Efficiency of Conditioned Aversion in Reducing Depredation by Crows

(1) Separate pairs of breeding crows (Corvus brachyrhynchos) were daily offered eight sweet-green and four plain-white chicken eggs at twenty-one independent sites in Illinois and Iowa during a 66-day field experiment. For 23 days the frequency of sweet-green eggs containing a tasteless but illness-producing dose of the cholinesterase inhibitor Landrin ranged at randomly assigned treatment sites from 100%, 50%, and 12.5% to 0% at control sites. White eggs never contained Landrin nor did eggs of either kind during the 20-day 1986 post-test and the 6-day post-test of 1987. (2) Crows at control sites consumed eggs freely while those at 100% sites avoided sweet-green but consumed plain-white eggs. Crows abandoned all 50% sites and most of the 12.5% sites. Although predation resumed at 100% sites during the post-test, abandonment continued at 50% and 12.5% sites. (3) In 1987 crows again visited every site, consuming from 91% to 98% of available eggs at control, 12.5%, and 100% sites but those at 50% sites continued to avoid eggs (P < 0.001), suggesting that the same crows exploited sites in succeeding years and aversion to eggs was retained by crows of the 50% group. (4) The most effective and efficient programme for reducing depredations by crows should include the placement of an adequate but not necessarily large number of illness-causing baits. If predators confound illness induced by one class of baits with other foods then local abandonment may result.