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Ecology and Management of Islands, Peninsulas and Structures for Nesting Waterfowl

Duck Nesting Success In and Homing To An Eletric-Fenced Area In Iowa

James Hansen, Ronald Andrews, Theodore LaGrange, and Alan Hancock
Iowa Department of Natural Resources
Clear Lake, Iowa 50428

Duck nesting on the Ventura Marsh Wildlife Area's uplands was studied over a 10-year period, 1979-88. Nests were located by means of a hand-held rope-drag the first 8 years and by a chain drawn between 4-wheel ATV's the last 2. Beginning in 1981, an effort was made to capture and band all nesting ducks with a net or trap during the second half of incubation. Hen mallards (Anas platyrynchos) were also marked with nasal saddles and since 1981, newly-hatched ducklings, primarily mallards, were web-tagged.

In response to low nesting success, an electric fence was erected prior to the 1983 nesting season. The fence surrounds 45 acres of nesting cover and a 5-acre marsh and lies next to a 370-acre marsh. From 1983 through 1985 the fence consisted of 9 alternating hot and ground wires, but for the last 3 nesting seasons a 2-inch mesh poultry netting "skirt" has been an added predator barrier. Large predator live-traps were used each year to capture and remove any predators that entered the fence. In 1987 and 1988 smaller live-traps were used to remove short-tailed weasels (Mustela erminea).

The primary nesting species were mallard and blue-winged teal (Anas discors). Nest success was quite low on the area's uplands from 1979 through 1982, with Mayfield nest success rates for mallards of 0, 0, 18, and 5 percent for the 4 years. Predation was the main cause of low nest success with striped skunks (Mephitis mephitis) and raccoons (Procyon lotor) believed to be the principal predators.

Results of the electric fence at first were encouraging with Mayfield nest success rates in 1983 and 1984 of 50 and 46 percent, respectively compared to 26 and 18 percent outside of the fence. Mayfield nest succes inside the fence declined to 16 percent in 1985 when a skunk entered the fence. No skunks or raccoons were known to enter the fence after the poultry-net barrier was added. Some apparent weasel predation, which may have occurred earlier, was confirmed in 1985, and became even worse in 1986, when Mayfield nest success dropped to 9 percent. In 1987 the removal of 43 short-tailed weasels resulted in an increase to 55 percent in the Mayfield nest success rate inside the fence, significantly greater than the 12 percent outside. In 1988, 25 short-tailed weasels were removed. Mayfield nest success was 46 percent inside the fence, significantly greater than the 10 percent outside.

The number of duck nesting attempts inside the fence ranged from 41 to 63 from 1984 through 1987, but in 1988, after a year with good nesting success the number of attempts soared to 102, of which 45 were mallards.

The marking of ducklings and hens has allowed us to document homing to the fence, especially for mallards. Thus far we have Captured 9 hen mallards on nests inside the fence that had been web-tagged as ducklings from 9 different broods inside the fence. Adult female mallards also home to the fence.

Our study emphasizes the importance of dealing with all types of egg predators to have a successful electric fence. We think that high nest success rates can be maintained, and that this, coupled with the homing of ducklings and successful hens, will allow the number of nests and ducklings produced to continue to increase.

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