Passage Rates and Timing of Bird Migration in Montana
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Rates and timing of bird passage in the proposed Norris Hill Wind Resource Area (NHWRA) and vicinity in southwestern Montana were investigated using two marine surveillance radars between August 1995 and August 1996. The scanning radar array displayed movements in a horizontal plane within 360while the vertical radar displayed altitudes of birds in and out of the NHWRA to the east and west. Radars were also used to record raptor movements within NHWRA in summer. Spatio-temporal profile of migration was determined by adjusting observed numbers of events by detection probability by radar, derived from point- and line-transect bird sampling techniques. Autumn migration was more protracted than vernal migration. Altitude of birds flying in and within 2 km east and west of NHWRA averaged 209 m in autumn and 388 m in spring. Higher altitudes in spring were a function of birds ascending after leaving Ennis Lake, whereas birds were descending to visit the lake in autumn. More birds passed over valleys and swales than high points. Passage rate decreased with declining barometric trend in autumn (headwinds), but the reverse was true in spring (tailwinds).
