An emerging consideration for utility-scale solar development in the U.S. is the potential for impacts to birds, including protected species. The Endangered Species Act, the Migratory Bird Treaty Act, and state or local regulations provide a framework for limiting and mitigating any potential impacts to these species by solar and other forms of development. In thinking about impacts to birds, it is important to note that concentrating solar power (CSP) and photovoltaic (PV) plants use significantly different technologies with consequently different potential impacts.

Potential avian impacts can include habitat loss or alteration, collision with structures, noise or light disturbance, and - at CSP facilities only - thermal injuries due to solar flux. Thermal injury at CSP plants and collisions at both types of plants have gained particular attention in recent years because the solar industry monitors for and reports these incidents more formally than many older energy industries and also because regulatory agencies apparently did not expect these kinds of impacts to occur.

Many of the impacts to birds are not yet well understood, and research is under way to identify possible avoidance and minimization strategies. Unfortunately, the absence of knowledge regarding the mechanism of impacts has created difficulties for developers, as regulatory agencies push them to devote time and money to solutions that may not effectively address impacts. Despite the attention on potential impacts of solar development on birds, it is important to note that the magnitude of known impacts is similar to or lower than that for other forms of energy that have documented their impacts. Following is a brief review of the potential impacts to avian species, the current status of knowledge and suggested directions for future investigation to address knowledge gaps.



Habitat loss or alteration

Like any other form of development, solar energy requires modification of land cover that may affect its value as habitat for birds. Habitat impacts can be related to clearing for construction and maintenance requirements at operational facilities. The nature and extent of habitat clearing varies by facility, but it generally consists of grading and compaction of areas for collector arrays at PV facilities and heliostats or parabolic mirrors at CSP facilities; erection or burial of electrical and communication lines; creation of erosion control features; and fencing of the facility.

There is also potential for a solar project to create new habitat for some species. Birds may be present within a project area as a result of shade and nesting opportunities associated with structures.

Our understanding of habitat-related impacts to birds is limited because they are generally indirect and the causes are difficult to evaluate. In addition, there have been concerns about ravens using facility structures for nesting and resultant increases in desert tortoise predation. Impacts to other avian species from raven predation are likely negligible.

Due to the design allowing for long mirrors to track the sun in tandem, the land beneath CSP trough facilities needs to be graded nearly level, compacted and maintained in that state. Although similar constraints apply to construction of CSP tower facilities, the smaller scale of individual heliostats does not place the same premium on maintenance of level, vegetation-free conditions, and in some cases, heliostats can be installed without grading or foundations. According to a study published by Rebecca Hernandez and others in 2014, CSP facilities in California produce an average of 35 W/m2 of land, suggesting that large amounts of clearing are needed for utility-scale CSP.


Evaporation ponds are generally needed for wastewater derived from heliostat and mirror cleaning, dust control, and generator operations, and they are likely to create habitat for aquatic invertebrates, which may in turn attract waterbirds and insectivorous birds. Due to the potential for heat transfer fluid (HTF) overflow and concentration of anti-fouling and anti-scaling chemicals, evaporation ponds are often netted in an effort to exclude birds from the water. Netting presents trade-offs, as it can create entanglement hazards for birds while reducing contamination hazards.

Although the need for maintaining level, vegetation-free conditions is reduced at a PV facility, there is still permanent alteration of the land due to initial grading and compaction. The land-use efficiency of PV is nearly the same as CSP, indicating the need to clear large areas for such facilities. The need for evaporation ponds is somewhat lower than for CSP because water is typically only used for cleaning and dust control; therefore, the potential for pond-related impacts to birds is lower.




