Proponents of Prop 2 imply that close confinement was a major factor in these bird's deaths. However, Prop 2 opponents assert that Norco Ranch was in violation of many California laws already in place. A 2004 study of California egg farms in the journal "Avian Diseases" finds comparatively low Salmonella prevalence in indoor housing systems, commonly used in California, as compared to cage-free and free-range housing systems. The researchers state that this low Salmonella prevalence in California egg farms reflects the "distinct geographic, climatic, production and management characteristics" of the state's egg farms. 98 percent of egg farms adhere to the California Egg Quality Assurance Plan, which is a pathogen reduction program for Salmonella in California. The study states, "The highest prevalence [was] in the free-range birds kept on the dirt floors." The California study notes that "feral cats, rodents, skunks, opossums, wild birds, and other wildlife" were seen near the free-range hens’ feeding areas, and that rodents "were considered to be the biological vectors and amplifiers" of salmonella on the egg farm in the study. A 2003 study from the "Journal of Applied Microbiology" and a study published in the journal "Applied and Environmental Microbiology" support the conclusion that wild animals are a significant and dangerous vector for salmonella. Supporters of Prop 2 note that furnished cages for egg-laying hens have already been developed in Europe, which allow birds to move freely and display natural behaviors. The waste material in these systems is far less concentrated than with battery cages, and the animals are healthier and calmer with a stronger natural immunity to disease. Opponents of Prop 2 note that a process called "traceback" is conducted by the Food and Drug Administration (FDA) and helps to maintain a safe food supply. The FDA's guidance to its staff for conducting tracebacks has sections entitled "Farm Investigations" and "Egg Processor/Packer Investigations," which contain detailed protocols explaining who goes on the farm, how the investigation is carried out, biosecurity procedures and other important steps to ensure that should an outbreak from eggs occur, the traceback would successfully reveal the original source. These opponents to Prop 2 say that California already has adequate and exemplary disease control techniques. In July 2008 the University of California, Davis conducted a study through their University of California Agricultural Issues Center (AIC). The study concluded that "the best evidence from a variety of sources suggests that (non-organic) non-cage systems incur costs of production that are at least 20 percent higher than the common cage housing systems".