Use of Radar to Study the Movements of Marbled Murrelets at Inland Sites

Abstract

A modified vehicle-mounted, X-band marine radar system was used to study the movements of marbled murrelets (Brachyramphus marmoratus) at inland and coastal sites in northern California during July. The ability of the radar to discriminate murrelets from other targets, and to estimate abundance was assessed. Murrelets were detected by radar at distances up to 1.3 km. Radar recorded the distance, ground speed, flight direction, and flight behavior (such as circling). The average ground speed of murrelets was 77 km/hr (range = 56-105 km/hr). Ground-based observers recorded an average of 67% of the murrelets within 700 m at inland sites that were recorded by radar. Using ground speed as a identification criterion, radar correctly distinguished murrelets from other bird species 87.8% of the time at coastal sites and 97.8% at inland sites. The only species contributing to identification error at inland sites was the band-tailed pigeon (Columba fasclata). Radar has advantages over round-based observers as it does not rely on murrelets to vocalize for detection and can detect murrelets over a large area, regardless of variability in light conditions, observers' auditory and visual abilities, fog, and background noise. The benefits of using radar to understand the inland flight behavior of murrelets include better interpretations of ground-based observer survey data, better estimates of the number of birds using an area, collection of data that are not biased by murrelet vocalization rates, increased understanding of landscape level flight behaviors and use of flight corridors, 24-hr sampling ability, and a more detailed analysis of seasonal and daily changes in abundance inland sites.