Collisions are a major cause of avian fatalities at industrial sites (e.g., communication towers and power plants) and residential or office buildings (e.g., homes and tall buildings). Scott Loss and his co-authors published a 2015 review of human-caused bird fatalities in the U.S. that indicates annual avian mortality from anthropogenic causes - such as from cats (1.4 billion to 4 billion), buildings (365 million to 1 billion), power lines (9 million to 69 million) and automobiles (80 million to 340 million) - greatly exceeds that from wind energy in total impact (140,000 to 644,000); no comparable estimates are available for solar energy, but initial indications are that it is comparable to wind.

Renewable projects, however, are generally the only facilities making systematic searches and reports to document collisions. A 2015 study published by the U.S. Department of Energy noted that collisions have been documented at both CSP and PV facilities, and collisions account for the majority (54%) of known fatalities at solar projects; however, there is currently no estimate of solar’s total impact comparable to that of the aforementioned anthropogenic causes. Additionally, only renewable energy projects routinely perform systematic studies and produce scalable estimates of fatality rates, and such rates are needed from other industries to facilitate meaningful comparisons of impacts.

One hypothetical explanation of collisions at solar projects is that avian species may mistake the mirrored surfaces of the heliostats or the dark surfaces of PV panels for open water, which is generally a source of foraging habitat. This is referred to as the “lake effect.” Studies of plate glass on buildings indicate that reflective glass can cause collisions if it reflects the environment, appears to be water or gives other false cues regarding the solid state of the windows; the same may be true of heliostats, mirrors or PV panels. There is currently no empirical evidence supporting this hypothetical explanation for collisions at solar facilities.

Studies of collision fatalities at CSP facilities indicate that birds can collide with mirrors, heliostats, and towers at both trough and tower facilities. The collision risk for CSP towers is similar to that of any tall structure, and no studies have indicated that the structures associated with CSP facilities pose a larger risk.

Monitoring results from some large PV facilities have indicated that birds can collide with PV panels, and collision accounts for a greater proportion of fatalities at PV facilities compared with CSP. Still, the overall fatality rate associated with PV appears to be lower than that associated with CSP.

It has been hypothesized that polarized light pollution (PLP), which is produced by smooth, dark surfaces, may indicate to birds that the surfaces of PV panels are water or attract high concentrations of insects (prey source for some species of birds). All empirical evidence that birds make use of polarized light indicates that migrants may use patterns of twilight polarization to orient (choose direction of migration), but there is no evidence that birds use polarized light to navigate (i.e., move about the landscape), and use in navigation presents the only means for PLP to cause avian collisions. Thus, the potential role of PLP in avian collisions is far from clear.


Noise and light

There is evidence that increased anthropogenic noise and light associated with a variety of forms of development can impact birds, and these potential impacts apply to solar development, as well. Elevated construction noise levels may alter bird behavior (e.g., foraging and breeding) and may cause disturbance potentially leading to nest failure or abandonment. Temporary sources of noise during construction consist primarily of heavy equipment. During operations, the main sources of noise vary by facility type.

For CSP systems, the primary sources of noise will likely be the boiler feedwater pumps, cooling tower, condenser and recirculation pumps, which are generally located within the power block in a power tower facility. Lesser levels of noise are also associated with the substation. At a CSP facility, tall towers and condensers must be lighted for aviation safety; additionally, the power block and tank areas, water treatment area, and condenser areas are typically lighted and may impact birds at night.

Noise is less of a concern at a PV facility compared with CSP. The primary sources of noise at PV facilities may include inverters or substations. Aviation lighting is typically not needed at a PV facility, and lighting is generally much more limited than at CSP facilities.

The overall change in ambient lighting conditions due to artificial lighting may disturb the nesting, foraging or migratory activities of birds. However, these potential impacts do not differ significantly from other industries or anthropogenic impacts in general.

Large energy projects are often constructed in areas that have previously been undeveloped and, therefore, unlighted. During operations and maintenance, exterior lighting is installed to provide for safe access to facilities, as well as for visual surveillance. Exterior lighting is generally concentrated around the power transformers, the main access roadway, the parking areas within the power block area and the main entrance gate, but solar fields are not illuminated.


