Passage rates and timing of bird migration in Montana

Rates and timing of bird passage in the proposed Norris Hill Wind Resource Area (NHWRA) and vicinity in southwesteren Montana were investigated using two marine surveillance radars between August 1995 and August 1996. The sca nning radar array displayed movements in a horizontal plane within 360 degr ees while the vertical radar displayed altitudes of bir dr in and out of th e NHWRA to the east and west. Radars were also used to record raptor moveme nts within NHWRA in summer. Spatio-temporal profile of migration was determ ined by adjusting observed number of events by detection probability by rad ar, derived from point- and line-transect bird sampling techniques. Autumn migration was more protracted than vernal migration. Altitude of birds flyi ng in and within 2 km east and west of NHWRA averaged 209 m in autumn and 3 88 m in spring. Higher altitudes in spring were a function of birds ascendi ng after leaving Ennis Lake, whereas birds were descending to visit the lak e in autumn. More birds passed over valleys and swales than high points. Pa ssage rate decreased with declining barometric trend in autumn (headwinds), but the reverse was true in spring (tailwinds).