Effect of Density and Concealment on American Crow Predation of Simulated Duck Nests

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Abstract
Predation by American crows (Corvus brachyrhynchos) on simulated duck nests was studied in the Saskatchewan [Canada] parklands during 1983-84. Density dependent predation on partially concealed nests became evident at about 1 nest/ha and reached a maximum at about 6 nests/ha in plots that contained an exposed "decoy" nest. Crows found few or no hidden (> 60% concealed) nests that were not associated with a nearby decoy nest. Experimental nests usually were found while crows searched cover on foot after having detected the decoy nest while flying. Concealment conferred little or no protection from a walking crow. The maximum protection by nesting cover would be realized at low nest densities or in dense cover that would deter walking crows and conceal nests. Intensive farming on the prairies and parklands leaves natural nesting cover in small patches. Therefore, duck nests sometimes occur in densities that would be vulnerable to density dependent predation by crows and perhaps other predators. Our results indicate that nest losses could be reduced by land-use practices that allow ducks to disperse their nests. An alternative, at least for some species, is the development of dense cover such as patches of low shrub.

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