With eggs the size of coffee beans, hummingbird nests tempt predators. However, a new study has shown that the tiny birds have adapted, parrying threats by taking advantage of another predatory bird.

In a study published in the journal Science Advances, researchers reported that around 80 percent of hummingbird nests within Arizona's Chiricahua Mountains are built close to hawk nests. To be precise, the hummingbird nests are built inside a "cone of safety" provided by hawk nests. The cone extends downward, deterring predator jays because they will want to avoid getting too close to hawk territory and become prey themselves.

It's still unclear how hummingbirds came to realize that building nests beneath hawk nests would benefit them, but it appears to be working because a hummingbird nest in close proximity to a hawk nest has a survival rate of 31 percent. That may appear low but it's significant compared to the 6 percent that nests away from hawk territory have.

Harold Greeney and colleagues first wrote about the survival rates in 2009 in The Wilson Journal of Ornithology. They arrived at the results of the current study after going back, seeking to answer why hummingbirds actually fare better when in hawk territory.

"This is the first time anyone's figured out not just who all the players are, but exactly what it is about those players that's making [this] happen," said Greeney.

The researchers observed black-chinned hummingbirds in the river valleys and canyons of Chiricahua, walking around the woods to find the nests. At first it wasn't apparent that proximity to hawk nests was a matter of safety; the researchers thought that maybe it just so happened that hummingbirds and hawks determined the same spots as ideal nesting grounds. They discovered the "cone of safety" after comparing data on nest heights.

Based on their records, the researchers found that hummingbirds that build their nests within a 560-foot radius of a hawk nest could boost their survival rates up to 52 percent.

Greeney and his colleagues are keen on observing the hummingbirds more to see how females pick out their nesting spots. Given the size of the birds, they are difficult to track. The researchers are still in the process of figuring out an effective tracking method to help further their study.

Photo: Robert Nunnally | Flickr

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