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Landscape analysis of distribution and demographic consequences in a brood parasite-host system Jewell, Kelly Jean

Abstract

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.

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