What's the right search plot size and shape for wind turbine carcass searches?

Q: What's the right search plot size and shape for wind turbine carcass searches?

The IFC PCFM Handbook recommends one of three plot types (full circular, full square, or road-and-pad / RAP). Search dimensions scale with the turbine's maximum blade tip height; transect spacing falls between 4 and 20 m depending on target species and vegetation. Bigger plots catch more carcasses; RAP plots save effort but require larger area corrections.

The right search plot gives a wind farm operator the largest catchment of fall-zone carcasses that can realistically be walked at the program’s search interval. Plot size, plot shape, and transect spacing together determine how much of each turbine’s true fatality count will ever be available to find. The IFC PCFM Handbook is direct that bigger plots win on detection but cost more effort, and that picking plot type at the start of the program is one of the higher-stakes design decisions an operator makes:

Bigger plots contain more of the carcasses that fall below turbines, resulting in higher detection probability and more precise fatality estimates, but they require a greater level of effort per search.

— IFC PCFM Handbook, §3.3.3.1

The three plot types

Plot type	When to use	Search dimensions
Full circular / square	Sparse vegetation under 30 cm, accessible terrain	Radius = half the maximum blade tip height; First Year default ~80 m
Road-and-pad (RAP)	Vegetation over 30 cm, dense cover, or unsafe terrain	Restricted to gravel pad, crane pad, and access roads; rest of fall zone treated as unsearched

Transect spacing inside the plot typically falls between 4 m and 20 m. Smaller target species (bats, small passerines) and denser vegetation require tighter spacing; the First Year default is 6 m (§3.3.4).

The RAP tradeoff

RAP plots are cheaper to walk and produce a higher searcher efficiency rate on the ground that is searched, but they leave most of the fall zone outside the plot boundary. The IFC PCFM Handbook is explicit about what that costs:

RAP plots have the benefit of being rapid and relatively easy to search and generally lead to higher searcher efficiency than the areas off the RAP. However, there is a tradeoff, because the proportion of the carcass fall area around a turbine that is covered by RAP surfaces is smaller than a full plot. Consequently, only a small proportion of the potential carcasses available for searchers to find will land within a RAP plot. This will result in low detection probability and fatality estimates with wide confidence intervals.

— IFC PCFM Handbook, §3.3.3.2

Larger rotor diameters spread the fall zone wider, which is why full-plot radius scales with maximum blade tip height rather than a fixed number. RAP designs can still meet lender requirements when the unsearched-area correction (DWP) is calculated and applied properly, but the fatality estimate becomes very sensitive to that correction.