Abundance of Red-eyed Vireos (Vireo olivacea) relative to foliage density and forest structure

We evaluated the relationship between Red-eyed Vireo (Vireo olivaceus) abundance and foliage density and forest structurein a northern hardwood forest on Douglas Lake, Michigan. We found no relationship between the abundance of Red-eyed Vireos and forest structure complexity. Although our results suggest that Red-eyed Vireo density does not increase with forest structure, as predicted, our sampling protocol and short period that we had for sampling may have been insufficient to identify Red-eyed Vireo distribution patterns relative to forest structure. I grant the Regents of the University of Michigan the non-exclusive right to retain, reproduce, and distribute my paper, titled in electronic formats and at no cost throughout the world. The University of Michigan may make and keep more than one copy of the Paper for purposes of security, backup, preservation and access, and may migrate the Paper to any medium or format for the purpose of preservation and access in the future. Signed, Introduction Density of breeding foliage-gleaning birds varies spatially across the landscape and temporally at all scales. One likely factor that affects densities of these breeding birds is foliage volume, which in turn may be associated with availability of insects and other invertebrate prey consumed by foliage gleaning birds. One way to test the relationship between foliage volume and abundance of foliage gleaning birds is to compare the abundance of breeding birds with estimates of foliage volume. We predict that the density of a common inhabitant of northern hardwood forests, the Red-eyed Vireo (Vireo olivaceus), will increase with increased forest structure. Red-eyed Vireos prefer to nest and forage in northern hardwood forests composed of deciduous trees, including red maples (Acer rubrum) and aspens (Populus grandidentata and P. tremuloides) (Pettingill 1974, Kendeigh 1948). The Red-eyed Vireo gleans insects, mainly Lepidoptera larvae (Robinson and Holmes 1984) from foliage (Marshall et al. 2002), so we predict that increased density of forest structure provides more foliage, thus supporting increased insect abundance and therefore an increased abundance of Red-eyed Vireos. The purpose of this study is to compare areas of less structure to areas of greater structure to determine its effect on Red-eyed Vireo density, controlling for forest type. Because increased forest structure increased insect populations (Gray 1993), we expect forest structure to have a positive relationship with abundance of Red-eyed Vireos. Materials and Methods Data were collected in a northern hardwood forest along Douglas Lake, Pellston, Michigan (45 ̊ 33’ 45” N, 84 ̊ 40’ 38”, elevation 721 ft) at the University of Michigan’s Biological Station (Jurik 1986); the forest varied in canopy and understory density. We sampled vertical forest structure and abundance of Red-eyed Vireos at 12 circular plots, each with a 25-meter radius and located 18.3 m from the shoreline of Douglas Lake. Each site was separated by at least 200 meters to reduce the chance of recounting Red-eyed Vireos. To measure foliage density and ultimately forest structure at the sites, the poletouching method (Shemske and Brokaw 1981, Kennedy et. al. 2010) was used. Measurements were taken at the center of each site and at the four cardinal directions on a 25-meter radius. A seven-meter pole was held up and leaves contacting the pole at four height intervals (0-1 m, 1-2 m, 2-5 m, and 5-7 m) were counted. Above seven meters, the height of the tallest tree above the pole was estimated. The number of major branches (those connected to the trunk of a tree) directly above the seven-meter pole was also counted. The average number of leaf touches across the five sampled locations in each site was combined with the average number of major branches across the site to calculate a measure of Forest structure. Point counts were performed to estimate abundance of Red-eyed Vireos. Observers were randomly assigned to a partner and sites were assigned randomly to each pair of observers. Each group located the center of each site, recorded the site number, time of day, date, current temperature, amount of wind, and sky condition. Point counts were not performed during rain, thunder or lightning. Point counts were 10 minutes long. All observations started were completed between 06:00-10:00 (Latta and Sondreal 1999). During the 10 minutes, all Red-eyed Vireos detected visually or acoustically within our 25-meter radius were recorded. To avoid double-counting birds we noted when birds were interacting, including countersinging, and mapped movement of birds within the