In 2009, researchers Harold Greeney, founder of the Yanayacu Biological Research Station in Ecuador, and Susan Wethington, a co-founder of the Hummingbird Monitoring Network in Patagonia, Arizona, uncovered remarkable facts about Black-chinned Hummingbirds in southeastern Arizona:
They tend to cluster their nests near the active nests of Cooper’s Hawks or Northern Goshawks. What’s more, the hummingbirds are five times more likely to fledge their young when they nest within 300 meters (328 yards) of active nests of the accipiters. Greeney and Wethington reported their discovery in the Wilson Journal of Ornithology.
Black-chins have nothing to worry about from hawks, of course: Goshawks prey on large birds as well as rabbits and other mammals, while Cooper’s Hawks go after doves, pigeons, jays, robins, and other medium-size birds, as well as small mammals.
Greeney and Wethington wrote that it was likely the presence of the raptors reduced the density of hummingbird predators, but they didn’t know which predators stayed away.
Today, the scientists and seven other researchers, including biologists and authors Noel and Helen Snyder, report in the journal Science Advances that the nesting hawks deter Mexican Jays — notorious nest-robbers — and indirectly boost the breeding success of Black-chins.
For three nesting seasons in the Chiricahua Mountains of southeastern Arizona, the team mapped 342 hummingbird and 12 hawk nests with GPS technology. Only 20 percent of the hummingbird nests were built in areas without active hawk nests.
When hawks were present, jays foraged at different heights above the ground than they would have otherwise. The change in the jays’ foraging created a cone-shaped area around and under hawk nests that provided a safe haven for nesting hummingbirds.
The researchers say the shape of the predator-free space occurs because jays are much safer from hawks when they are at least as high above ground as the hawks. The hawks, of course, hunt from perches within the canopy in horizontal or descending chases.
The result, as with the researchers’ first study, is that Black-chins are five times more likely to fledge young when they nest in the presence of accipiters.
When Greeney and Wethington reported their initial findings in 2009, they said it was the first time so-called protective nesting associations had been described in a hummingbird. Their new study, they say, is the first to show a clear case of predator interactions that cascade through a food chain and affect a species in a separate food chain.
“Hummingbirds don’t pay much attention to hawks and vice-versa,” says Greeney. “But with jays as the intervening trophic level, hawks are indirectly having a very large impact on hummingbirds.” — Matt Mendenhall, Managing Editor
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