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The authors investigated different techniques to study bird and bat fatalities and the numbers and passage rates of migrant birds and bats at the Montezuma Hills Wind Resource Area (MHWRA) in central California.This study comprised daily carcass searches at 48 turbines, with marine radar, full-spectrum acoustic monitoring and night-vision techniques to determine nocturnal flight directions, passage rates, and flight altitudes of birds and bats during fall 2009 and 2010.Mean nocturnal passage rates ranged 326-454 targets/km/hr, a higher rate than reported elsewhere in the US.Only 2-6% of the total nocturnal birds and bats recorded by radar passed through the high risk area at altitudes < 125 m agl.Migrating bats and birds ("radar targets") flew in specific directions at altitudes > 125 m agl, but showed no predominate flight directions < 125 m agl.Few bat acoustic data and no night-migrating bird acoustic data were collected.Six nocturnal-migrant bird fatalities and 53 bat fatalities were observed during the two 40-day survey periods.Trial-specific, carcass-detection ratios ranged from 0.20-0.50.Carcass-removal trials indicated that many small bird (45%)and bat (39%) carcasses disappeared within 24 hours.Bat fatalities were correlated to occurring mid season, on warmer nights, and with higher barometric pressure.The probability of finding a hoary bat (Lasiurus cinereus) fatality decreased as wind speeds increased, whereas the opposite was true for Mexican free-tailed bats (Tadarida brasiliensis mexicanus).Similarly, the probability of finding a hoary bat fatality increased as the average altitude of radar targets decreased, but the opposite was true for Mexican free-tailed bats.Bat fatalities were clustered, and more numerous at turbines southeast and downwind of the nearest Eucalyptus grove.Bird fatalities also showed clustering and were more numerous at turbines southeast of the nearest riparian area.