The California Wind Energy Association (CalWEA) evaluated the procedures in the California Guidelines for Reducing Impacts to Birds and Bats from Wind Energy Development (the Guidelines) for estimating fatality of birds and bats associated with wind energy facilities. The research sought to improve the accuracy of methods for estimating the number of bird and bat fatalities by evaluating the effect of time dependency on the probability of scavenging and removal of bird and bat carcasses (carcass persistence) and detection by searchers (searcher proficiency). Researchers used data collected from the Altamont Pass Wind Resource Area from January 7 to April 30, 2011, to calculate traditional carcass persistence and searcher proficiency functions and to create new functions in which searcher proficiency and carcass persistence are modeled as a function of time and carcass age. This study is the first to document quantitatively that searcher proficiency and carcass persistence are time-based processes. The report offers lessons and implications for experimental designs and the field monitoring recommendations provided in the Guidelines. The study also investigated the fatality estimation equation provided in the Guidelines and three other prominent equations from the literature that are used to adjust fatality observations for searcher proficiency and carcass persistence. The report examines both the common and equation-specific assumptions inherent in these fatality estimators, evaluates them in light of data from the field experiment, and finds that each of the fatality estimation equations can result in positive or negative bias, depending on the length of search interval relative to carcass persistence time. A new equation incorporating carcass persistence from one search interval to the next is proposed. This project will help reduce conflict in the siting process and make sound wind project permitting decisions easier by improving the accuracy of fatality estimates and the ability to accurately compare them with those from other wind facilities.