Nesting burrowing owl density and abundance in the Altamont Pass Wind Resource Area, California

Abstract

About 600 burrowing owls (Athene cunicularia) are killed annually at wind turbines in the Altamont Pass Wind Resource Area (APWRA), California, USA. Understanding the biological significance of this toll requires population data, including an estimate of the number of breeding pairs in the APWRA. An empirical model of breeding density predicted 35–75 pairs within the 16,760 ha of the APWRA, but that prediction proved low in spring 2011, when we detected 78 breeding pairs of burrowing owls in 46 randomly selected plots totaling 2,563 ha. We estimated that 3.201 breeding pairs/km2 occupied the 46 plots we surveyed in April–May. Extrapolating this density to the area of the APWRA led to an estimate of 537 pairs (90% CI = 320–753 pairs) across the APWRA, or 10× the model prediction. Counts of chicks emerged from burrows averaged 1.2 chicks/nest on the 46 plots, but these counts were minimum numbers. We estimated the APWRA supported at least 1,836 burrowing owls in 2011 (90% CI = 1,082–2,590), indicating the local population could conceivably be the sole source of fatalities attributed to wind turbines in the APWRA. Measures to conserve burrowing owls in the APWRA could include ground squirrel conservation and repowering to safer wind turbine models and careful siting to avoid areas densely populated with burrowing owls. More generally, management decisions based on comparisons of breeding-pair density should also consider whether the density estimates were made in habitat fragments, sites selected for known high density, or in randomly or systematically selected plots sampling large areas, because the type of study area strongly influences the density estimate. © 2013 The Wildlife Society.