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UBC Theses and Dissertations

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UBC Theses and Dissertations

Mechanisms regulating ecological responses to resource pulses within cavity-nesting bird communities Norris, Andrea Rose

Abstract

Resource pulses may influence mechanisms that can regulate consumer populations directly through bottom-up effects on resource availability and indirectly via top-down effects of inter-specific interactions. Although these are well documented in food webs, the responses within nest webs (communities structured around nesting cavities in trees) have received little attention. Bark beetle (subfamily: Scolytinae) outbreaks represent food pulses that may lead to secondary pulses of nest cavities and increases in fecundity and competition among insectivores if excavating new nest cavities allows exploitation of novel habitats that increase reproductive output for both excavators and obligate secondary cavity nesters (SCNs), and if increased territory quality leads to greater energetic expenditures on territoriality. Using observational and experimental approaches, I examined how a large-scale outbreak of mountain pine beetle (Dendroctonus ponderosae) influenced the production of cavities, fecundity, and competition within nest webs at 30 sites in interior British Columbia, Canada, from 1995-2009. I used 1,018 nests of two species that varied in their ability to excavate cavities and specialize on bark beetles, Red-breasted nuthatch (Sitta canadensis; facultative excavator and bark beetle specialist) and Mountain chickadee (Poecile canadensis; SCN and generalist insectivore). I found that nuthatches excavated more cavities in lieu of using old cavities, maintained a constant clutch size throughout the breeding season (~6 eggs), and fledged up to 100% more young per nest, at sites and in years with increasing beetle abundance. Chickadee clutches were initiated earlier, mean clutch size increased from 5 to 7 eggs, and fledgling success doubled with a dual pulse of food and nest sites. I examined intra- and inter-specific territoriality by simulating conspecific and heterospecific territorial intrusions using 974 presentations with song playbacks, from 2004-2008. Chickadees, although typically subordinate to nuthatches when competing for food, attacked all intruders more frequently (24% of 397 responses elicited) than nuthatches (8% of 372 responses). Both species showed increasing territoriality with increasing beetle abundance. Overall, my research suggested that species compensated both reproductively and behaviourally in response to resource pulses, and that plasticity in foraging and nesting behaviours can promote the resilience of wildlife communities in highly variable forest environments.

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