Juvenile Golden Eagle (Aquila chrysaetos) Drops Rock near Dusky Grouse (Dendragapus obscurus)

There are no known reports of a Golden Eagle (Aquila chrysaetos) dropping objects on or near potential prey items such as Dusky Grouse (Dendragapus obscurus). I observed such behavior at close range in Grasshopper Creek canyon on the Salmon–Challis National Forest in south-central Idaho. The top of Grasshopper Creek canyon, located at an elevation of approximately 2900 m, consists of a roughly 1-km wide, flat, mostly open meadow surrounded by scree slopes on three sides, which rise abruptly to high mountain peaks and ridges. At the lower edge of this meadow, the terrain drops off steeply and the canyon is spanned by a 5ha stand of mostly dead whitebark pine (Pinus albicaulis) on the west side and a 10-ha stand of aspen (Populus tremuloides) in the center and on the east side of the canyon. Below the tree stands is a grass/forb meadow interspersed with clumps of stunted subalpine fir (Abies lasiocarpa) and occasional currant bushes (Ribes spp.). There are no roads or established trails in Grasshopper Creek canyon. At about 1115 H on 13 September 2010, I was sitting in a small patch of grass/forb meadow below the stand of whitebark pines, when I noticed a juvenile Golden Eagle hovering on an updraft about 30 m overhead, facing downcanyon. The eagle apparently noticed me when I looked up and it turned back up-canyon. Ten minutes later, I saw the eagle again hovering roughly 175 m up-canyon, about 25 m above the ground, and near a prominent, single dead tree. Watching through binoculars, I saw it slowly lower its right foot, pause, and then drop a fist-sized, light-colored object that looked like a rock. The eagle hovered for another 2–4 sec while staring at the ground, then veered off and turned back up-canyon. This appeared to be a deliberate and purposeful action by the eagle to drop a rock on a specific location and observe the results. I moved toward the location where the eagle dropped the presumed rock. Upon approaching to within 3 m of the spot, I flushed two Dusky Grouse, one from the base of a currant bush and the other from the lower limb of the prominent tree adjacent to the bush. I did not locate the dropped rock. About 2 hr later, I flushed an adult Golden Eagle from a live whitebark pine about 200 m up-canyon from where the juvenile eagle dropped the rock. Later that evening, a friend again saw two Dusky Grouse at about the same spot where I had seen them previously. Golden Eagles and other raptors have been observed repeatedly dropping and catching objects such as sticks, moss, and dead prey while soaring, exhibiting behavior that is often interpreted as play (Gordon 1955, Palmer 1988). As part of an agonistic encounter, a male Osprey was observed picking up and then dropping a rock on another, perched male Osprey that had invaded its territory (Roche 1996). In Spain, an immature Golden Eagle dove playfully with another eagle, then landed, picked up small stones in its foot, flew to a height of 40 m, and slowly released them for no apparent reason (Romer 1987). A Common Raven (Corvus corax) threw stones from atop a cliff at researchers under its nest in apparent nest defense (Janes 1976). The differences between my observation and those previously reported for raptors appear to be intent. I believe there are three possible explanations for the behavior I observed: (1) the eagle purposefully dropped the object with the intent of affecting the grouse; (2) the eagle behaved deliberately in dropping the rock but with no specific intent to affect the grouse; or (3) it was a purely random event. My distinct impression was that the eagle acted purposefully with the intent of either killing or stunning a grouse, or flushing it to facilitate capture. I provide additional details of the observation in Koch (2012). Thanks to Karen Steenhof, Carol McIntyre, Terry Rich, and an anonymous reviewer for their reviews of this manuscript, suggested edits, and guidance. —Edward D. Koch (email address: ted_koch@fws.gov), U.S. Fish and Wildlife Service, 1340 Financial Boulevard, Reno, NV 89502 U.S.A.