AVERSIVE-CONDITIONING TO REDUCE RAVEN PREDATION ON CALIFORNIA LEAST TERN EGGS

We examined the potential of using conditioned aversion to reduce pred ation by Common Ravens (Corvus corax) on eggs of the endangered Califo rnia Least Tern (Sterna antillarum browni). In 1991, we placed Japanes e quail (Coturnix japonica) eggs injected with 30 mg (active ingredien t) methiocarb near eight raven nest sites at the U.S. Marine Corps Bas e, Camp Pendleton, CA. All raven pairs learned to avoid the treated eg gs within 4-5 days, and at seven sites egg removal was minimal through out 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 le arned to avoid quail eggs placed near their nests dominated access to the site, actively excluded neighboring ravens, and resumed taking qua il eggs. This response indicated that the previously acquired aversion was site-related and not transferrable to eggs encountered in a diffe rent location. From 15 April to 4 June 1992, we placed quail eggs inje cted with 30 mg methiocarb at three California Least Tern colonies at Camp Pendleton. Methiocarb-treated eggs were taken by ravens at each l ocatio, but no tern eggs were lost to ravens. Ravens that nested near the Least Tern colonies actively defended their territories against in truding 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 prope r deployment of eggs treated with methiocarb or a similar aversive age nt can be a useful, nonlethal means of reducing raven predation at Lea st Tern colonies.