A decade of change in avian abundance and richness in an urban forest, Forest Park in Portland, Oregon

Background/Question/Methods

It is important to understand how expanding human development affects surrounding natural areas. Researchers have been studying the impact of urbanization on avian populations. Few studies have looked at long-term changes in avian community structure. This study examined changes in avian abundance and richness in an urban, forested park between the 1990s and the 2000s. The goal was to ascertain what changes occurred in avian richness and diversity over a decade and then to try to determine causes for any noted changes. In 1993, 24 sites were randomly selected in Forest Park in Portland, Oregon; one additional site was situated in the Ancient Forest Preserve northwest of the park. The fixed-point method was used to count birds. All birds seen or heard within a 50-meter radius from the center of the site were counted during a 10-minute interval. Birds were counted three times a year at each site in June of 1993, 1994 and 1995. Counts were repeated in June of 2003, 2004 and 2005. Data was lumped by decade of study and the results compared using paired t-tests. Data is also being analyzed for possible impact of location of site along an urban-rural land use gradient. 

Results/Conclusions

I found significantly greater abundance for all birds, as well as for urban birds, edge birds and forest birds. I did not find any significant differences in species richness. Bird species that significantly increased in abundance included American robins, winter wrens, song sparrows and purple finches; species that decreased in abundance included brown creepers, orange-crowned warblers, black-throated gray warblers, black-headed grosbeaks and woodpeckers. I found significantly fewer bark foragers and raptors and more resident birds, ground foragers and birds that produce multiple broods of offspring per season. Some of these observations are in agreement with the results from the Breeding Bird Survey, but others are in conflict. The observed changes may be the result of increased urbanization of surrounding areas; the results of this analysis will be presented at the conference.