Influence of skunk removal on nest success and breeding populations of upland nesting ducks in the lower Flathead Valley Montana

Forman, Kurt J., M.S., Spring 1993 Wildlife Biology Influence of skunk removal on nest success and breeding populations of upland nesting ducks in the lower Flathead Valley, Montana. (60 pp.). Director: Dr. I.J. Ball'^<=^^ As part of an ongoing study, duck nest success, nest densities, and breeding pair numbers were studied during 1991 and 1992 on the Ninepipe skunk (Mephitis mephitis) removal area and Pablo National Wildlife Refuge (control area) in northwestern Montana. Results were compared with baseline data collected in 1986 and 1987 prior to skunk removal (Hall in prep.). Skunk trapping success (N captures/1000 trap-nights), in 1991 (5) and 1992 (3) was substantially lower than in 1988 (15), the first year of skunk removal (Pengeroth 1991), indicating that skunk densities had declined considerably during the removal period. Although Mayfield nest success varied substantially between years on both study areas, the 1991-92 average at Ninepipe was significantly higher than at Pablo (45% vs. 14%, Z=4.24, P<0.005). Conversely, nest site Visual Obstruction Readings (VCRs) were significantly higher at Pablo than at Ninepipe in 1991 (3.9 dm vs. 3.2 dm, t=3.19, P<0.005) and 1992 (4.0 dm vs. 2.8 dm, t=4.67, P<0.005), suggesting that upland habitat conditions were not responsible for high nest success at Ninepipe. Apparent nest densities (nests/100 ha) at Ninepipe ranged from 8 in heavily grazed pasture to 175 in managed cover. Breeding pair densities (pairs/ha surface water) at Ninepipe in 1991 (13.9) and 1992 (11.9) were among the highest ever recorded in glaciated wetland habitats. Comparisons to baseline levels indicated that skunk removal was effective. With years combined, nest success at Ninepipe during this study was approximately double the pre-skunk-removal average of 1986-87, and 1992 nest densities in managed cover were over 4 times higher than in 1986. The number of dabbling duck pairs surveyed on standardized areas at Ninepipe by National Bison Range personnel increased from a 1982-88 average of 234 to 753 in 1992. ii Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ACKNOWLEDGEMENTS Funding for this study was provided by the U.S. Fish and Wildlife Service: National Bison Range, Region 6, U.S. Prairie Pothole Joint Venture, Montana Cooperative Wildlife Research Unit; Confederated Salish and Kootenai Tribes; Montana Department of Fish, Wildlife and Parks; and Ducks Unlimited. I wish to thank my committee chairman. Dr. J. Ball for project initiation, encouragement, timely advice, and critical review. I also thank my other committee members, Dr. D. Pletscher and Dr. E. Greene for assistance in project design and critical review. Special thanks to Bill Swaney for his patience, enthusiasm, and friendship during both field seasons. I am especially grateful to Nate Hall for providing invaluable advice on all aspects of the study and for furnishing data from 1986 through 1990. Field technicians, Wendy Wilson, Dan Breneman, Henry Normandeau, Fred Courville, Kevin Podruzny, and Keith Wesley all provided reliable and much appreciated assistance with data collection, and Don Shepard diligently assisted with skunk trapping. I thank John Grant of the Montana Dept, of Fish Wildlife and Parks for allowing access to state land, logistical assistance, and use of maintenance facilities.