Habitat usage of breeding songbirds in urban Columbus, Ohio
Lana Cecile Milbern, MS
Advisor: Stephen N. Matthews, PhD
Urbanization can have profound influences shaping patterns of songbird diversity.
For many species, urbanization poses considerable challenges, yet in many instances urban areas provide important habitat for songbirds, most notably riparian urban areas. With renewed interest to maintain functioning urban forests, understanding the interactions between songbirds and their urban environments is critical to making informed land management decisions.
My research addresses habitat usage of breeding urban songbirds in Columbus, Ohio, focusing on the relationship between urban songbird presence and behavior and the vegetation in urban riparian forests. Previous songbird studies have been conducted in the Columbus metropolitan area since 2001 and have focused primarily on the impacts of urbanization on abundance, nesting, and survival. My goals for this study were to add to this knowledge base by examining potential correlations between songbird territory density and structural vegetative characteristics in urban forests and to investigate the foraging strategies of urban songbirds.
The first component of my study examines the relationship between the breeding territory densities of individual songbird species and the vegetation structure of urban sites. Given that vegetation structure has been linked to urban songbird abundance and survival, the number of trees, the size of trees, and the density of exotic or native stems in a site may influence the territory density of certain species. I tested the hypotheses that overall songbird territory density will be greater in areas will fewer exotic stems and Neotropical territory density will increase with structural diversity. I conducted spot maps in urban riparian forests in Columbus, Ohio from late April to August in 2019 and compared these data to spot maps conducted by my colleagues in 2007 and 2011 to determine territory densities for the most common species, which included the Blue-gray Gnatcatcher, Blue Jay, Carolina Chickadee, Carolina wren, Gray Catbird, and the Northern Cardinal. We also conducted vegetation surveys during each year to quantify vegetation structure and composition changes. I developed models for each common species to predict territory density based on the measured vegetation variables. I found that large trees and year, with 2007 as the reference year were positive predictors of Blue- gray Gnatcatcher territory density and, contrary to my hypothesis, invasive exotic stems were not a negative predictor. I also found that exotic stems were a positive predictor of Northern Cardinal territory density and that medium trees were a negative predictor. My results suggest that preserving large trees is important to maintaining breeding habitat for Blue-gray Gnatcatchers in urban areas.
The second component of my study investigates the foraging strategies of breeding songbirds in urban riparian forests. Previous studies in different study systems have shown that songbirds exhibit tree-species specific foraging preferences, which may explain the avian community structure of those habitats. Given that plant species composition varies across urban forests and includes greater numbers of exotic shrubs than natural forests, identifying how songbirds use resources for foraging is important to informing restoration efforts in regards to their conservation. I hypothesized that songbird species, such as Neotropical migrants, that have been historically more sensitive to urbanization would exhibit the strongest tree-species foraging preferences. I tested this hypothesis by comparing the tree species foraging choices of four songbird foraging guilds with the availability of each tree species. I conducted songbird foraging surveys from April to August in 2018 and 2019 and vegetation surveys to determine the tree and shrub species composition at urban riparian sites in Columbus, Ohio. I found that out of aerial insectivores, bark foragers, ground foragers, and foliage gleaners, ground foragers exhibited the strongest foraging preferences. Ground foragers showed the highest foraging preference for black cherry and the highest aversion to honey locust, osage- orange, pawpaw, and sugar maple. Considering that all four foraging guilds exhibited preferences for or did not exhibit aversions to American elm, black cherry, and silver maple, restoration efforts should consider the potential of these species (along with other goals and feasibility) to support the greatest diversity of urban songbirds.
My research shows that the structural and floristic aspects of vegetation in urban forests influence songbird foraging strategies. While territory density differences were less systematic, they do reinforce the importance that vegetation structure can have on bird assemblage. This study increases the knowledge of how breeding songbirds respond to urbanization in their territory and foraging choices, and thus, adds to the understanding of how urban forests can be restored to benefit songbirds.