Radar Monitoring of Vernal Bird Migration in Central Montana

Abstract

Marine surveillance radar was configured to determine distance from radar and height of birds within a 5600-m long east-west transect near the town of Judith Gap in spring 2002. Located in a physiographic gap between two mountain ranges, data were intended as control for impact studies related to proposed wind power development in the vicinity. A single echo or target displayed on the radar screen was considered an event and consisted of a single bird or many birds of one or several species. Highest event rate recorded was 716/hour on the night of 18 May and was the larger and second of two prominent peaks evident during the season. Detectability of events was a function of proximity to radar. Event rate was adjusted for radar detectability and defined as activity rate. Mean activity rate at night was over 2.5 times that of daylight. Daylight activity rates/100-m segment of transect were inversely correlated with topographical relief. Median height of night events (278m) was 1.5 times higher than daylight events (P < 0.001 ). Median heights of daylight events/I 00-m segment of transect were inversely correlated with topographical relief(P < 0.001). A larger (P < 0.001) proportion (29.3%) of events in daylight were at or below maximum height of rotor swept area (105 m) of wind turbines proposed in the vicinity than night events (18.5%). Although an estimated 2 million birds may have passed over the radar transect in spring 2002, as many as 6. 7 million may have passed through the Gap. Event rate may be an appropriate metric for impact analysis of future wind resource development.