Estimating Nest Densities for Marbled Murrelets in Three Habitat Suitability Categories in the Ursus Valley, Clayoquot Sound

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Book Chapter

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We investigated nest densities and breeding habitat preferences of Marbled Murrelets (Brachyramphus marmoratus) in unfragmented old-growth habitat in the Ursus Valley, Clayoquot Sound in 1998, 1999 and 2000. The study covered three habitat suitability categories, ranked as Excellent, Good and Sub-optimal by the habitat suitability model of Bahn and Newsom (this volume Ch. 6). Our objectives were to: 1) compare habitat structure and abundance of potential murrelet nest structures in the habitat categories; 2) document micro-habitat and forest structures which describe murrelet nest habitat preferences; and 3) estimate murrelet nest densities by climbing randomly selected clusters of trees with potential nest platforms. Trees were sampled in a manner similar to stratified cluster sampling. In total, 44 vegetation plots were randomly selected and 467 trees with potential nest platforms were climbed. Vegetation plot data indicated that trees in habitat rated as Excellent had thicker epiphyte growth, were taller and had greater diameter at breast height than trees in Good or Sub-optimal habitats. Tree density was lower and canopy closure was higher in Excellent habitat than in Good and Sub-optimal habitats. Good and Excellent habitats had higher densities of platforms and higher densities of trees with platforms than Suboptimal habitat. Trees with platforms climbed in Excellent habitat were taller, had larger diameters, greater numbers of mossy platforms per tree and more abundant and thicker epiphyte cover than trees with platforms climbed in other habitat classes. Of 240 trees with potential nesting platforms that were climbed in Excellent habitat, five nests were found; no nests were found in Good (n = 139 trees) or Sub-optimal (n = 88) habitats. The five nests found included one used in the current year and four used in previous years. All nest site characteristics were within the ranges found in other nest sites in BC. Within Excellent habitat, trees with nests (n = 5) had significantly larger stem diameters than trees that had potential nest platforms but no visible nests (n = 235); no other tree characteristics were significantly different. The density of trees (± SD) with potential nest platforms was 30 ± 14, 37 ± 27 and 12 ± 11 per ha in Excellent, Good and Sub-optimal habitats, respectively. Considering only nests active in the year of discovery, nest density was 0 for Good and Sub-optimal habitats and 0.11 ± 0.12 per ha (95% CI = 0 to 0.35) in Excellent habitat. Future work should increase sample sizes and optimize study design in order to improve density estimates, and should examine the applicability of these findings to fragmented and more modified landscapes.