Source-Sink Dynamics of Urban Bird Populations: Apparent Refuge or Ecological Trap?

EPA Grant Number: F5F71438
Title: Source-Sink Dynamics of Urban Bird Populations: Apparent Refuge or Ecological Trap?
Investigators: Smith, Sarah B.
Institution: Portland State University
EPA Project Officer: Zambrana, Jose
Project Period: September 1, 2005 through September 1, 2008
Project Amount: $111,344
RFA: GRO Fellowships for Graduate Environmental Study (2005) RFA Text |  Recipients Lists
Research Category: Academic Fellowships


My research investigates the demography of the Spotted Towhee (Pipilo maculatus), a resident bird species in the Northwest, in six variously sized urban greenspaces in Portland, Oregon. My preliminary research suggests that while reproductive success (measured as number of young fledged) in these greenspaces may be adequate to promote population growth, greenspaces may act as ecological traps that ultimately exhibit negative growth rates due to high fledgling mortality. The primary goal of this study is to explore the source-sink dynamics of several urban Spotted Towhee populations, and to determine the roles of reproduction, survival, immigration, emigration, and predation in explaining the general abundance trends for towhees in Portland’s network of urban greenspaces. The results of this study will help identify the primary factors that limit bird populations in urban environments.

The goal of this study is to explore the demography and source-sink dynamics of several Spotted Towhee populations in Portland, Oregon. I will determine the roles of reproduction, survival, immigration, and predation in explaining variation in towhee abundance in small urban greenspaces.


The work will be done in six forested greenspaces ranging from 1 to 24 hectares. To assess fecundity in each greenspace, I will color-band all mated pairs and locate their nests. I will calculate productivity as the average number of female offspring per female per year (assuming a 50:50 sex ratio). I will color-band nestlings, and affix a radio transmitter to twenty randomly selected nestlings. Once these nestlings fledge, they will be monitored with radio receivers to follow their movements and determine their rate of survival and causes of mortality. Annual survival of adults and juveniles will be calculated for the six study plots. I will estimate immigration rates of adult birds by recording the number of unbanded birds (immigrants) appearing in each greenspace from year to year. I will also determine the male age structure of each population (using plumage characteristics to distinguish between first-time breeders and experienced males). To determine the dominant predators within these urban greenspaces, I will randomly choose five nests from each breeding attempt in each study area, and place motion-sensored digital cameras near the nests so that predation events can be recorded.

Expected Results:

This study will be among the first comprehensive multi-year studies of the demography of a bird species in an urban park system in the United States. Studies of this sort are critical for maintaining wildlife populations in the face of increasing urbanization. I have the strong support of the Portland and Lake Oswego Parks Associations, who hope to use my results to inform their management decisions, and I hope that my results can be useful for managers in other cities across the United States as well.

Supplemental Keywords:

birds, predation, source-sink dynamics, Spotted Towhee, survival, reproductive success, urbanization,, Scientific Discipline, Ecosystem Protection/Environmental Exposure & Risk, Habitat, Environmental Monitoring, Ecology and Ecosystems, habitat dynamics, source sink dynamics, environmental sources of mortality, habitat use, avian habitat quality, avian community dynamics, reproduction, birds, environmental stress, mortality

Progress and Final Reports:

  • 2006
  • 2007
  • Final