Above: Grasshopper Sparrow (Ammodramus savannarum), photo by Ezra Staengl. (Not taken at Oakencroft Farm.)
March 4, 2021
Oakencroft Farm is a small, privately owned farm located in the Piedmont Uplands of central Virginia, near the town of Earlysville. Greg Shriver and Sarah Goodwin of the University of Delaware surveyed Oakencroft’s breeding bird community in 2009, with the goal of establishing baseline data that could be used to inform conservation decisions and track changes in bird populations on the farm over time (Shriver & Goodwin, 2009). Shriver and Goodwin used 29 fixed radius point counts to calculate density per hectare of 37 species in late May, at the start of the breeding season (Shriver & Goodwin, 2009). The presence of this past quantitative data creates a unique opportunity to examine how species composition and density has changed at Oakencroft over the last eleven years. Center for Urban Habitats resampled the 29 points from the 2009 study in May of 2020, with the goal of recalculating the density of as many species as possible in order to understand how Oakencroft’s bird populations have changed over time.
The farm contains several habitat types, but the vegetation is generally open and early successional in character. Hedgerows of bushes or mature trees border pastures and fallow fields. Shallow, marshy drainages intersect the property, all eventually emptying into Brown’s Creek, which flows into Ivy Creek off the property. A strand of rock outcrops runs through the center of the property, and a small pond is also present. Unsurprisingly, the birds at Oakencroft are generally associated with these open, edge habitats and small patchwork grasslands. However, species associated with forest and wetland habitats are also present at Oakencroft, occupying the appropriate niches in the landscape, and contributing to the farm’s impressive bird species richness.
As mentioned in the 2009 study, grassland associated birds are in particularly swift decline, and so should be a conservation priority at Oakencroft and elsewhere in the region (Rosenberg, K. V. et al., 2019; Shriver & Goodwin, 2009). The 2009 study also mentioned that prairie warbler, field sparrow, and grasshopper sparrow, all species that have been documented at Oakencroft Farm, are conservation priority species associated with grassland or shrub habitats (Kearney, R. F., 2003; Shriver & Goodwin, 2009). A different (more inclusive) ranking of Virginia’s Species of Greatest Conservation Need (SGCN), compiled by the Department of Recreation and Conservation and the Department of Game and Inland Fisheries, also includes brown thrasher, chimney swift, eastern kingbird, eastern meadowlark, eastern towhee, and northern flicker (VDGIF, 2015). In addition, both ranking systems include a few species of primarily forest associated songbirds found at Oakencroft, including yellow-billed cuckoo and eastern wood pewee (Kearney, R. F., 2003; VDGIF, 2015). Oakencroft is home to an important and declining set of birds. Continuing to monitor, protect, and restore those birds should be a priority.
Between May 25 and May 29 2020, two Center for Urban Habitats field technicians (Drew Chaney & myself) made two visits each to the Oakencroft property. During our two visits, we each surveyed all 29 of the fixed radius point counts established in 2009 (Shriver & Goodwin, 2009; Also, see Appendix A). The point locations are relatively evenly spaced throughout the property. On each of our visits we started work at 7:30 AM and finished around 12:30 PM. The weather was relatively consistent, warm and without precipitation.
On our separate visits to each of the 29 points, Drew and I each recorded every individual bird detected by sight or sound during a ten-minute count. We recorded if each individual was between 0 and 50 meters from us (close distance class), or greater than 50 meters from us (far distance class). We also recorded which 2.5-minute quarter of the ten minutes each bird was detected in, but this data was not used in our analysis. The data was compiled in the same format as was used for the 2009 paper, collapsing across observer and days, in order to facilitate direct comparison (G. Shriver personal communication, 2020). We calculated density for each species following Buckland (1987), using binomial methods. The formula we used for density is given below.
Density (D) = n ln (n/n2)/(r2 πm)
Where n = the total number of birds (of a species) detected, n2 = the total number of birds detected greater than 50 meters from the point, r = 50 meters (the radius of the close circle), and m = total number of counts (29*2 = 58).
We then calculated the variance of the density according to the formula below in order to calculate standard error for each species’ density.
All species for which density was calculated in both years (2009 and 2020) were graphed with their standard error bars. Additionally, all the species from the 2020 study were associated with a habitat code, either forest, edge, open, or wetland, following the method used by Valiela & Martinetto (2007). Species density was then graphed with standard error bars for each habitat category.
