Using Radar to Estimate Populations and Assess Habitat Associations of Marbled Murrelets

Abstract

I used high-frequency marine radar to count marbled murrelets (Brachyramphus marmoratus) entering 20 watersheds in Clayoquot Sound, Vancouver Island, British Columbia, Canada, in 1996-1998. My goal was to develop standard protocols for radar inventory and to explain landscape-level habitat associations of this threatened species. Dawn counts were consistently higher and less variable than dusk counts, but both sampling periods produced similar rankings of watersheds and proportionate numbers of murrelets. Most dawn surveys showed a unimodal pre-sunrise pulse of incoming murrelets, but a few dawn surveys showed post-sunrise pulses, likely caused by repeat visits by some birds. These post-sunrise pulses, although rare, inflated estimates of incoming murrelets and were avoided by restricting analyses to pre-sunrise counts. Dawn and dusk counts were higher on cloudy days (≥80% cloud cover) than on clear days, but among cloudy days there was no additional effect on counts caused by precipitation (thick fog or drizzle). Numbers of murrelets entering watersheds varied seasonally, reflecting the breeding chronology, but counts restricted to the core period covering incubation and chick-rearing (mid-May through mid-Jul) showed no significant seasonal effects. Counts varied among years at some stations, but when all stations were considered together, no significant inter-annual variation occurred. Murrelets sometimes flew over low ridges (200-600 m), taking shortcuts into watersheds or crossing from 1 watershed into another. I therefore adjusted the boundaries of some inland catchment areas (based on topography and likely flight paths) to match correctly counts made at the watershed mouths with the appropriate inland catchment area. Radar counts at 18 watersheds were significantly correlated with total watershed area, areas of mature (>140 year old) forest, and-most strongly-with areas of mature forest below 600 m. Logging produced negative impacts. Three of the 5 watersheds with extensive logging of low-elevation forest had fewer murrelets per area than unlogged watersheds or those that were <10% logged, but these differences disappeared once remaining low-elevation mature forests were considered. With the removal of old-growth forests, murrelets evidently moved elsewhere and did not pack into the remaining old-growth patches in higher densities.