| Species' ranges are dynamic, shifting in response to a large number of inter-related ecological and anthropogenic processes. Climatic variation is generally strongly tied to species' geographical range distributions, and yet the effects of confounded abiotic and biotic processes are not satisfactorily discriminated by observational studies completed at a single spatial scale. The aim of this dissertation is to understand what processes contribute most to range expansion and the population dynamics of a species at multiple spatial scales (from range-wide species' distributions, regional populations, and to site-level population dynamics). Continent- and province-wide surveys of breeding birds (i.e. BBS, 1976--2005, and OBBA, between 1981--85 to 2001--05), and an extensive rare species database compiled by Bird Studies Canada on the historic and present distribution of hooded warblers (Wilsonia citrina, 1949--2005), were used to examine the question: what processes influence range expansion in this species? At a regional extent, I conducted bird and habitat surveys in forest fragments of southern Ontario (2003--2005), to examine the breeding response of W. citrina to nesting habitat, forest amount, configuration, and connectivity. And, at the site level, nesting habitat classifications using remotely sensed data and randomization models were used to examine whether the spatial point pattern of W. citrina nests were clustered due to nesting habitat availability or conspecific social factors. My main conclusion was that the range of W. citrina is expanding due to a combination of factors, including but not limited to climate change. Indeed, the northward shift in the distribution of W. citrina over a 20 year period in southern Ontario (between OBBA Atlas 1 and 2) is more consistent with climatic variation acting as a constraint on range expansion. Biotic factors, such as the probability of establishment given surrounding local populations and conspecific attraction, however, were strongly related to W. citrina expansion and the species' distribution at all spatial scales examined. This dissertation provides an example of how studies of spatial and behavioural ecology can work in concert to investigate the underlying abiotic and biotic processes that affect species' distribution patterns at multiple spatial levels. |
