Reducing Mammalian Predation on Eggs by Using a Conditioned Taste Aversion to Deceive Predators

Abstract

When chicken eggs injected with 20-25 mg of emetine dihydrochloride were distributed along 3 different 0.7-1.0-km transects, mammalian predators that had been feeding on untreated eggs developed a conditioned taste aversion and reduced their consumption of treated eggs by >75%. The predators, primarily raccoons (Procyon lotor), opossums (Didelphis virginiana), and striped skunks (Mephitis mephitis), continued to avoid untreated chicken eggs placed subsequently. In another experiment, equal numbers of emetinetreated and untreated eggs were distributed simultaneously along 4 different transects. Egg depredation rates were compared to depredation rates along 4 nearby transects where only untreated eggs were distributed. Egg depredation rates at treated and untreated sites did not differ during the 4-week test. Thus, it might be possible to reduce egg depredation by mammals by distributing emetine-treated egg baits prior to the onset of egg laying, but not after laying has begun. J. WILDL. MANAGE. 54(2):360-365 Mammalian predators reduce the reproductive success of many birds (Ricklefs 1969). From a human perspective, predation of eggs is a problem when poultry, endangered species, gamebirds, or other highly valued species are victimized. For instance, 1 critical problem is high predation rates on duck nests in the midcontinental prairie wetland region (Klett et al. 1988). The usual, effective, and cost-efficient method to reduce egg loss is to remove predators through poisons, shooting, or trapping (Balser et al. 1968, Duebbert and Lokemoen 1980, Lokemoen 1984). The benefits of removal, however, are usually short-term because the predators removed are quickly replaced (Duebbert and Kantrud 1974, Lokemoen 1984, Greenwood 1986). Moreover, removal is often controversial because the predators are valuable, and their removal might inadvertently produce other problems (Sargeant et al. 1984). Another potential way to reduce egg predation is to teach predators that ingestion of eggs might be followed by gastrointestinal illness, causing the predators to develop a conditioned taste aversion (CTA) to eggs. CTA's have been tested as a method to reduce coyote predation on sheep, but the results have been mixed (Conover et al. 1977, 1979; Conover 1982; Burns and Connolly 1985; Ellins 1985; Forthman Quick et al. 1985; Lehner and Horn 1985; Wade 1985). Successful applications of CTA's have been limited primarily to reducing bird damage to seeds and ripening berries: situations where the chemical is directly sprayed on the food (Stickley and Guarino 1972, Guarino et al. 1974, Stone et al. 1974). Unfortunately, it is impractical to chemically treat the eggs of free-ranging birds so that a predator will become ill after consuming one. Spraying chemicals on eggs, for instance, may be ineffective because most predators do not eat egg shells, and injecting the chemical inside the egg could kill the embryo. Furthermore, searching for hidden nests would be laborious. An alternative is to deceive predators by scattering out eggs injected with an aversive-conditioning chemical before the onset of egg laying. Theoretically, predators should develop a CTA to the treated eggs, generalize the aversion to untreated eggs, and stop depredating all eggs (Conover 1984). Some researchers have tried to reduce predation on eggs using CTA's, but none have successfully reduced predation on untreated eggs (Hanners and Southern 1980, Nicolaus 1987). In the most successful test, American crows (Corvus brachyrhynchos) were averted from green-painted eggs after ingesting other green-painted eggs injected with a chemical that made them ill. They did not, however, stop eating unpainted eggs (Nicolaus et al. 1983). Recently, I found that a 25-50 mg dose of emetine dihydrochloride (emetine) not only averted captive raccoons from treated food (including chicken eggs) but also caused them to generalize the aversion to the same food when untreated (Conover 1989). Emetine is a toxic substance with medicinal uses (Klatskin and Friedman 1948, Sax 1984); it is one of the active ingredients in ipecac syrup that is administered to humans to induce vomiting following inges-