Good estimates of fatality are challenging because the relationship between the number of observed carcasses and the number of animals that were killed is not direct.
Estimating bird and bat fatalities from wind energy operation requires a good survey design to detect the fatalities and a reliable estimator to determine how many fatalities may have occurred.
There are several factors that contribute to the detection of a carcass including:
- When the carcass arrived;
- Fraction of turbines searched;
- Proportion of fatalities in the searched area (relative density of carcasses);
- Proportion of carcasses persisting to the next search; and
- Searcher efficiency.
Through survey design, some of these factors can be controlled to help increase the detection probability.
- Search as many turbines as possible.
- Target searches in easier visibility classes. Detection probability should be over 30%.
- Take into consideration the density weighting of carcasses (the probability of a carcass landing in the search area). Smaller carcasses generally fall closer to the wind turbine than larger carcasses.
- Use a minimum of 20 small birds and bats and 10 large birds for searcher efficiency and carcass persistence trials (trials can be designed so the same carcasses can be used for both). Place carcasses continuously throughout the study period, e.g., one or two every other day rather than several on one day, then none for a long period of time.
- To determine carcass persistence rates, place a small number of carcasses out each day and observe their persistence at day 1, 2, 3, 4, 7, 10, 14, 21 - (longer studies for larger carcasses or longer search intervals). It is important to check the persistence more often immediately after placing the carcass. Many small carcasses do not persist very long on the land and knowing the persistence pattern is important to accurately estimate how often searches need to be conducted and, ultimately, fatalities.
- Prior to initiating fatality surveys, it is highly recommended the survey design be tested in the Evidence of Absence Software program (http://pubs.er.usgs.gov/publication/ds881) to determine the detection probability.
- Use a non-biased fatality estimator to estimate fatalities of non-rare species. A recommended, free fatality estimator is available at: http://pubs.usgs.gov/ds/729/. A very important feature of the software is that it provides measures of uncertainty in the estimates it produces.
- For rare species (i.e. threatened and endangered species, eagles, or other species of concern), for which fewer than 10 fatalities are predicted, use the Evidence of Absence Software (http://pubs.er.usgs.gov/publication/ds881). The Software uses information about the search process and scavenging rates to estimate detection probabilities to determine a maximum credible number of fatalities, even when zero or few carcasses are observed.
The estimate of turbine-caused fatality reported will always be greater than or equal to what was observed at the wind energy facility.
- Proportion of wind turbines surveyed for different methods (i.e. complete searches out to 80-120 meters, modified road and pad, etc.).
- Wind turbine numbers searched.
- Sampling coverage (density weighted proportion of area searched for each carcass size class and each turbine).
- Search interval for each for different method (i.e. complete searches out to 80-120 meters, modified road and pad, etc.).
- Carcass persistence and searcher efficiency results for bats, small, medium, and large birds.
- Number and species of fatalities found.
- Estimate of fatality per turbine and for entire facility for bats, small, medium, and large birds.
- 95% confidence interval around estimates.
Remember, failing to detect and estimate a fatality (absence of evidence) cannot necessarily be interpreted as evidence of a fatality being absent (evidence of absence).
For more information see Huso and Dalthorp 2014 (http://onlinelibrary.wiley.com/doi/10.1002/jwmg.663/abstract).
Fatality Estimator User's Guide
USGS research statistician Manuela Huso and co-authors published new software and a user's guide designed to provide accurate and unbiased estimates of wildlife fatality at wind facilities.
The fatality-estimator software uses carcass counts and detection-rate information provided by the user. More importantly, the software provides measures of uncertainty in these estimates.
The estimates are critical to predicting potential fatality prior to construction, developing techniques to reduce fatalities, and assessing cumulative effects on wildlife populations.
The software is available for free at: http://pubs.usgs.gov/ds/729/.
Evidence of Absence Software
Evidence of Absence software (EoA) is a user-friendly application used for estimating bird and bat fatalities at wind farms and designing search protocols. The software is particularly useful in addressing whether the number of fatalities has exceeded a given threshold and what search parameters are needed to give assurance that thresholds were not exceeded. The software is applicable even when zero carcasses have been found in searches.
EoA uses information about the search process and scavenging rates to estimate detection probabilities to determine a maximum credible number of fatalities, even when zero or few carcasses are observed.
The software is available for free at: http://pubs.er.usgs.gov/publication/ds881