Effects of Predator Exclusion and Removal on Duck Production
in Iowa
RONALD D. ANDREWS, JAMES L. HANSEN,
THEODORE G. LAGRANGE, AND ALAN W. HANCOCK
Iowa Department of Natural Resources, 1203 North Shore Drive,
Clear Lake, IA 50428
Predator populations and duck nest success were studied on the Ventura Marsh and McIntosh Wildlife Areas in north-central Iowa during 1979-90. Nests were located by means of a rope and/or chain drag during this period. Predator populations were monitored by live-capture techniques and ear-tagging individual animals.
In response to low nest success, a smooth-wire electric fence exclosure was erected prior to the 1983 nesting season on the Ventura Marsh Area. The fenced exclosure 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 nine alternating hot and ground wires. From 1986 to the present, a 2-inch mesh poultry netting "skirt" has been an added predator barrier.
Nest predators were live-trapped and removed from the McIntosh Area from 1985 through 1988. The McIntosh Wildlife Area consists of a 200-acre upland including a 17-acre marsh and is located next to the shore of Clear Lake, which is 3,600 acres in size.
Striped skunks (Mephitis mephitis), raccoons (Procyon lotor), and short-tailed weasels (Mustela erminea) were the primary nest predators. The American crow (Corvus brachyrhynchos) became a more apparent egg predator within the exclosure in 1989.
On the Ventura Marsh Wildlife Area, Mayfield nest success rates ranged from 9 to 55% inside the exclosure and from 2 to 25% outside the exclosure. Mayfield nest success within the exclosure declined to 16% in 1985 when a skunk entered the exclosure. Some apparent weasel predation, which may have occurred earlier, was confirmed in 1986 when the Mayfield nest success dropped to 9%. Weasel removal in 1987-90 may be partially responsible for increasing nest success in 1987. However, in 1989 crow predation may have been responsible for the decreased nest success. When we began to mark nests differently, crow predation declined substantially.
During the predator removal period on the McIntosh Wildlife Area, Mayfield nest success rates ranged from 24 to 54%. Without removal, Mayfield nest success ranged from 6 to 59%. Number of skunks removed ranged from 10 to 19 (average 16), raccoons removed ranged from 9 to 17 (average 17), and opossums removed ranged from 1 to 11 (average 6). There does not appear to be any direct relationship between nest success and numbers of predators removed, probably because there an many other variables to consider.
Our study emphasizes the importance of dealing with all types of egg predators to have a successful exclosure or removal project. With proper construction and maintenance, electric fence exclosures can adequately eliminate nest predators such as skunks, raccoons, and opossums. During our study, red foxes did not appear to be a major nest predator. Apparently, during this period we did not have the associated fox populations that some of the Dakota studies had. Obviously it would be impractical to build a weasel "proof" exclosure, but with some minimal weasel trapping effort, weasels could be reduced. Crows apparently became a problem due to our nest marking method. We believe that electric fences are a promising management tool. On a local basis, fences are the most economically feasible, socially acceptable, and biologically compatible means of managing blocks of nesting cover to exclude nest predators and increase nest success for ducks.
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