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Providing evidence to improve practice

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

Key messages

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  • Five studies from the USA and Europe found reductions in consumption of eggs treated with various chemicals. A further ex situ study from the USA found that American kestrels Falco sparverius consumed fewer chicks when they were treated, but not to the point of losing body condition.
  • Three studies from the USA found some evidence that treating eggs with some chemicals may have reduced predation of eggs after treatment stopped, or  of untreated eggs, although two of these were only short term experiments and the third found that the effect was lost after a year.
  • Four studies from the Europe and the USA found no evidence for conditioning, or a reduction in predation of wild (untreated) eggs.


Supporting evidence from individual studies


A randomised, replicated and controlled before-and-after experiment at 21 sites in Illinois and Iowa, USA, in summer 1986 (Dimmick & Nicolaus 1990), found that predation of dyed-green chicken eggs by American crows Corvus brachyrhynchos over a 23 day period, was significantly reduced when 50% or 100% of green eggs (eight provided each day in total) were treated with Landrin (a tasteless but illness-inducing chemical). There was no corresponding reduction in consumption of green eggs at sites where they were not treated with Landrin. Sites where 12.5% of green eggs were treated had intermediate levels of predation (100% sites: 7.8 attacks/day before treatment vs. 1.2 attacks/day after provision of Landrin-treated eggs; 50% sites: 5.6 vs. 1.4; 12.5% sites: 6.0 vs. 3.4; control sites: 7.6 vs. 7.2). At 50% sites, crows also stopped predating un-dyed eggs and consumption was reduced at 12.5% and 100% sites but remained unchanged at control sites. Post-test trials (when green eggs were again distributed but did not contain Landrin) in 1986 found that crows resumed predation at 100% sites but not at 12.5% or 50% sites. Further tests in 1987 found that crows at all sites except 50% ones resumed predating green eggs.


A replicated ex situ trial in the USA (Avery & Decker 1994) found that consumption of Japanese quail Corturnix japonica eggs by 30 fish crows Corvus ossifragus (in five treatment groups) was affected by the injection of different chemicals into the eggs. Topically applying methyl anthranilate to the outside of eggs (alone or in conjunction with injecting 18 mg of methiocarb) reduced consumption compared to other treatments (injection of 18 mg of Carbachol or methiocarb or 18 mg methiocarb plus 100 mg methyl anthranilate). Post-treatment tests with untreated eggs found that only crows from the topical methyl anthranilate groups did not consume eggs on the first day following treatment stopping (only two of 12 birds from these two groups resumed consumption in the post-treatment phase, although seven others moved but did not eat eggs). In a separate experiment, 16 crows in two groups were given eggs injected with either 30 mg methiocarb or 40 mg carbachol for five days. These crows consumed more eggs than those exposed to topical methyl anthranilate treated eggs, but fewer than other previous treatments. Five of eight crows exposed to 30 mg methiocarb and three of eight exposed to 40 mg carbochol ate eggs in the post-treatment test period.



A replicated, randomised and controlled before-and-after experiment in California, USA, in 1991 (Avery et al. 1995) found that significantly fewer Japanese quails’ Coturnix japonica eggs were taken by ravens Corvus corax from artificial nest scrapes in a ‘test period’ following a ‘training period’ (approximately two weeks), when eggs were treated with methiocarb than during the training period itself (0-33% of eggs taken during the test vs. 9-67% during training). Fewer eggs were taken from four sites that contained treated eggs over the test period, compared to sites where eggs were untreated in the test period, but had been treated during training (3% of eggs taken at one of four treated sites only vs. 0-33% taken at untreated sites). A follow-up experiment found that one of the eight raven pairs previously conditioned to avoid quails’ eggs resumed predation of eggs (both treated and control eggs) when they were placed in a simulated Californian least tern Sterna antillarum browni colony. A further experiment found that when methiocarb-treated eggs were presented at ten sites within three least tern colonies in 1992, a total of 20 eggs were removed or broken over 1,450 ‘egg days’ and no tern eggs were predated by ravens at any of the colonies in 1992.



A controlled, replicated before-and-after study at a heronry in northern Italy in 1994 (Bogliani & Bellinato 1998) found that the percentage of greenish-brown hens’ eggs predated by hooded crows Corvus cornix was significantly lower than blue hens’ eggs when they were treated with Carbachol. There was no difference in predation rates either before or after the 12 day treatment period (before treatment: 100% of both blue and brown eggs consumed within one day, n = 40; treatment: 61% of untreated blue eggs consumed vs. 38% of treated brown eggs, n = 480; after treatment: 90% of both blue and brown eggs consumed, n = 40). During the conditioning period, consumption rates were similar until the tenth day and then only a single brown egg was consumed over three days.



A replicated, randomised and controlled ex situ experiment with 33 American kestrels Falco sparverius (Nicholls 2000) found that control (untreated) day-old chicks were preferentially chosen, compared with chicks treated with methyl anthranilate on two out of four experimental days (ten birds choosing controls first vs. one choosing methyl anthranilate treated chicks and nine choosing controls vs. two choosing treated). Birds showed a preference for controls over aminoacetophenone-treated chicks on one day (nine choosing controls vs. two choosing treated chicks). On all other days there was no difference in preference for treatment or control chicks. The total amount of food consumed was highest for control kestrels, intermediate for those fed on aminoacetophenone-treated chicks and lowest for those fed methyl anthranilate-treated chicks, however, kestrels did not appear to lower consumption to the point of threatening body condition: there were no significant differences between kestrel weights at the end of the trial. A further replicated, randomised and controlled experiment found that treating cockerels with the two chemicals and dyeing them either green or blue greatly reduced food intake. However, there was no difference between consumption of dyed and treated chicks and controls that were only dyed, suggesting that aversive conditioning was not occurring. This study is also discussed in ‘Use coloured baits to reduce accidental mortality during predator control’ and ‘Use repellents on baits’.



A controlled before-and-after study in a least tern Sterna albifrons colony in west Portugal (Catry & Granadeiro 2006)  found that 15 artificial nests containing methiocarb (an illness-inducing chemical) treated quails’ eggs were predated by carrion crows Corvus corone at the same rate as 15 artificial nests containing untreated quails’ eggs. During pre-treatment (no eggs treated), first treatment phase (six days with treated eggs in experimental nests and untreated eggs in control nests) and second treatment phase (a further eight days of treatment) all eggs were destroyed within 24 hours of placement, although many treated eggs were not consumed following removal.



A replicated before-and-after study on Vila Islet, Azores, Portugal, in 2003 (Neves et al. 2006), found that the number of methiocarb-treated domestic quails’ Coturnix coturnix eggs (11.25 mg methiocarb/egg) predated by yellow-legged gulls Larus michahellis from artificial tern nests in a mixed common tern Sterna hirundo and roseate tern S. dougalli colony, over six days was significantly lower than the number of untreated eggs taken in the previous three days (2.5 treated eggs predated/day vs. 10.6 untreated eggs predated/day). Once terns started laying, treated eggs were placed in 18 artificial nests at the colony and replaced if predated. No gulls were observed removing eggs over 13 days, but European starlings Sturnus vulgaris took both treated eggs and tern eggs. Predation of treated eggs declined over time, but there was no corresponding decline in predation on genuine tern eggs (days 1-6: 13% of tern eggs and 9.3% of treated eggs predated; days 7-13: methiocarb concentration increased to 22.5 mg/egg, 12.3% of tern eggs predated vs. 5.6% of treated eggs).


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.