Over our four days of survey effort, we recorded 68 species, 66 of which were detected at the 29 point count locations during the duration of the point counts. Below is the list of all 66 species detected at the point counts, with percentage of points detected, density, and standard error for each species. Density could not be calculated for species that were not present in both the close and the far distance classes, so it is not given for those species. In addition to those 66 species, belted kingfisher and willow flycatcher were detected on the property during our survey but not at the point count locations.
|Species||Percentage of Points Detected||Density (per ha)||Standard Error|
|eastern wood pewee||90%||0.420||0.091|
|great blue heron||3%|
|great crested flycatcher||41%||0.047||0.019|
|northern rough-winged swallow||7%|
Of the above 66 species, density was calculated for 48 species. Only five species had calculated densities above 1 bird per hectare. In order of descending density, these were cedar waxwing (1.540), indigo bunting (1.168), European starling (1.117), barn swallow (1.096), and northern cardinal (1.071). Cedar waxwing and European starling are species that are often found in large groups during the summer, which may explain why their densities are higher than many territorial songbirds. For instance, despite having the second highest calculated density, European starling was only detected at 45% of the points, suggesting that a few large flocks may have inflated its density value. Indigo bunting, barn swallow, and northern cardinal are all widespread native species that are extremely abundant on the property. Indigo bunting and northern cardinal were both detected at every point, while barn swallow was present at just over half (55%).
Seven species had a calculated density of less than .1 bird per hectare, the lowest group of densities calculated. Three of these species, pine warbler (0.049), scarlet tanager (0.024), and yellow-billed cuckoo (0.024), are primarily associated with forest, a sparse habitat type at Oakencroft. Two species, red-winged blackbird (0.053) and Canada goose (0.049), are associated with wetland habitats, which, other than the small pond, are also almost non-existent on the property. The other two, common yellowthroat (0.078) and great-crested flycatcher (0.047), are associated with edge habitats.
The midrange of density values is mostly comprised of open and edge associated songbirds, including many of the species identified as conservation priorities in the introduction. Grasshopper sparrow was detected at 69% of the points, with a density of 0.512 birds per hectare. Field sparrow was detected at 72% of the points, but only had a density of 0.173 birds per hectare. Prairie warbler, worryingly, was not detected at all during our survey. In general, the species from the list of Species of Greatest Conservation Need (SGCN), also seemed to fare well. Brown thrasher had a calculated density of 0.101, chimney swift of 0.823, eastern kingbird of 0.183, eastern meadowlark of 0.442, and eastern towhee of 0.116. However, density could not be calculated for northern flicker, suggesting a relatively low population size. Additionally, flickers were only detected at 10% of the points.
Comparison and Analysis
Overall, we detected 3 more species in 2020 than Shriver and Goodwin did in 2009 (68 as opposed to 65), and 3 more at the point count locations (66 as opposed to 63) (Shriver & Goodwin, 2009). In general, our calculated densities were very similar to those recorded in 2009. Of the 32 species for which density was calculated in both 2020 and 2009, 26 showed statistically similar densities between the two study periods. The standard error of the densities of the other six species did not overlap, suggesting that there may be a change in their density on the property over the past eleven years. These species were European starling, barn swallow, mourning dove, blue jay, northern mockingbird, and American crow. Below is a graph of all species for which density was calculated in both 2009 and 2020, with standard error bars (Fig.1).
Out of the six birds with potentially different densities between the two years, four (blue jay, northern mockingbird, mourning dove, and American crow) appeared to decrease, while only two (European starling & barn swallow) appeared to increase (Fig. 2). Interestingly, all six of these birds are associated with edge habitats.
As mentioned in the introduction, Oakencroft’s avifauna is strongly characterized by the farm’s grassland nature. Many of Oakencroft’s more abundant birds are associated with grassland habitats, and many of these same birds are local conservation priorities. Although the habitat classification scheme used by Valiela & Martinetto has no specific category for grassland (2007), their “open” category is roughly equivalent in this context. As can be seen in the graph below, all four species attributed to this habitat type showed similar densities between 2009 and 2020 (Fig. 3). This is especially important because the first two, eastern meadowlark and grasshopper sparrow, were two of the species identified as conservation priorities (Kearney, R. F., 2003; VDGIF, 2015).
