EPOW - Ecology Picture of the Week

Prey remains from a nest of Magellanic (or Magellan) Horned Owl 
(Bubo magellanicus), southern Argentina.
(No, the owl didn't swallow the pen; its here only for scale.)
This page also links to "Prey Remains of Magellanic Horned Owl
(Bubo magellanicus) at Perito Moreno, Southern Argentina"

Explanation:  In March of 2004, I found myself climbing the side of a large rock outcrop northeast of the town of Bariloche, southern Argentina.  I was in the steppe country of southern Patagonia, in Perito Moreno along a lagoon called Los Juncos.  On the side of the rock outcrop was sign -- drippy fecal "whitewash" -- of an active nest of Magellanic (or Magellan) Horned Owls.
  

One of the two rock outcrops at Perito Moreno, southern
Argentina, with nests of Magellanic Horned Owl.
  

Sign of a nest of Magellanic Horned Owls, hidden up on a 
ledge of the outcrop:  "whitewash" of dried fecal material.

Magellanic Horned Owls are large owls of the genus Bubo -- the same genus as the widespread Great Horned Owl of North, Central, and South America, from which the species was recently split.  Magellanic Horned Owls occur in southern South America, are mostly crepuscular and nocturnal, and their behavior is little studied.

As it turned out, no one was home in the nest at that time, so I collected feathers and owl pellets from the nest site and along the ground below the nest ledge, and also from a second nest on another nearby outcrop called "Elephant Rock" (which is shaped rather like an elephant from one perspective).  
  

"Elephant Rock" at Perito Moreno, 
southern Argentina.

  
Interestingly, on Elephant Rock I also discovered two other active nests, on high ledges, built of heavy sticks by Black-chested Buzzard-eagles.  Apparently, these eagles and Magellanic Horned Owls can occupy the same cliff site at the same time!
  
  

Two nests of Black-chested Buzzard-eagles on Elephant Rock.
Eagles build heavy nests of sticks, whereas Magellanic Horned Owls 
typically nest on the bare rock ledge and do not pile sticks.

Owl pellets (or castings) are the regurgitated remains of the animals that the owl eats.  I later dissected the pellets to determine what kinds of small mammals these Magellanic Horned Owls were preying upon. The prey remains -- typical in an owl pellet -- included skulls, jaw bones, whole legs and leg bones, hairs, and fractured bits of other bones.

Overall, these prey items I identified are typical of Magellanic HOrned Owls in this area of Argentina.  Additional prey remains I found included European hares (an introduced species there) and tuco-tucos (gopher-like digging rodents).  

Acknowledgment:
     Many thanks to my two hosts in Argentina, Martin Swieykowski and Andy MacKinnon, for bringing me to this wonderful site.  Thanks also to two local owl biologist and ornithologist colleagues, Dr. Ana Trejo and Dr. Valeria Ojeda, for providing the information on the location and on biology of Magellan Horned Owls of the region.

Information:
     König, Weick and Becking.  1999.  Owls: A Guide to the Owls of the World.  Yale University Press.
     Trejo, A., and D. Grigera. 1998. Food habits in the great horned owl (Bubo virginianus) in a Patagonian steppe in Argentina. Journal of Raptor Research 32(4):306-311.
     Trejo, A., M. Kun, M. Sahores, and S. Seijas. 2005. Diet overlap and prey size of two owls in the forest-steppe ecotone of southern Argentina. Ornitologia Neotropical 16:539-546.