Direct comparison of tilted and untilted radar for monitoring Marbled Murrelet Brachyramphus marmoratus populations

Abstract

We compared detection rates for Marbled Murrelets (Brachyramphus marmoratus) using an untilted 5-kW radar unit and a similar 10-kW scanner with its waveguide tilted upwards to scan an arc of approximately 25°. Depending on the site, the tilted scanner detected 29%-2925% more murrelets than the untilted scanner (mean: 76% more). The increase in total detections from the tilted scanner was higher at four interior sites (184%) than at three coastal sites (60%). Of 1016 detections analyzed in detail, 27% were detected by both scanners, 56% were detected by the tilted scanner only and 17% were detected by the untilted scanner only. Because tilting the radar scanner allowed more murrelets to be detected, such an arrangement could be useful for monitoring populations where lower numbers are detectable (e.g. inland sites).