In discussing the risks wind energy can pose to wildlife, we define risk in terms of three components: hazard, vulnerability, and exposure.
The Risk Framework
A hazard is any activity or thing that causes an adverse impact. Many human-made structures pose a collision threat to birds, including wind turbines – even when they are stationary. Bats are not likely to collide with stationary turbines but can collide with or be struck by rotating turbine blades. Wind energy facilities can also constitute a hazard if a species changes its use of the surrounding landscape. For example, species may avoid a wind energy site during the construction phase of a project and may not habituate to and the presence of turbines once the facility is operational. The presence of turbines and access roads may result in fragmentation of a species’ habitat, or change the balance of prey and predator species, affecting the survival or reproductive success of species of concern. The size and number of turbines within a project and the project footprint are aspects of the hazard that we consider when assessing risk.
Vulnerability pertains to the consequences of being exposed to a hazard. In the case of collisions with wind turbines, the consequence to an individual animal is injury or death. In the case of habitat impacts, vulnerability is a measure of how species’ use of habitat, survival, and reproductive success rates are affected by the presence or proximity of wind energy facilities. Whether individual fatalities render populations vulnerable to decline depends on the size and reproductive rate of the population, and cumulative impacts to populations from additional sources of take. A species’ population is more vulnerable to an impact if a small level of take can lead to changes in population size. Endangered and threatened species are considered more vulnerable to impacts because their populations are already facing smaller sizes and conservation threats. Species with low reproduction rates also tend to be more vulnerable to impacts from wind energy because they tend to have lower rates of reproduction. As an example, Golden Eagle populations, for which individuals take several years to reach maturity and only produce one to two chicks per year, are more vulnerable to impacts from wind energy than Mallard populations, because Mallards are able to reproduce early in life and can produce multiple broods of a dozen eggs annually.
Exposure considers the amount of time and number of individuals within a population are likely to interact with wind energy facilities. In the case of collisions, exposure considers the frequency that birds or bats enter the collision risk zone of a wind turbine and the amount of time spent within that zone. In the case of habitat impacts, exposure is a measure of the likelihood that a wind facility is constructed in habitat used by a species. Site assessment activities that measure species occurrence within a site provide a cursory measure of exposure when estimating risk.
How we evaluate risk
Collision risk models (CRMs) consider a species’ predicted exposure and a measure of hazard to estimate the likelihood of collision within a wind energy project. Collision risk models also consider species flight altitudes and other behaviors that might put an individual of a species at high probability of collision should it encounter a turbine—known as collision probability.
The U.S. Fish and Wildlife Service’s uses a CRM to predict eagle take for a wind facility’s Eagle Incidental Take Permits. The Service’s Eagle Conservation Plan Guidance eagle take prediction model is a function of eagle exposure – defined as eagle minutes within a certain height range at a potential wind facility site – measured pre-construction, the number of turbines, and collision probability. The Service recently updated the model for golden and bald eagle collision risk to reflect new information gathered at wind energy projects that have pre-construction take predictions and post-construction fatality data.
The Service’s CRM is used to predict annual eagle fatalities for a proposed site, enabling wind companies to apply for the appropriate permits and plan for compensatory mitigation to offset any predicted fatalities, specifically of golden eagle. Once the site is in operation, post‐construction fatality monitoring can be used to update the collision probability input for the model.
We do not have collision risk models developed for all species that may be exposed to wind turbines, For these species, projects may have to rely on more qualitative evaluations of risk, as is often done for habitat-sensitive species, based on information of species use in and around a proposed project site, species behavior and life history, and collision trends from other wind energy facilities.
Whereas risk from collision has direct, measurable links to survival and mortality for populations, the consequences to wildlife from risks of habitat-related impacts is less clear. Evaluating risk in this case still considers hazard and exposure, but is less quantifiable than collision risk. Exposure considers the number of individuals, leks, or nests observed within or near a wind energy facility. To avoid or minimize exposure, developers are often required or requested to place infrastructure away from critical habitat features. Important areas to avoid can include leks (mating grounds) used by sage-grouse or prairie-chickens.