Like fake owls perched on buildings, the presence of hawks up in trees seems to deter nest-robbers from preying on hummingbird babies and eggs, according to new findings published in Science Advances this week.
Previous observations in Arizona have found that nests of black-chinned hummingbirds (Archilochus alexandri) are often clustered around the nests of northern goshawks (Accipiter gentilis) and Cooper’s hawks (Accipiter cooperii). However, it’s been difficult to pinpoint exactly how hummingbirds and hawks interact. Hawks could be eating hummingbirds, but these tiny, zippy birds might not be worth the effort.
Using GPS trackers over three nesting seasons, a team led by Harold Greeney from the Yanayacu Biological Station & Center for Creative Studies mapped 342 hummingbird nests and 12 hawk nests to study nesting patterns and survival rates in the Chiricahua Mountains of southeastern Arizona. Even though the two never interact, hawks indirectly affect hummingbird survival.
Tiny hummingbird eggs (pictured right) are full of fat, protein, and calcium. And notorious nest-robbing Mexican jays (Aphelocoma wollweberi) are always looking to bring them back to feed their own nestlings. These jays are about 40 times bigger than the hummingbirds. However, there are top predators perched up in those trees. Mexican jays forage at different heights above the ground to avoid being spotted by the hawks if they’re around. Since hawks hunt in horizontal or descending chases, the jays are much safer when they’re at least as high or higher above the ground as their predators.
This change in foraging behavior created a cone-shaped, jay-free area around and under hawks' nests. These turned out to be safe havens. Hummingbird nests near inactive hawk nests lost all but 8% of their young, Science reports, but those in the enemy-free zone had a 70% success rate.
Mexican Jays are always looking to bring home hummingbird babies and eggs to feed their own nestlings. Harold F. Greeney, Yanayacu Biological Station
All images: Harold F. Greeney, Yanayacu Biological Station