Novel Scavenger Removal Trials Increase Wind Turbine—Caused Avian Fatality Estimates

Abstract

For comparing impacts of bird and bat collisions with wind turbines, investigators estimate fatalities/megawatt (MW) of rated capacity/year, based on periodic carcass searches and trials used to estimate carcasses not found due to scavenger removal and searcher error. However, scavenger trials typically place ≥10 carcasses at once within small areas already supplying scavengers with carcasses deposited by wind turbines, so scavengers may be unable to process and remove all placed carcasses. To avoid scavenger swamping, which might bias fatality estimates low, we placed only 1–5 bird carcasses at a time amongst 52 wind turbines in our 249.7‐ha study area, each carcass monitored by a motion‐activated camera. Scavengers removed 50 of 63 carcasses, averaging 4.45 days to the first scavenging event. By 15 days, which corresponded with most of our search intervals, scavengers removed 0% and 67% of large‐bodied raptors placed in winter and summer, respectively, and 15% and 71% of small birds placed in winter and summer, respectively. By 15 days, scavengers removed 42% of large raptors as compared to 15% removed in conventional trials, and scavengers removed 62% of small birds as compared to 52% removed in conventional trials. Based on our methodology, we estimated mean annual fatalities caused by 21.9 MW of wind turbines in Vasco Caves Regional Preserve (within Altamont Pass Wind Resource Area, California, USA) were 13 red‐tailed hawks (Buteo jamaicensis), 12 barn owls (Tyto alba), 18 burrowing owls (Athene cunicularia), 48 total raptors, and 99 total birds. Compared to fatality rates estimated from conventional scavenger trials, our estimates were nearly 3 times higher for red‐tailed hawk and barn owl, 68% higher for all raptors, and 67% higher for all birds. We also found that deaths/gigawatt‐hour of power generation declined quickly with increasing capacity factor among wind turbines, indicating collision hazard increased with greater intermittency in turbine operations. Fatality monitoring at wind turbines might improve by using scavenger removal trials free of scavenger swamping and by relating fatality rates to power output data in addition to rated capacity (i.e., turbine size). The resulting greater precision in mortality estimates will assist wildlife managers to assess wind farm impacts and to more accurately measure the effects of mitigation measures implemented to lessen those impacts.

Use of bird carcass removals by urban scavengers to adjust bird-window collision estimates

“…For example, Erickson et al (2000) reported an average survival time of 25.0 days in Oregon, whereas Smallwood et al (2010) reported 4.45 days in California. Villegas-Patraca et al (2012) reported similar results in Mexico to those of Smallwood et al (2010).…”

“…For example, Erickson et al (2000) reported an average survival time of 25.0 days in Oregon, whereas Smallwood et al (2010) reported 4.45 days in California. Villegas-Patraca et al (2012) reported similar results in Mexico to those of Smallwood et al (2010). Each of these studies had larger average survival times than our study, which suggests that the results from wind turbine studies are not accurate for use in estimates of urban bird-window collision fatalities.…”

“…However, in that series of papers there were numerous caveats related to the accuracy of the various mortality estimates. Many reviews identify the removal of carcasses before an observer has a chance to detect them as one of the largest biases in estimating bird mortality from anthropogenic sources (Smallwood et al 2010, Hager et al 2012.…”

“…To date, estimates of carcass removal rates by scavengers and the bias created in estimates of avian mortality rates from human activities have been estimated primarily at wind farms, at power lines, and in pastures (Balcomb 1986, Tobin and Dolbeer 1990, Wobeser and Wobeser 1992, Ragg et al 2000, Kostecke et al 2001, Smallwood 2007, Prosser et al 2008, Ponce et al 2010, Smallwood et al 2010, Villegas-Patraca et al 2012, Bispo et al 2013, Smallwood 2013. Two studies have estimated carcass removal by scavengers in an urban landscape (Table 1).…”

Canadian Estimate of Bird Mortality Due to Collisions and Direct Habitat Loss Associated with Wind Turbine Developments

“…Several factors could lead to overestimates of searcher efficiency, including use of inappropriate carcasses that may be more conspicuous or larger than species that would be expected to be found during carcass searches (Labrosse 2008), concentrating carcasses in more exposed habitats within the search area, and failing to ensure that searcher efficiency trials occur without the knowledge of the observer. Scavenger rates may be biased if carcasses used in the trials are not fresh (Smallwood 2013), are not representative of the species being detected, or if too many carcasses are used at one time for trials, i.e., scavenger swamping (Smallwood et al 2010). All of these could result in underestimates of mortality.…”