UBC Theses and Dissertations

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UBC Theses and Dissertations

Resource availability and limitation for a cavity-nesting community in mature conifer forests and aspen groves in interior British Columbia Aitken, Kathryn Elizabeth Helen

Abstract

Nest-site availability limits cavity-nesting populations in harvested forests, and woodpeckers are often considered keystone species because they influence the abundance of other cavity-nesters in the community. However, little is known about the relative importance of excavated versus non-excavated holes for cavity-nesters, and the extent of nest-site limitation in mature forests. I analyzed data from 1371 holes used by 29 bird and mammal species between 1995-2006. Excavated cavities were more abundant than non-excavated and were smaller and higher above ground, but were used in proportion to their availability. To test the hypothesis that nest-site availability limited cavity-nester abundance in mature forests, I conducted two multiyear, replicated before-after/control-impact (BACI) experiments in which I altered nest-site availability. In coniferous forests, which had low cavity densities (1.9/ha) and low occupation rates (9%) prior to treatment, I added nest boxes within the size ranges of the most common excavators (northern flicker Colaptes auratus and red-naped sapsucker Sphyrapicus nuchalis). Densities of mountain chickadees (Poecile gambeli), red squirrels (Tamiasciurus hudsonicus), and northern flying squirrels (Glaucomys sabrinus) increased following box addition and returned to pre-treatment levels following box removal. In aspen groves, which had high cavity densities (16/ha) and relatively high occupancy rates (44%) prior to my experiment, I blocked the entrances of high quality cavities (those with a high probability of occupancy). Total nest abundance declined by 49% on treatment sites following cavity blocking and returned to pretreatment levels once cavities were reopened. Nest abundance of European starlings (Sturnus vulgaris), a dominant secondary cavity-nester, declined by 89% and failed to recover posttreatment. Conversely, nest abundance of mountain bluebirds (Sialia currucoides; a subordinate secondary cavity-nester) increased following cavity blocking and remained high following reopening. While woodpeckers provide an abundant supply of cavities in some mature forests, non-excavated holes may release secondary cavity-nesters from the constraints of excavator nest-site preferences. Additionally, while nest-sites may appear to be abundant and potentially non-limiting at the community level, individual species preferences, as well as interspecific interactions, may influence true nest-site availability, particularly for mountain chickadees, starlings, and small mammals.

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