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Providing evidence to improve practice

Action: Use aversive conditioning to reduce nest predation by mammalian predators Bird Conservation

Key messages

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  • One study from the USA and three  ex situ experiments found evidence for lower consumption of eggs treated with repellent chemicals.
  • However, when untreated eggs were provided simultaneously with or after treated eggs, no studies found evidence for continued lower predation. I.e. aversive conditioning did not occur. In addition, a study from the USA found no effect of repellent chemicals on predation rates of genuine nests.


Supporting evidence from individual studies


A replicated and controlled before-and-after experiment in southern Connecticut, USA, in June-September 1986 (Conover 1990), found that distributing 40 eggs treated with 20-25 mg of emetine dihydrochloride along 0.7-1.0 km transects at three second growth deciduous forest sites each week for three weeks reduced consumption of eggs by mammalian predators (raccoons, opossums, skunks and rodents) by >80% during treatment and for the following three week period (from >75% of eggs predated daily to <15%). There was no corresponding decrease at five control sites, where only untreated eggs were presented (daily predation rates rose from 3% to 90%). However, a randomised, replicated and controlled paired sites study in July-September 1987 found that egg predation was not significantly different at four experimental sites, where 10 eggs treated with 20-25 mg of emetine dihydrochloride and 10 untreated eggs were placed in set locations twice a week, compared to control sites, where only untreated eggs were provided.



A randomised, replicated and controlled experiment on ten captive coyotes Canis latrans (each tested with ten treatments over ten consecutive three day trials) in Utah, USA (Hoover & Conover 1998), found that no differences in food consumption, time delay before eating or amount of time spent eating when one of ten volatile chemicals was placed adjacent to food (so that they could be smelt but not ingested) at either the first or second exposure or in post-exposure trials. However, it also found that injecting eggs with 1 ml of one of ten volatile chemicals reduced the amount of egg consumed during both first and second exposures, compared to control eggs, for all chemicals except ammonia. However, egg consumption during post-treatment trials was unchanged following treatment and all eggs in post-treatment trials were opened. The ten chemicals tested were: allyl sulphide (garlic), ammonia, capsaicin (chilli pepper), chloroacetophenone (chemical mace), cinnamaldehyde (hot cinnamon), ethyl acetate, isoamyl nitrite (smelling salts), napthaldehyde (mothballs), pulegone (mint extract) and undecanone (commercially available dog repellent). When injected, pulegone, allyl sulphide and cinnamaldehyde reduced the amount of egg consumed significantly more than the other chemicals.



A replicated, controlled experiment with 12 captive coyotes Canis latrans (Hoover & Conover 2000) found that they preferentially consumed eight untreated eggs from untreated nests, compared to four untreated eggs from nests sprayed with pulegone (mint extract) or four eggs sprayed with pulegone, over a three day period. A second trial with 29 coyotes found that, during a five-day conditioning period when coyotes were presented with eggs injected with 1 ml pulegone, they opened and consumed fewer eggs each day (from 100% to <40% opened, <8% consumed). However, after the conditioning period, coyotes continued to eat 100% of untreated eggs when presented with them, either singly or alongside pulegone injected and sprayed eggs.



A randomised, replicated and controlled ex situ experiment in the UK (Messi et al. 2002) found that administering thiabendazole orally to 33 rats after they ate either a chicken Gallus gallus domesticus or quail Cortunix coturnix egg reduced the rate that they subsequently fed on either chicken or quail eggs, compared to control rats. Experimental rats ate 83% fewer eggs over eight post-conditioning tests and spent 80% less time eating eggs. No rats offered the same type (chicken or quail) of egg as in the experiment ate it in the first post-conditioning trial and only 20% of those offered the alternative egg ate it. All effects grew weaker over the eight post-conditioning tests, with most experimental combinations being indistinguishable from controls after eight tests.


Two randomised, replicated and controlled experiments in April-July 1996 at a mixed ring-billed gull Larus delawarensis and California gull Larus californicus colony in Idaho and California, USA (Conover & Lyons 2005), found that none of three aversive conditioning strategies reduced the number of eggs from experimental nests predated, compared to control nests. At a colony in Idaho, a trial with 110 repeats of each treatment found that, following a two week conditioning phase, where 45 pulegone (mint extract) injected chicken eggs were distributed and replaced around the colony, neither placing two drops (1 ml each) of pulegone on the edge of gull nests, nor spraying 2 ml of pulegone around the periphery of a nest reduced the number of eggs predated (26-38% of nests with pulegone drops were predated, 25-34% of sprayed nests and 22-37% of controls). At a colony in Idaho and two in California, a trial with 275 replicates found that, after a similar conditioning phase, burying a cup containing 2 ml of pulegone (mint extract) so that its lip was flush with the ground of the nest did not reduce the number of eggs predated, compared to control nests (30-31% predation for treated nests vs. 33-35% for controls).


Referenced papers

Please cite as:

Williams, D.R., Child, M.F., Dicks, L.V., Ockendon, N., Pople, R.G., Showler, D.A., Walsh, J.C., zu Ermgassen, E.K.H.J. & Sutherland, W.J. (2019) Bird Conservation. Pages 141-290 in: W.J. Sutherland, L.V. Dicks, N. Ockendon, S.O. Petrovan & R.K. Smith (eds) What Works in Conservation 2019. Open Book Publishers, Cambridge, UK.