Harold F. Greeney, Yanayacu Biological Station
Sometimes it pays to have big, bad neighbors. Weighing in at about 3 grams, black-chinned hummingbirds (Archilochus alexandri) can do little but stand by and watch Mexican jays 40 times their weight chow down on their eggs. So in the mountains of southeastern Arizona, the hummers have learned to build their nests near goshawk and Cooper’s hawk nests (Accipiter gentilis and Accipiter cooperii). Almost five times bigger than the jays (Amphelocoma wollweberi), the hawks enjoy these birds for lunch. So to avoid hawks swooping down and surprising them, the jays only forage above the hawks’ nests. Thus a cone-shaped safe zone exists below the 20-meter-high hawk nests, extending out about 100 meters, researchers report today in Science Advances. Of 342 hummer nests studied over three years, 80% were near hawk nests—and for good reason. The researchers monitored hummingbird egg and fledgling survival near six active and six inactive hawk nests. Those hummers unlucky enough to be near inactive nests lost all but 8% of their young, while those in a “good” neighborhood had a 70% success rate, they report. Hawks could eat the hummingbirds, but these morsels are too small and agile to be worth the effort, the researchers note. This phenomenon, in which one species is changing the behavior of another and benefitting a third species is called a trait-mediated trophic cascade, and is similar to what happened in Yellowstone National Park when the introduction of wolves changed the behavior of elk, which may have benefited shrubs and trees that the elk fed on.