| Summary: | Source-sink dynamics may occur where variation in predation or parasitism is
sufficient to cause spatial variation in population performance. Brood parasitic brownheaded
cowbirds Molothrus ater locally reduce host reproductive success, but little is
known about their regional effects on demography. I aimed to predict, using detailed
empirical data, the effect of cowbirds on the spatial dynamics of a preferred host, the
song sparrow Melospiza melodia. My objectives were to: (1) predict the distributions of
parasite and host based on landscape features; (2) predict parasitism rates from cowbird
occurrence and relate this to sparrow fecundity; and (3) map expected population growth
rates (λ) for song sparrows given parasitism, testing if spatial refuges from parasitism
exist.
I used logistic regression to predict cowbird occurrence by analyzing >500 avian
point counts in the Southern Gulf Islands (SGI), BC. I also evaluated alternate
hypotheses for landscape features thought to influence cowbird distribution elsewhere.
Cowbird occurrence in the SGI was best predicted by proximity to potential feeding areas
and landcover. The best logistic model included landcover, cattle, and distance to urban
and agriculture. Autologistic regression improved model performance, and models using
widely available data performed only slightly worse than those including all data. All
models of cowbird occurrence compared favorably with parasitism rates observed in 12
populations studied from 1 -8 years.
To estimate the regional effect of cowbirds, I employed data from 10 islands
studied from 2-8 years to estimate λ in song sparrow populations subject to parasitism.
Predicted growth rates were strongly influenced by cowbird distribution and land use, and were consistent with independent estimates of local population trend and findings relating
parasitism rates to landscape features elsewhere in their range.
My results suggest that the ratio of source to sink populations within a study
region will depend on land use patterns, their effect on cowbird distribution, and the
influence of parasitism on host reproduction. I use my results to suggest improvements
for future habitat modelling studies, management to limit cowbird distribution and
identify host refuges, and an approach to estimate the regional effect of enemies on the
spatial population demography of species of conservation concern. === Forestry, Faculty of === Graduate
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