Comparison of two models for estimating mortality from baitings with Compound DRC-1339 Concentrate avicide

In the U.S., DRC-1339 baitings for blackbirds (Icteridae) are generally done under the pesticide label, Compound DRC-1339 Concentrate ‒ Staging Areas. DRC-1339 is a slow-acting avicide and gives the birds enough time to leave the baiting sites. Carcass searches and other forms of onsite counts are ineffective. Instead, linear models (LM) are used. The LM are based on esophageal analyses of several blackbird species collected while feeding at staging area bait sites. Biases and large variances can occur with this type of sampling. As an alternative to the LM, we developed a semi-mechanistic model (SM) that combined mechanistic modeling of environmental and biophysical processes with statistical modeling of DRC-1339 toxicities, avian physical and physiological traits, and foraging behavior. We used simulated baiting scenarios in Missouri and Louisiana to quantify and compare mortality between the LM and SM. The SM accounted for meteorological and regional effects on feeding rates, and we ran the SM scenarios for both mild and inclement weather conditions during January, a month when DRC-1339 baitings frequently occur. Mortality was calculated for males and females of three blackbird species. We used brown rice as the delivery substrate in a mix consisting of 11.34 kg untreated and 0.45 kg 2% DRC-1339 treated rice (1:25 dilution ratio). Compared to the LM, estimates by the SM ranged from 5% higher for male common grackles [Quiscalus quiscula L] to 59% lower for male brown-headed cowbirds [Molothrus ater Boddaert]. On average, the SM was 29% lower (x̅ = 8635, SE = 274.6) than the LM (x̅ = 12,131, SE = 1530.8, P \u3c 0.001). Mortality estimates by the SM were 21% lower (x̅ = 7630, SE = 235.2, n = 12) under inclement than mild conditions (x̅ = 9641, SE = 276.3, P \u3c 0.001). Latitudinal difference between the states did not affect mortality estimates produced by the SM (P \u3e 0.65). Unlike the LM, the SM used avian physiological and behavioral responses to environmental and meteorological conditions based on individual characteristics of the modeled blackbird species. It represents a scientifically rigorous and broad-scale approach that can be applied at all staging area baitings regardless of region or time-of-year. The SM will produce much lower mortality estimates compared to the LM when brown-headed cowbirds are the major species using staging area sites