Inland habitat associations of Marbled Murrelets on southwest Vancouver Island, British Columbia

Abstract

We used a combination of standardized audio-visual surveys, made over nine years, and vegetation analysis to determine habitat associations of Marbled Murrelets (Brachyramphus marmoratus) breeding in inland coniferous forests. Our study, in Carmanah-Walbran watersheds on southwestern Vancouver Island, British Columbia, covered extensive contiguous tracts (>16,000 ha) of apparently suitable habitat that supported a large breeding population of murrelets, a threatened species. Indicators of stand occupancy (circling and subcanopy flights) from audio-visual surveys were consistently associated with known nest habitat indicators (availability of platform limbs, cover and thickness of epiphytes on tree limbs, variable canopy structure). Both murrelet detections and nest microhabitat indicators were associated with a suite of macrohabitat variables, indicating that the most suitable habitat was low-elevation old-growth forest with widely spaced large trees. Biogeoclimatic productivity units, based on soil moisture and nutrient regimes, were useful proxies of habitat suitability, but tree species composition, timber volume, and tree height, variables commonly available in timber inventory maps, were not. Using two principle component factors derived from habitat characteristics, we clustered the 27 survey stations into three groups that differed significantly in occupied and subcanopy detections of murrelets and in nest habitat indicators. This is a useful method for combining multivariate measures for classifying and mapping habitat for murrelets.