First Advisor

Michael T. Murphy

Term of Graduation

Spring 2008

Date of Publication


Document Type


Degree Name

Doctor of Philosophy (Ph.D.) in Biology






Towhees -- Behavior -- Oregon -- Portland, Birds -- Behavior -- Oregon -- Portland, Urban ecology (Biology) -- Oregon -- Portland



Physical Description

1 online resource (2, ix, 137 pages)


I studied the demography, reproductive biology, and mating behavior of Spotted Towhees (Pipilo maculatus) in four urban parks in relation to park area, the presence of recreational trails, and the effects of habitat edges. Despite high towhee abundance in the parks, two of the four are population sinks with levels of yearly reproductive productivity that do not offset adult survival. The other two parks have sufficient reproduction to adequately result in stable or growing population sizes. Park area and the proportion of habitat edge had no influence on population growth rate, reproductive productivity, or survival. However, trail density was negatively related to population growth rate, and this appears to be due to the negative effects of trail density on adult survival.

Experienced females who nested near recreational trails exhibited high nest success, and while there was no such relationship between nest success and a nest's distance from habitat edge, nests near edges fledged significantly more offspring, were less likely to suffer partial brood losses, and nestlings near edges bordering residential areas were significantly heavier than those in the park interior. This suggests that urban habitat edges, particularly those bordering residential areas where anthropogenic food sources like bird feeders are present, may provide abundant food resources for species that are able to utilize these sources, and these supplemental foods may improve the overall reproductive output for individuals nesting nearby.

Consistent with previous results that edges provide supplemental food for urban towhees, I found that female condition was negatively correlated with distance to habitat edge. The probability of extra-pair paternity (EPP) was highest both near habitat edges and in the park interior, but the proportion of extra-pair young (EPY) in the nests of females who engaged in this activity showed the opposite pattern: broods at intermediate distances from edges had the highest proportion of EPY. Communal food resources near habitat edges may create opportunities for chance extra-pair mating encounters, but females in food-poor interior habitats may engage in EPP more regularly as they leave their territory in search of food resources.


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