Each year from 2012–2016, we conducted helicopter surveys to monitor the distribution, success, and productivity of Golden Eagles (Aquila chrysaetos) breeding within 16 km of California Valley Solar Ranch (CVSR) in San Luis Obispo County, California. This 250-megawatt photovoltaic solar plant was built over 2 yr on 642 ha of disturbed grassland and desert scrub habitat on Carrizo Plain, and was operational in October 2013. Beginning in 2012 coincident with initial CVSR development, the study area and region experienced a severe drought that persisted through the end of the 2016 breeding season. During the study, we detected and monitored 19 variably occupied Golden Eagle breeding territories. We confirmed a similar number of occupied territories in 2012 (15) and 2016 (16), but our ability to reliably confirm pairs declined during the drought and shifting territory dynamics occurred in several areas. Between 2012 and 2015, the proportion of pairs that laid eggs (breeding rate) and fledgling production per occupied territory (productivity) declined by 93–95% (13 pairs laid eggs in 2012; only 1 pair did so in 2015). In 2016, the breeding rate increased sharply again to 63% of occupied territories (87% in 2012) following the return of average rainfall during the preceding winter, but the proportion of pairs that fledged young and productivity rebounded to notably lesser degrees. The eagle breeding rate and productivity trends were strongly correlated with current-year measures of primary plant productivity and rainfall totals from the previous water-year (June–July). Breeding performance metrics for five pairs with territories situated on the periphery of Carrizo Plain and 13 pairs located elsewhere in the study area away from possible development disturbance did not differ statistically. This suggested that CVSR development and initial operation did not adversely affect eagle breeding performance during the study; however, we cannot draw definitive conclusions from this 5-yr study about how CVSR development might have influenced the local eagle breeding population. Instead, we concluded that a severe 5-yr drought dramatically reduced eagle breeding activity in the study area (and region) and confounded our effort to evaluate the effects of the solar development.
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Vol. 54 • No. 2