animal is an island unto itself.”