U.S. Department of Agriculture: Animal and Plant Health Inspection Service


Date of this Version

January 2001


Application of slow-acting toxic bait is one method of reducing local populations of depredating blackbirds. Estimating numbers of birds killed in such baiting operations is difficult because affected birds die off-site and are seldom recovered. We conducted bioassays and flight pen studies of red-winged blackbirds (Agelaius phoeniceus) with a slow-acting, toxic brown rice bait to determine whether bird mortality could be predicted accurately using the Poisson and binomial discrete probability distributions. Bioassays confirmed that brown rice treated with 2% (w/w) 2-chloro-p-acetotoluidide was effective as a 1-particle lethal bait for redwings. Within a 0.2-ha flight pen, we offered this bait diluted 1:99 with untreated particles to three 60-bird flocks and three 20-bird flocks in a simulated baiting operation. Across the 6 test groups, we recorded bird mortality not different from that predicted using the Poisson distribution. We obtained the same estimates using the binomial distribution. Although a number of factors could influence the relevance of our findings to field use, the application of discrete probability distributions appears superior to using estimates of bait consumption as a means for evaluating blackbird mortality due to slow-acting toxic baits.