Effect of density and concealment on American crow predation of simulated duck nests

Predation by American crows (Corvus brachyrhynchos) on simulated duck nests was studied in the Saskatchewan 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. J. WILDL. MANAGE. 50(1):9-14 Intensive agriculture on the northern Great Plains is considered a major factor responsible for low recruitment rates and depressed dabbling duck populations (Sellers 1973, Higgins 1977, Batt 1979). Increased predation of waterfowl nests has been linked with the extensive conversion of native upland to cultivated crops (Johnson and Sargeant 1977). Information about the relationship between intensive agricultural land use and increased predation is required to predict future trends and develop management strategies to maintain prairie duck populations. Intensive manipulation to reduce predation and/or direct management of predators will become increasingly important as managers seek ways to produce more ducks from less habitat. Although the American crow is one of the most important agents of nest loss for upland nesting ducks in the parklands (Smith 1971), little is known about its foraging strategies. Information is needed about how different predators find nests (Keppie and Herzog 1978) and what factors influence a predator's foraging decisions (Orians 1981). Such information is essential for developing sound management practices to improve waterfowl production. Several European studies concluded that corvids found proportionately more prey (eggs in most cases) as prey density increased (Tinbergen et al. 1967, Croze 1970, GSransson et al. 1975, Andersson and Wiklund 1978, Bijlsma 1980, Hill 1984a). A mathematical description of the relationship was provided by Taylor (1976). Studies concerned with the effect of conc alment on predation rates have yielded conflicting results, perhaps because different predators and/or hunting methods were involved (Keith 1961, Dwernychuk and Boag 1972, Jones and Hungerford 1972, Schranck 1972, Byers 1974, Erikstad et al. 1982). Little is known about interactions of nest density, concealment, and predation. In this paper we describe how nest density and concealment affected predation by American crows on simulated duck nests. We thank the student assistants who helped with fieldwork and the farmers who allowed us to work on their land. We are indebted to M. R. McLandress for helpful review of a draft of the manuscript.