Overwater nesting structures for ducks have been used primarily to increase mallard (Anas platyrhynchos) production where success of ground-nesting hens was low (<15%), such as in the Prairie Pothole Region (PPR) in North America. However, managers have inquired about recommended numbers of nest structures for small wetlands (i.e., <2 ha) based on duck-production and benefit–cost evaluations, but such data were not available. Therefore, we conducted an experiment in the PPR in Manitoba, Canada, in 2001 and 2002 to test the effects of different treatment numbers of nest structures/wetland (1, 2, or 4) and wetland area (≤0.4 or 0.45–1.5 ha) on use of structures by nesting ducks, nest success, and number of ducklings produced, as well as to evaluate cost–benefits of the structures. Across duck species, mean use of structures by nesting hens increased 56% between years (F1,107 = 14.29, P <0.001); however, by year 2 use did not differ among treatment numbers of structures and averaged 78% (t157 =−0.15–1.08, P≥0.28). Apparent nest success averaged 99% across treatment numbers of structures in 2001, but it did not differ among treatments (F2,72 = 0.93, P = 0.40) and decreased to an overall 48% in 2002, largely due to egg predation by corvids. Mean number of ducklings departing structures did not vary between years (F1,54 = 2.38, P = 0.13), but treatment number of structures influenced mean number of ducklings (pooled over 2001 and 2002) exiting structures (F2,54 = 4.54, P = 0.02). Mean numbers of ducklings departing 1 and 2 structures/wetland did not differ (t54 = 0.29, P = 0.77; pooled x̄ = 4.61 ducklings), but each respective mean was nearly twice that from 4 structures/wetland (t54 = 2.26–2.51, P≤<0.03). Neither wetland area nor any interactions influenced any analysis (F1–2,54–157 = 0–1.11, P≥0.33). We concluded that 2 structures/wetland was cost-effective ($2.16/fledged duck [U.S.]) for consideration among other management strategies to increase nest success of mallards in the PPR; however, managers should monitor duckling production from structures to determine whether our initial recommendation from a single study area warrants change.
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