Hummingbirds live hard lives. Their metabolism is the fastest of any warm-blooded animal, requiring a steady supply of nectar to avoid starvation. And on top of that, the tiny birds must somehow protect their eggs from bigger, stronger predators like jays.
In the mountains of southeast Arizona, for example, black-chinned hummingbirds are no match for nest-raiding Mexican jays, which outweigh them by a factor of 40. But the hummingbirds have an ace up their sleeve: They hang out with hawks.
Goshawks and Cooper's hawks build their nests high in trees, giving them a prime vantage point for swooping down on prey — including Mexican jays. Hawks rarely try to hunt hummingbirds, which are too small and agile to be worth the effort. Hummingbirds can thus protect their progeny just by building nests within a cone of safety created by hawks, since jays tend to avoid the raptors' nests.
Scientists reported in 2009 that these hummingbirds have a habit of clustering near hawk nests, a phenomenon that has been featured in recent nature documentaries. But a new study, published in the journal Science Advances, offers fresh insight into the relationship. Not only does it reveal how important hawks can be for hummingbirds' survival, but also how ecosystems in general are like Jenga: All the pieces affect each other, even if they aren't directly touching.
Led by Harold Greeney of the Yanayacu Biological Station in Ecuador, the study is based on three seasons of research in Arizona's Chiricahua Mountains. The authors studied a total of 342 black-chinned hummingbird nests, 80 percent of which were built within the safety cone of an active hawk nest. Hummingbirds living near inactive hawk nests lost all but 8 percent of their eggs, Science reports, while those located in hawks' safety cones had a survival rate as high as 70 percent.
The closer a nest is to an active hawk nest, the safer it seems to be. Living within 984 feet (300 meters) boosted hummingbirds' nest success to 19 percent, and that rose to 52 percent for nests within a radius of 560 feet (170 meters).
On top of this correlation, the researchers also saw what happens when hawks are removed from the equation. Goshawks and Cooper's hawks may be apex predators, but even their nests are sometimes raided by raccoon-like mammals known as coati. This can lead them to abandon their nests and move elsewhere, taking their safety cones with them. Without active protection from hawks overhead, hummingbird nests that were previously safe can be decimated by jays.
A young Cooper's hawk scans the forest for prey. (Photo: Dawn Huczek/Flickr)
This research reveals two "strong patterns," the researchers write: "hummingbirds prefer to nest in association with hawk nests, and realize greater reproductive success when the associated nest is occupied by a hawk." While it's possible the hummingbirds intentionally seek out hawks for home security, Greeney tells New Scientist he doubts the birds really understand what's going on.
"They simply return to sites where they've previously had good breeding success," he says, "and this happens to be under the hawk nests."
Either way, this is an example of a "trait-mediated trophic cascade," the researchers write. That unwieldy term refers to apex predators like hawks changing the behavior of "mesopredators" like jays, creating a ripple effect with subsequent changes further down the food chain. It's similar to the effect of wolves being reintroduced in Yellowstone National Park, which altered the behavior of elk enough to prevent overgrazing and promote forest growth. And while none of the species in this study are endangered, their complex dynamic illustrates why top predators in general are often key to the success of their entire ecosystem.
"Such indirect effects are important for structuring ecological communities," the researchers note, "and are likely to be negatively impacted by habitat fragmentation, climate change, and other factors that reduce abundance of top predators." Or, as Greeney tells Slate, "For conservation, no animal is an island unto itself."