Factors Affecting Egg Predation by American Crows

Abstract

We studied predation by breeding American crows (Corvus brachyrhynchos) on artificial duck nests in upland and overwater habitats from April to July in 1986 and 1987 in the prairie pothole region of southwestern Manitoba, Canada. Predation was higher on nests placed within home ranges of breeding crows than on nests placed at random locations outside of home ranges. However, artificial nests placed >700 m from crow nests, yet within home ranges, were relatively safe from predation. Predation was greater on upland than on overwater nests. Nests in low vegetation were most vulnerable to crows, but increases in cover height above 20-50 cm did not substantially reduce predation. J. WILDL. MANAGE. 54(3):433-437 The possibility that egg predation has depressed waterfowl nesting success throughout the prairie pothole region (Cowardin et al. 1985, Greenwood et al. 1987) is of concern to waterfowl managers because this region is an important breeding area (Smith et al. 1964, Batt et al. 1989). Increasing nest success in the prairie pothole region is a goal of the North American Waterfowl Management Plan (Can. Wildl. Serv. and U.S. Fish Wildl. Serv. 1986). Electric fences are sometimes used to exclude predators from habitat managed for nesting waterfowl (Lokemoen et al. 1982). However, these fences cannot exclude avian species such as American crows, which are recognized as important predators of waterfowl eggs (e.g., Kalmbach 1937, Johnson et al. 1989). Egg predation by crows seldom has been investigated in detail, yet such data are needed to improve waterfowl management decisions in areas where crows are potential predators. Sugden and Beyersbergen (1986, 1987) found that crows that searched on foot located more artificial nests when nests were clumped rather than dispersed and were located in vegetation short and sparse rather than tall and dense. We compared rates of predation by crows on artificial duck nests: (1) during different stages of crow nesting cycles, (2) in upland and overwater sites, (3) in different vegetation types, (4) within and outside breeding crow home ranges, (5) at varying distances from active crow nests, and (6) in different heights of vegetation. Our study was not designed to estimate predation rates on wild duck nests. Rather, we used artificial nests as a tool to examine how nest site characteristics affected predation rates by crows, but Gdtmark et al. (1990) showed that predation rates on artificial and natural nests can be similar. This project was funded by the North American Wildlife Foundation through the Delta Waterfowl and Wetlands Research Station, and Iowa State University through the Department of Animal Ecology and the Iowa Agriculture and Home Economics Experiment Station. We thank J. M. Buenger, R. J. Keith, and D. M. Richardson for assistance in collecting data, and D. C. Glenn-Lewin, R. J. Greenwood, D. H. Johnson, E. E. Klaas, and L. G. Sugden for providing critical comments on the manuscript. M. G. Anderson, A. B. Sargeant, and L. G. Sugden provided valuable insight during the formative stages of the study. This is Journal Paper J-13149 of the Iowa Agriculture and Home Economics Experiment Station, Ames, Iowa, Project 2466.