Breeding Owls Defecate Strategically

With a four-foot wingspan, the Eurasian eagle owl is a big bird with a big appetite — and a fecal output to match. Yet the owl's body waste does not always go to waste.

New research suggests that breeding eagle owls defecate strategically, using their excrement to erect “No Trespassing” signs within their territories.

Vincenzo Penteriani and Maria del Mar Delgado of the Doñana Biological Station in Seville, Spain, report that when breeding eagle owls answer the call of nature, they squirt their white feces onto the vertical faces of dark rocks in exposed locations, rather than onto more abundant—but less contrasting—pale rocks. When not breeding, the owls let go indifferently onto the ground. After the two scientists erased feces on both exposed, dark rocks and more inconspicuous spots by spray-painting over them, they noted that the owls only re-marked the exposed, dark rocks, usually within a day.

Among eagle owls, fights between territory owners and intruders are nasty and often fatal, so it pays to advertise ownership clearly. In fact, the scientists say the owls may post a second form of signage: they lay out the brightest feathers of their avian prey at plucking sites, something they never do with the dull fur of their most frequent victims, rabbits.

The reaction of intruders to such marks remains to be tested, but Penteriani and Delgado say the evidence suggests that owls use feces and feathers as visual supplements to their hooting. Most birds use their voices alone to stake territorial claims.