Action: Control predators not on islands for wildfowl
- Six out of seven studies, mostly from North America found higher reproductive success of ducks when mammalian predators were removed. A before-and-after study found higher survival of captive-bred brown teal Anas chlorotis following feral cat Felis catus control.
- One meta-analysis from the USA and Canada found that ducks on sites with mammalian predator removal did not have higher reproductive success and trends in reproductive success were no more positive than on sites without predator control.
Supporting evidence from individual studies
A controlled study at two prairie and forest sites in Minnesota, USA, between 1959 and 1964 (Balser et a. 1968) found that using predator control significantly increased the survival of duck nests (59% of 247 nests vs. 29% of 112) and the number of ducklings produced (from 4,858 to 7,571) compared to the same sites when control was not used. The ‘survival’ of artificial wildfowl nests also increased (81% of 654 nests vs. 34% of 699). The duck species were blue-winged teal Anas discors, mallard A. platyrhynchos, and gadwall A. strepera. Raccoons Procyon lotor, striped skunks Mephitis mephitis and red fox Vulpes vulpes were removed using strychnine-treated eggs and trapping. The authors note that the two sites may not have been far enough apart to prevent predator immigration from control areas to treatment areas, so results may be conservative.
A controlled study on two wetland sites in south Manitoba, Canada, in June 1966 (Lynch 1972) found that a higher proportion of artificial nests survived in an area where nests contained strychnine-treated eggs, than in an area with non-poisoned eggs over a 16 day period (84% survival of 215 nests, with predated nests replaced every four days vs. 66% of 225 nests, predated nests replaced every four days). A total of 33 striped skunks Mephitis mephitis and 15 Franklin’s ground squirrels Poliocitellus franklinii were killed.
A controlled, replicated study in South Dakota, USA, between April and August 1971 (Duebbert & Kantrud 1974), found that duck egg hatching success was significantly higher, and more ducklings were produced, on both idle fields and active agricultural land when predators were removed, compared to control sites (with predator removal: 85-92% hatching success of 324 nests, producing 22 ducklings/ha on idle fields and 0.7 on active farmland vs. without predator removal: 51-68% of 245 nests and 4.7 ducklings/ha on idle fields and 0.5 on active farmland). Dabbling duck Anas spp., diving duck Aythya spp. and ruddy duck Oxyura jamaicensis nests were studied. Predators removed through poisoning, trapping and shooting were red fox Vulpes vulpes, raccoon Procyon lotor, striped skunk Mephitis mephitis and American badger Taxidea taxus.
A replicated, randomised, paired site study from May-June in 1987-1990 in 15 pairs of waterfowl production areas (61-201 ha) consisting of equal wetland and grassland habitats in Minnesota and North Dakota, USA (Sargeant et al. 1995), found that four duck species exhibited higher nest success and daily survival rate in sites where predators were removed. Mean daily survival rate of nests was significantly higher in predator-removal sites than control sites (0.94 compared to 0.91). Mean hatching rate was 13.5% for predator-removal sites and 5.6% for control sites but there was considerable variation in both treatments (1-58% and 1-62% respectively). Nest predation rate was significantly lower in predator-removal sites than control sites (91% compared to 96%). However, hatch rate was not correlated to the number of predators removed. Ducks species analysed were mallard Anas platyrhynchos, blue-winged teal Anas discors, gadwall Anas strepera and northern pintail Anas acuta.
A 1996 meta-analysis of 58 studies from the Prairie Pothole Region of the USA and Canada between 1935 and 1992 (Beauchamp et al. 1996) found that the nesting success of dabbling ducks Anas spp. did not differ significantly between sites where predator removal was practiced and those without removal. In addition, there was a significant decline in nesting success over the study period, but the rate of this decline did not differ between sites with predator removal and those without. Two of the studies analysed are described above (Balser et al. 1968, Duebbert & Kantrud 1974).
A randomised, replicated and controlled study in northern North Dakota, USA, in 1995-6 (Garrettson & Rohwer 2001) found that survival rates of dabbling duck Anas spp. nests was higher on eight mixed agriculture and wetland sites (each 41.5 km2) with predator removal than on eight sites without predator removal (42% survival for 1,584 nests vs. 23% for 1,122). Trapping and shooting removed 2,404 predators between 1994 and 1996, most of which were raccoons Procyon lotor, striped skunks Mephitis mephitis and red fox Vulpes vulpes.
A replicated study in southern Saskatchewan, Canada, in 2000-1 (Pearse & Ratti 2004), found that survival rates of mallard Anas platyrhynchos ducklings was 41-50% higher in four 41km2 grassland-wetland sites where predators were removed (average survival rate of 59% for 686 ducklings from 78 broods), compared with four sites without predator removal (40% survival). Survival was measured until 30 days old, with a total of 686 ducklings from 78 broods studied. A total of 509 predators were removed: red foxes Vulpes vulpes, striped skunks Mephitis mephitis, raccoons Procyon lotor, coyotes Canis latrans, American badgers Taxidea taxus and American mink Neovison vison.
A before-and-after study of a reintroduction programme in the north of North Island, New Zealand (O'Connor 2005) found far higher survival rates of captive-bred brown teal Anas chlorotis in 2004 than in 2003, following a more extensive feral cat Felis catus control programme in between releases. This study is discussed in more detail in ‘Release captive-bred individuals’.
- Balser D.S., Dill H.H. & Nelson H.K. (1968) Effect of predator reduction on waterfowl nesting success. The Journal of Wildlife Management, 32, 669-682
- Lynch G.M. (1972) Effect of Strychnine Control on Nest Predators of Dabbling Ducks. The Journal of Wildlife Management, 36, 436-440
- Duebbert H.F. & Kantrud H.A. (1974) Upland duck nesting related to land use and predator reduction. The Journal of Wildlife Management, 38, 257-265
- Sargeant A.B., Sovada M.A. & Shaffer T.L. (1995) Seasonal Predator Removal Relative to Hatch Rate of Duck Nests in Waterfowl Production Areas. Wildlife Society Bulletin, 23, 507-513
- Beauchamp W.D., Nudds T.D. & Clark R.G. (1996) Duck nest success declines with and without predator management. The Journal of Wildlife Management, 60, 258-264
- Garrettson P.R. & Rohwer F.C. (2001) Effects of mammalian predator removal on production of upland-nesting ducks in North Dakota. The Journal of Wildlife Management, 65, 398-405
- Pearse A.T. & Ratti J.T. (2004) Effects of Predator Removal on Mallard Duckling Survival. The Journal of Wildlife Management, 68, 342-350
- O'Connor S. (2005) Captive breeding and release of brown teal Anas chlorotis into the Moehau Kiwi Sanctuary, Coromandel, New Zealand. Conservation Evidence, 2, 72-73