| Summary: | Cavity nesting communities are structured in a complex hierarchy of interdependencies
based on the creation of and competition for nest-sites. This structure has been called a nest
web. Although cavities are persistent and may be used multiple times, few studies have
examined cavity use in relation to availability, or patterns of nest-site reuse. My objectives
were to: 1) determine cavity availability and use in continuous and naturally-fragmented
forests, and 2) examine nest-site reuse by cavity nesting guilds and species.
To examine cavity availability and use, I measured nest-site characteristics in continuous
forests and naturally-fragmented aspen (Populus tremuloides) stands in British Columbia. I
examined cavity selection at the community, guild and species levels using resource selection
indices. Continuous forests had fewer cavities and lower cavity occupancy rates (9-10%)
than fragments (35-44%). However, cavity characteristics did not differ between those
habitats. Overall, cavity nesters preferred live, unhealthy trees with few holes. Nest-site
selection was influenced by tree and habitat attributes, rather than cavity characteristics such
as orientation. Low overall occupancy rates suggested that there was a surplus of cavities.
To examine nest-site reuse, 193 cavities were monitored between 1995-1999. Eight
percent of cavities were destroyed during the study, mainly due to tree blowdown. Cavities
were occupied two years in a row, rather than intermittently. Reuse rates were highest for
cavities occupied by secondary cavity nesters (48%) and were lowest for those used by weak
excavators (17%). Although woodpeckers were the main providers of cavities for secondary
cavity nesters in the community, only 28% of cavities used by woodpeckers were occupied
the following year. Reuse rates varied considerably among species within all guilds. Deep
cavities with large entrances and those in aspen were reused most often, as were those in
aspen groves and close to forest edges.
I suggest that large-scale attributes such as proximity to foraging habitat are better
indicators of nest-site suitability than microhabitat characteristics. Thus cavity-nesting
communities should be managed at a larger scale than individual nest trees or cavities.
Because cavity abundance does not reflect cavity suitability, counts of holes will not predict
the ability of a habitat to sustain cavity-nester populations.
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