We examined the potential of using conditioned aversion to reduce predation by Common Ravens (Corvus corax) on eggs of the endangered California Least Tern (Sterna antillarum browni). In 1991, we placed Japanese quail (Coturnix japonica) eggs injected with 30 mg (active ingredient) methiocarb near eight raven nest sites at the U.S. Marine Corps Base, Camp Pendleton, CA. All raven pairs learned to avoid the treated eggs within 4-5 days, and at seven sites egg removal was minimal throughout the 4-5 week trial period. Later, we established a simulated Least Tern colony on the beach, and a pair of ravens that had previously learned to avoid quail eggs placed near their nests dominated access to the site, actively excluded neighboring ravens, and resumed taking quail eggs. This response indicated that the previously acquired aversion was site-related and not transferrable to eggs encountered in a different location. From 15 April to 4 June 1992, we placed quail eggs injected with 30 mg methiocarb at three California Least Tern colonies at Camp Pendleton. Methiocarb-treated eggs were taken by ravens at each locatio, but no tern eggs were lost to ravens. Ravens that nested near the Least Tern colonies actively defended their territories against intruding ravens thereby excluding them from the tern colonies as well. In contrast to 1989-1991, when up to 10 ravens had been removed yearly for predator control, none was killed in 1992. We conclude that proper deployment of eggs treated with methiocarb or a similar aversive agent can be a useful, nonlethal means of reducing raven predation at Least Tern colonies.