In general, the species identified as conservation priorities saw little change in their densities between the two reports (Fig. 4). In addition to eastern meadowlark and grasshopper sparrow, field sparrow, brown thrasher, and eastern towhee showed similar densities. Density could be calculated in only one year, and not the other, for the other four species, prairie warbler, northern flicker, chimney swift, and eastern kingbird. However, three of those species, all but the prairie warbler, were detected on both surveys. It’s possible that the densities of those three species may have changed between the two surveys, as not being able to calculate a density for a species usually indicates that it was less abundant than a species for which the density was calculated. Chimney swift in particular appears strikingly more abundant in 2020 than 2009, with a high density in 2020 and only 2 birds detected in 2009.
The prairie warbler had a calculated density of .172 birds per hectare in 2009 but was completely absent on our surveys in 2020. Oakencroft’s edge and early successional habitats still look like good habitat for prairie warbler, so its disappearance is particularly troubling. Future summer surveys could confirm if it might still be breeding on the property, or if it’s truly gone as our results indicate (for a complete comparison of the species only detected in one study period and not the other, see Appendix C).
With prairie warbler being the only notable exception, Oakencroft’s breeding bird populations, including those of many of the conservation priority species, seem to have been relatively stable between 2009 and 2020. Given the recent, widespread population declines of many native songbirds, and grassland species in particular (Rosenberg, K. V., 2019), this stability seems positive. Continuing work to restore Oakencroft’s habitats and monitor its biodiversity will ensure it continues to serve as important habitat for birds and other organisms.
Thanks to Greg Shriver for sending me an excel spreadsheet with the original data and formulas from 2009.
Buckland, S. T. (1987). On the Variable Circular Plot Method of Estimating Animal Density. Biometrics, vol 43, no 2, pp. 363-384.
Kearney, R. F. (2003). Partners in Flight Landbird Conservation Plan Physiographic Area 10: Mid-Atlantic Piedmont. Accessed February 2021 (https://www.partnersinflight.org/wp-content/uploads/2017/03/Phys-Area-10-Mid-Atlantic-Piedmong.pdf)
Rosenberg, K. V., Dokter, A. M, Blancher, P. J., Sauer, J. R., Smith, A. C., Smith, P. A., Stanton, J. C., Panjabi, A., Helft, L., Parr, M., Marra, P. P. (2019). Decline of the North American avifauna. Science, 366(6461), pp. 120-124. DOI: 10.1126/science.aaw1313
Shriver, G. & Goodwin, S. (2009). Birds. Private Report.
Valiela, Ivan & Martinetto, Paulina. (2007). Changes in Bird Abundance in Eastern North America: Urban Sprawl and Global Footprint? BioScience, vol 57, pp. 360-370. DOI: 10.1641/B570410.
Virginia Department of Game and Inland Fisheries. (2015). Species of Greatest Conservation Need. Accessed in February 2021 at http://www.bewildvirginia.org/species/
Appendix A: Point Count Locations
Point 1 (38.087620, -78.553127)
Point 2 (38.086408, -78.554377)
Point 3 (38.085038, -78.555521)
Point 4 (38.083542, -78.557171)
Point 5 (38.082276, -78.557801)
Point 6 (38.080202, -78.558543)
Point 7 (38.078772, -78.558977)
Point 8 (38.077310, -78.560087)
Point 9 (38.086540, -78.551388)
Point 10 (38.084875, -78.552890)
Point 11 (38.083285, -78.554019)
Point 12 (38.081917, -78.554822)
Point 13 (38.080622, -78.556002)
Point 14 (38.078997, -78.556725)
Point 15 (38.077183, -78.557532)
Point 16 (38.085913, -78.550174)
Point 17 (38.083818, -78.550798)
Point 18 (38.082016, -78.552385)
Point 19 (38.080414, -78.553110)
Point 20 (38.078608, -78.554826)
Point 21 (38.076851, -78.555208)
Point 22 (38.083356, -78.549467)
Point 23 (38.082045, -78.551151)
Point 24 (38.079820, -78.551541)
No Point 25
Point 26 (38.076557, -78.553284)
Point 27 (38.083672, -78.547199)
Point 28 (38.084392, -78.544673)
Point 29 (38.082538, -78.546080)
Point 30 (38.081906, -78.548439)
Appendix B: Birds Graphed by Habitat Association
Appendix C: Species Seen Only in One Study Period and Not the Other.
Species seen only in 2009 and not in 2020.
|Seen Only in 2009||Density 2009||Total Count 2009|
Species seen only in 2020 and not in 2009.
|Seen Only in 2020||Density 2020||Total Count 2020|