METHODS McIntyre,
Blodwyn M. 2000. The breeding bird populations of Sweet Briar College.
Naturalist-in-Residence Report Number 1. Sweet Briar College, Sweet Briar VA
24595.
Introduction Methods Results and Discussion Conclusions Literature cited
METHODS
Five land cover types define the primary vegetation cover of Sweet Briar College. These classifications include Hardwood forest, Pine forest, Sanctuaries, Fields, and Main Campus. The percent of Sweet Briar College land in each of the five cover types was calculated using aerial photographs and GIS Maps.
Fifty sample plots were located according to two criteria: First, the number of plots in each land cover type was proportional to the percentage of that land cover type. Second, a stratified random sampling procedure was used to insure an even geographical coverage and random sampling of the land. To achieve these objectives the property was divided into five zones of equal area with ten sample plots in each zone.
Sample plots were placed by generating random numbers corresponding to Universal Transverse Mercator (UTM) coordinates between the North-South and East-West boundaries of each zone. A map showing the 50 randomly located points and the five zones was made using the existing GIS map of Sweet Briar College land (Figure 2).
A Global Positioning System (GPS) was used to pinpoint exact locations of the 50 plots in the field using the UTM coordinates of the points generated. A permanent marker (rebar stake colored pink) identified the location of each plot in the field. The GPS coordinates and the rebar stakes will simplify site relocation.
At each plot birds were surveyed twice during the breeding season, which in Virginia is from approximately May 20 to July 4. Birds included in the survey are listed in the Results in Table 3. Birds were surveyed using the singing male point count method, wherein all birds seen and heard are recorded for a specified period of time at a specific location. Three and eight minute point counts were conducted at each of the 50 plots. This method is commonly used for counting breeding birds since the males sing during the breeding season. Most observations were auditory rather than visual. All bird counts were conducted between 5 am and 10 am, and between June 1 and July 2, 2000. These restrictions are imposed to control sampling bias. The time of day parameters restrict sampling bias since many birds stop singing at approximately 10 am. The date restrictions insure that all breeding birds are in their breeding locations and actively breeding. Sampling conducted outside the aforementioned dates could result in counting birds that are passing through the area to northerly breeding sites, or under sampling the actual number of breeding birds due to the fact that breeding behavior has terminated.
Habitat data was collected at each of the 50 plots. Sixteen different habitat variables were measured and are listed in Table 1. Quantifying these variables and calculating the statistical correlation between each habitat variable and the presence or absence of the bird suggests which variables are most important to the bird species in breeding habitat selection. Combining these data and assessment of mapped locations of bird observations can contribute to bird species management.
Table 1. Habitat data collected at each sample plot.
SLOPE
BASAL AREA
CANOPY HEIGHT
CANOPY COVER (%)
GROUNDCOVER DENSITY
SHRUB DENSITY:
- DECIDUOUS
- CONIFEROUS
- BROADLEAF EVERGREEN
TOTAL SHRUB DENSITY
NUMBER OF TREE SPECIES
FOREST TYPE
AVERAGE DIAMETER OF TREES
MAXIMUM DIAMETER OF TREES
NUMBER AND AVERAGE DIAMETER OF SNAGS_
NUMBER AND AVERAGE DIAMETEROF DOWNED LOGS
PRESENCE OF INVASIVE SPECIES
Habitat data was collected at each of the 50 sample sites using basal area plots. Basal area is a measurement of surface area of trees per acre, which indicates the density and size of the trees on the plot. This sampling technique recommends a plot radius according to the basal area of the trees. A ten-factor basal area prism was used to include or exclude trees from the inventory, which determined the radius of the plot. The average plot radius was approximately 20m.
Canopy height is, as the name implies, the height of trees that form the forest canopy. Canopy height was calculated by averaging the heights of two canopy trees on the plot. Slope is grade of the incline or decline of the plot and was measured in degrees from 0 to 90. Canopy cover quantifies the amount of sky obscured by the canopy and was measured with a densiometer, which calculates the percent of sky visible through the canopy. Ground cover was measured as the percent of the area that was occupied by ground cover species from 0 to 0.6 m from the surface.
Shrub density was calculated according to three species groups: broadleaf evergreens, deciduous trees and shrubs, and coniferous trees and shrubs. Shrubs included any woody stem less than 2.5cm in diameter counted along a 16m randomly oriented transect through the plot center.
The number of trees on a plot indicates diversity. Snags are standing dead trees, which often provide nesting sites for cavity nesting birds. The diameter of a snag will determine which bird species can nest in the cavity. Similarly, downed logs provide nesting sites for ground nesters.
Invasive species are non-native species that may impact an ecosystem by out-competing native species. Eleven species of invasive plants were counted at each plot. These species include Ailanthus altisima (Tree of Heaven), Vinca major, English ivy, Japanese Honeysuckle, Multiflora Rose, Garlic Mustard, Winged Burning Bush, Russian Olive, Winterberry, Wineberry, and Japanese Grass. Invasive species are associated with human disturbance and may impact forest ecology. These species are a concern as they have been found throughout the Sweet Briar land holdings.
The bird species selected for further analysis included the following Neotropical migrants: Ovenbird, Wood thrush, Worm-eating Warbler, Pine Warbler, Prairie Warbler, Common Yellowthroat, Yellow-bellied Cuckoo, Eastern Wood Peewee and Acadian Flycatcher. Also included were the Brown-headed Cowbird, a brood parasite of Neotropical migrants, and four Woodpeckers: Pileated, Red-bellied, Downy and Hairy. Bird observations for each of the 14 species were plotted on an existing GIS map of Sweet Briar (Figures 3 - 15). T-tests were conducted to determine which habitat variables were significantly correlated (with 95% confidence or a < .05) with the presence or absence of the bird species at the 50 plots.
Introduction Methods Results and Discussion Conclusions Literature cite
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