Cost-effectiveness of single- versus double-cylinder over-water nest structures

Minnesota waterfowl management plans prescribe widespread deployment of mallard (Anas platyrhynchos) nest structures. We compared 53 single- and 57 double-cylinder nest structures from 1996 to 2003 because managers used both structure types but were uncertain about their respective cost-effectiveness. More nests occurred in double-cylinder structures, but numbers of successful nests and hatched ducklings were comparable for both types. Nest success in single- and double-cylinder structures was 92.8% and 79.4%, respectively, with nest abandonment being >4.5 times greater in doubles. Structure damage occurred only at ice-out and was greater for doubles. However, relative risk of failure for double- versus single-cylinder structures was similar (1.26; 95% CI = 0.91-1.75) and increased with size of the open-water area containing the structure. Modeling indicated approximately 95% of recruits from nest structures were additional recruits. A case-history approach indicated doubles produced an additional recruit for $23.11 versus $23.25 for singles. However, these estimates were sensitive to assumptions used to apportion costs between structure types and ignored structure-placement influences. Placement affected cost-effectiveness significantly, with structures placed in open-water areas >10 ha being more cost-effective. Results also suggested singles might be more effective than doubles when placement is considered. Lower nest abandonment alone might make single-cylinder structures the better choice.