Unlikely Heroes

When my nursery-school teacher asked me to draw a picture of my family, I inserted two imaginary brothers tall enough to be twice as big as my—actual—scary older sister. Those made-up siblings, I fantasized, would stop my big sis from being mean to me. For the aptly named black-chinned hummingbird, that wish is a reality: The 3.5-inch-long bird hangs out around hawks to protect itself from nest-robbing Mexican Jays. Hummingbirds in the Chiricahua Mountains of southeastern Arizona have figured out that if they build their nests close to their enemy’s enemy—Cooper’s hawks and northern goshawks—their eggs are more secure from jays on the hunt.

For three breeding seasons, ornithologist Harold Greeney and his team mapped 342 hummingbird nests and 12 hawk nests using GPS. The scientists found that hummingbird nests built within 300 meters of active hawk nests were more likely to produce young hummingbirds that fledged, or took flight, than nests built farther away.

Here’s why Greeney’s team thinks the close approach works: Both Cooper’s and goshawks prey on the midsized jays but don’t bother with tiny hummingbirds. When jays enter hawk’s nesting territory, they alter their foraging and flight patterns to avoid becoming dinner themselves, keeping at or above the height of active raptor nests. Through 3-D analysis of the jay's movements, Greeney’s team discovered that there’s a “cone of safety” for hummers: an invisible barrier radiating down and around active hawk nests.

Of the 342 hummingbird nests, 69 fell within the safety cone, while the remaining 273 were outside the perimeter, more than 300 meters from the hawk nest. Among the in-crowd, 52 percent of the nests were successful. Those on the outside saw only 19 percent of nests produce fledglings.

An important note: Jays know whether their hawk enemies are inhabiting a nest. When predators destroyed four active hawk nests in one study area and the raptors left, the success of hummingbird nests nearby plummeted to nearly zero.

The raptors aren’t intentionally acting as bodyguards. “Hawks don’t give a heck” about hummingbirds, says Greeney. “They have a large effect on hummingbirds, but only if jays are there. Jays are the link.” There are other accounts of a top predator inadvertently protecting a smaller bird it doesn’t actually interact with, but Greeney and his team wanted to nail down how it works. One thing is certain, at least when it comes to black-chinned hummingbirds in the Chiricahua Mountains: Harm hawks—whether through habitat fragmentation, climate change, or some other factor—and the little guys will likely suffer, too.