Does Removal of Skunks and Raccoons Increase Duck Nesting
Success: Case Reports for Two Montana National Wildlife Refuges
STEPHEN J. MARTIN
U.S. Fish and Wildlife Service, Benton Lake National Wildlife Refuge,
P.0. Box 450, Black Eagle, MT 59414
I evaluated the effectiveness of removing striped skunks and raccoons to increase upland duck nest success on Medicine Lake and Benton Lake National Wildlife Refuges (NWRs) in eastern and central Montana, respectively, during 1985-89. Striped skunks and raccoons were the principal nest predators at both refuges and the target species of the spring trapping programs. Red fox are known to contribute to low nest success in the Prairie Pothole Region but were not a significant nest predator on the refuges. High coyote populations on both areas were believed responsible for depressing red fox numbers. Target species were removed with 220 size conibear traps placed in wooden cubby box sets and with live traps. Traps were baited with fish or canned cat food.
A total of 144 and 118 skunks and 52 and 43 raccoons were removed at Medicine Lake and Benton Lake NWRs, respectively. The average number caught per year included 29 and 30 skunks and 10 and 11 raccoons for the two areas, respectively. Due to yearly variations in the number of traps and length of trapping periods, the number of skunks and raccoons caught were combined to determine trapping success (number of animals caught per 100 trap-nights). These figures allowed for comparisons between years and served as an index to relative abundance of skunks and raccoons.
Upland duck nest success was determined using standardized methods to locate nests and the modified Mayfield method to calculate success. Duck nest success at Medicine Lake NWR ranged from 28 to 60%. Between-year comparisons reveal an inverse relationship between predator relative abundance and nest success. As relative abundance decreased during 1985- 87, the nest success increased. Likewise when the relative abundance increased (1987-88), the nest success decreased. Endemic diseases (rabies, distemper) were believed to have dramatically reduced skunk and raccoon populations at Medicine Lake NWR from 1985 to 1987, and were believed to be more responsible for the increase in nest success than predator removal. Duck nest success at Benton Lake NWR ranged from 19 to 74%. The lowest success occurred in 1985 prior to the initiation of predator removal in 1986. Nest success increased to 72% in 1986 following the spring removal of 62 skunks and 18 raccoons. Nest success remained above 70% during 1986 to 1988. The inverse relationship between abundance and nest success was less evident at Benton Lake NWR although when predator abundance increased two-fold in 1989 the nest success dropped from 72 to 56%.
Removal of skunks and raccoons can be effective in increasing duck nest success as demonstrated at Benton Lake NWR. Success of predator removal at Benton Lake NWR was attributed to the smaller size of the area. Also, a series of dikes and roads bisecting the refuge allowed for easy trap placement along major predator travel corridors. Conversely, Medicine Lake NWR was characterized by having three times the acreage and contained large acreage that was not as effectively covered by traps. Removal of striped skunks and raccoons can dramatically increase nest success, but site-specific characteristics such as red fox populations need to be considered when designing removal programs.
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