Search

Depredation by common ravens often exceeds stakeholders’ levels of acceptance. Common sources of excessive raven damage include loss of young livestock and nest depredation of species of concern, such as desert tortoise, greater sage-grouse, and least tern. USDA/APHIS Wildlife Services applies the avicide DRC-1339 (3-chloro-p-toluidine hydrochloride, CPTH) formulated in an intact-boiled egg bait to reduce such damage under the DRC-1339 Concentrate Livestock, Nest & Fodder Depredations label (EPA No. 56228-29). Although considered extremely effective on corvids, DRC-1339 is a slow acting toxicant and ravens may succumb away from the bait site, preventing the use of carcass recovery as a means to estimate take. To estimate take, Wildlife Services employs a model comprised of a bioenergetics module to predict consumption combined with a toxicological module that predicts mortality based on a probit analysis. This model has recently been revised to improve and standardize take estimates for common raven management activities using the egg baits covered by this label. Common ravens exhibit complex behaviors that impact the amount of bait consumed when offered DRC-1339 egg baits. The approaches to capturing this behavior in the model are presented along with take estimates associated with a range of baiting scenarios.

DRC-1339 (CPTH, 3-chloro-p-tolouidine) is an avicide registered to reduce local populations of selected bird species at feedlots, dairies, and staging areas near rice fields, and to prevent livestock depredation. Additionally, two registrations are specifically for controlling gulls and pigeons. U.S. Department of Agriculture Wildlife Services personnel historically have used a variety of methods to estimate take, including counting carcasses and quantifying reduction in bird activity. Because this avicide is slow-acting and birds usually succumb away from the bait site, carcass recovery provides poor estimates of take. Because of natural variability in bird numbers and activity, quantifying reductions in bird activity is also a poor gauge of efficacy. Scientists at the U.S. Department of Agriculture APHIS Wildlife Services National Wildlife Research Center developed and continue to adapt and refine a model using a bioenergetics approach to estimate consumption and the resulting mortality from DRC-1339 bait consumption. The model estimates take for the major baits used under the feedlot and staging area labels for European starlings, blackbirds, brown-headed cowbirds, and common grackles. Consumption of homogeneous baits (rice, cracked corn, fat pellets) has been well characterized, and take (i.e., mortality) is accurately predicted in the model. However, predictions of take when french fries were used as a bait for controlling starlings at feedlots in Washington State were less accurate. Thus, we modified the model to capture the feeding behavior of birds using this highly heterogeneous bait by removing the dependence on bait mass and simplifying the calculation of dose ingested. The modified model calculates dose directly from the amount of DRC-1339 per calorie consumed instead of from the DRC-1339 concentration per bait consumed. In this paper, we compare estimates of take based on french fry bait mass distributions and caloric contents determined for five different french fry bait types.