Thermal or solar flux

CSP systems concentrate solar flux, which refers to the strength of the solar radiation at a given point in space. Solar flux in excess of 25 kW/m2 is generally considered harmful to birds.

Injuries related to solar flux have received a great deal of attention, primarily due to one CSP tower facility in the U.S. that reported an estimated fatality rate from solar flux of 690 birds per year (1.83 birds/MW/year) in a survey report docketed with the California Energy Commission (CEC); however, this fatality rate is comparable to the average collision-fatality rate for wind energy facilities in the U.S. Solar flux has been implicated in approximately 30% of bird fatalities at CSP facilities.

Because PV does not concentrate solar flux, thermal injuries have not been observed at PV facilities.


Other potential impacts

Avian species are commonly impacted by hazardous material releases at non-solar industrial facilities, and there is potential for such impacts associated with the HTF used at CSP trough facilities. HTF is a liquid that flows through tubes in the parabolic mirrors of trough systems, transferring the solar heat to generate steam to run the steam turbines. In some cases, HTF is petroleum based.

In 2014, approximately 50 fatalities were recorded in a report docketed with the CEC as occurring from HTF contamination of the evaporation ponds at a CSP facility after exclusion netting was destroyed in a wind storm and birds were able to access the ponds. Evaporation ponds may also present risks of poisoning due to the concentration of naturally occurring agents or additives to the water.

Additionally, the large scale of reflective surfaces and the use of dry cooling systems may impact the thermal microclimate within and around utility-scale solar facilities.

However, most of the aforementioned impacts are not unique to solar energy development.


Implications for utility-scale solar

A variety of avoidance and minimization strategies for avian impacts may be employed at solar facilities, including 1) netting of evaporation ponds; 2) installation of visual, acoustic or chemical deterrents; 3) minimization of lighting; 4) installation of perch deterrents; 5) increasing visibility of above-ground lines and fencing; and 6) changing storage position of pivoting heliostats or PV panels during non-generating hours if evidence suggests this can change collision probability.

The effectiveness of some standard avoidance and minimization strategies is unknown. Additionally, some strategies, such as perch deterrents, have been shown to have limited effectiveness. Further research is imperative - and currently under way - to understand the mechanisms of avian collision risks with solar energy facilities and to critically test hypothetical explanations (e.g., “lake effect” and PLP) before investing heavily in avoidance measures based on those hypotheses. Research into the minimization of avian collisions with window glass may provide insights that can be applied to solar facilities. The National Renewable Energy Laboratory has developed simulation modeling to understand the distribution of harmful levels of solar flux around CSP towers, and this may aid the development of operational modifications to reduce risks of this form of impact.

A potentially productive development is the formation of an Avian-Solar Collaborative Working Group by several federal and state agencies in the U.S., including the Bureau of Land Management, U.S. Department of Energy, and U.S. Fish and Wildlife Service (FWS), to address issues related to potential impacts of utility-scale solar development on avian species. Furthermore, there is a parallel need for development of programmatic guidelines similar to the FWS Wind Energy Guidelines that would help to promote consistency and standardization in the treatment of potential avian impacts within the industry across the various technologies in use.

We also hope that the self-monitoring commitments of the renewable energy industry will provide an example followed by other industries so that monitoring and mitigation costs may be spread equitably rather than concentrated within a single industry.


All three co-authors are part of DNV GL Energy’s environmental and permitting services department, where Chris Farmer, Ph.D., is principal biologist, Amanda Klehr is project biologist and Ellen Crivella is global head of the practice. A full list of information sources used in this article is available from the lead author at christopher.farmer@dnvgl.com.

Environmental & Siting Issues

Does Utility-Scale Solar Pose A Major Threat To Birds?

By Chris Farmer, Amanda Klehr & Ellen Crivella

Understanding a project’s potential effects on birds could help the solar industry mitigate environmental impacts.











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