The Use of DRC-1339 Treated Eggs Attached to Elevated Platforms
for Raven Control in the Mojave Desert of California
JOHN MAESTRELLI AND PETER BUTCHKO
USDA/APHIS/ADC, 750 Windsor Street, Room 207, Sun Prairie, WI
53590; USDA/APHIS/ADC, 1942 South Court Street, Suite 2,
Visalia, CA 93277
Avian population surveys in the Mojave Desert of California showed a 15-fold increase of common ravens (Corvus corax) over the past 20 years due primarily to increased human activities. While the raven population was expanding, desert tortoise (Gopherus agassizii) numbers, primarily the younger age classes, were disappearing from large areas of the desert. Studies indicated that ravens were actively preying on tortoises less than 7 years of age. Over 250 tortoise shells with holes pecked in the carapace were found under a single Joshua tree used as a raven roost.
A raven reduction program utilizing DRC-1339 treated chicken eggs was initiated to reduce predation on the tortoises. A 23-gauge hypodermic needle was used to inject 1 ml of a 10% solution of DRC-1339 into unshelled hard-boiled eggs, the equivalent of eight lethal doses. Because of concerns about secondary poisoning when using DRC-1339 treated eggs, a delivery system was needed to eliminate the chance of secondary poisoning due to the ravens' propensity to cache scavenged eggs. Also, the treated eggs would have to remain in place overnight to take advantage of the early-morning feeding behavior of ravens while at the same time being unavailable to ground-dwelling scavengers.
Platforms, l6xl6 inches with 2x4 inch sides and plywood bottoms were attached to 6-foot high 4-inch posts that were sunk 1 foot into the ground. Twenty-three platforms were placed around a sanitary landfill near 29 Palms, California, where approximately 125 ravens were feeding. Treated hard-boiled eggs were displayed in two ways: enclosed in chicken wire sleeves or threaded on 18-gauge wire attached to the platforms. Both delivery methods were readily accepted by ravens and both prevented egg caching. The wire threading method was selected due to the ease of attaching the eggs to the platforms. Two eggs, with replacements, were wired to each of the 23 platforms for 6 days. A total of 75 eggs were consumed and 78 raven carcasses were retrieved near water sites up to 5 miles from the landfill. Over 50 hours of extensive searches in the vicinity of the landfill failed to reveal any non-target mortality.
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