| In the California deserts, avian monitoring studies documented collision-related fatalities at solar facilities. These raw data are adjusted to correct for detection probabilities and scavenger removal to produce estimates of total facility-caused mortality for all bird species. These estimates are used to determine potential impact of the solar facility on birds. However, this approach ignores avian mortality that occurs naturally in the same environment, i.e., the actual impact of the facility should be evaluated in terms of estimated facility-caused mortality that exceeds background rates. While this is a potentially important factor, few studies have estimated the magnitude of background fatality, and we are aware of no such studies from California deserts. Gathering background avian mortality data was integrated into an existing study on desert tortoise populations. Transects were conducted across the Mojave and Colorado desert in desert tortoise conservation areas, typically the least developed areas in the region. The avian mortalities detected during surveys were corrected for detection probabilities and scavenger pressure in the region to provide an estimate of background avian mortality (and 95% confidence interval) in desert tortoise habitat during the monitoring period. This background rate is compared to mortality rates documented at solar facilities.
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