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Ovenbird. Photo by Peter Caulfield via Maine Audubon
Written by Marin Harnett and Gavi Mallory
Background
Ovenbirds (Seiurus aurocapilla), a migratory warbler species, are named for the domed, pizza-oven-shaped nest they build on the forest floor during the breeding season. They are identifiable by their black and orange striped crown and their loud tea-cher, tea-cher call. Ovenbirds breed in internal forests with significant canopy cover, ranging from eastern Appalachia north through Canada (Gallo et al., 2020). While many bird species thrive in post-disturbance forest ecosystems, ovenbird populations have consistently been found to decline in these conditions. In areas where ovenbirds have previously built nests, their absence during breeding time is an indicator to observers that changes in the forest structure have shifted habitat availability from one set of species to another. For land owners, understanding the impacts of timber harvest on bird species can aid in developing management plans that support ecological diversity on a landscape scale.
At Holt Research Forest, bird censuses were conducted in 1984, 1986, 1987, and most recently in 2022. Technicians completed the counts in the 1980s before a group selection timber harvest on site between 1987 and 1988. The most recent bird census followed a significant harvest completed between late 2020 and early 2021. This study compares data from Holt Research Forest bird censuses to understand how timber harvest impacts ovenbird abundance in southern Maine oak-pine forests. We used the 1980s census data to describe pre-harvest abundance and the 2022 data to describe abundance post-harvest.
Methods
Study Area
Holt Research Forest’s 100-acre study area is divided into 40 1-hectare blocks. Each block is further divided into quadrats. In late 1987, harvesting occurred in 10 blocks, which is the primary reason for focusing on data before this as pre-harvest data. The most recent harvest, completed in late 2020 and early 2021, was larger scale, with harvest activities occurring in over 85% of the study area. The latest harvest was conducted to promote oak-pine forest regeneration and allow selected trees to continue growing with reduced competition. Though many trees were left on the landscape, the forest structure was significantly altered with decreased canopy cover creating a more open structure.
Figure 1. (a) Holt Research Forest and Study Area Overview. (b) Study area post-harvest (outlined in red). Areas outlined in pink represent control (unharvested) areas. All other areas are managed and were included in the recent harvest.
Ovenbird Censusing
Bird censusing was completed along established walking paths or transect lines. Bird counts were completed early in the morning on days with no precipitation or high winds. Data collectors walked transect lines quietly and slowly to avoid startling birds, stopping to record observations as necessary. For the summer of 2022, a complete census indicated data collection along either the first three or last three transect lines. This definition of complete census was a modification from earlier data collection methods on-site and was made due to time constraints. Data was collected once along each transect line during July 2022, totaling two recorded censuses.
Results
Post-harvest strip censusing revealed very few ovenbirds present within the study area. During the census completed in mid-July, eight ovenbirds were recorded. A census completed later in July recorded three ovenbirds. In contrast, the 1984 census recorded 17 ovenbirds, 1986 recorded 28, and 1987 recorded 18.
Figure 2. Ovenbirds counted in July in 1984, 1986, 1987, and 2022.
The location of ovenbird observations in 2022 aligned with expectations considering the species’ preferred habitat and breeding grounds, characterized by significant canopy cover (Gallo et al., 2020). Following the 2021 harvest at Holt Research Forest, study blocks containing some control (unharvested) areas were 3J, 4J, 4D, 3E, 7I, and 7G. In the 2022 census, ten out of eleven total ovenbird observations occurred in a block that contained or was adjacent to one of these unharvested control areas.
Figure 3. Ovenbird observation proximity to unharvested (or control) regions of Holt Research Forest.
Discussion
The observed decrease in ovenbird abundance following timber harvest is consistent with findings in similar studies (Lambert & Hannon, 2000; Perry et al., 2018). Given the preference of ovenbirds toward forests with a high percentage of canopy cover, harvest was expected to lead to a decline in ovenbird abundance. It is not surprising that the majority of ovenbirds recorded post-harvest were observed within or near unharvested areas.
Though expected, it is critical to consider various factors that may have influenced these results. One of these factors is observer bias, which is possible given that different observers than the 2022 censuses completed the censuses in the 1980s. Observers may follow slightly different patterns in speed and navigation when following transect lines, as well as slight variations in timing (both dates and time of day). Additionally, as discussed above, a slightly altered census pattern was used in 2022 due to time constraints. This shift in methodology may have influenced results. Finally, factors outside of the study may influence ovenbird abundance. One study observed that climate change factors such as habitat degradation had a greater impact on ovenbird abundance than harvesting (Haché et al., 2016). Given these factors, we can interpret that a combination of factors, including the timber harvest, led to decreased abundance of ovenbirds within the study area at the Holt Research Forest.
While timber harvest has consistently been found to decrease ovenbird abundance, such disturbance is incredibly beneficial to other bird species. In fact, an earlier study at Holt Research Forest found that bird species with a preference for early successional, or young forest, habitat, including Eastern Wood-Pewee (Contopus virens), Winter Wren (Troglodytes troglodytes), Pine Warbler (Dendroica pinus), and White-throated Sparrow (Zonotrichia albicollis), increased in abundance following the 1987 timber harvest (Campbell, 2007). Continued research could more holistically describe how the 2020-2021 timber harvest impacted forest birds at Holt Research Forest.
References
Cambell, S.P., Witham, J.W., Hunter M.L. (2007). Long-term effects of group-selection timber harvest on abundance of forest birds. Conservation Biology, 21(5), 1218-1229. https://doi.org/10.1111/j.1523-1739.2007.00768.x
Gallo, S., Bryan, R., Mahaffey, A., Morrill, R., Morgan, D., Shultz, A., Stockwells, S., Wiley, J. (2020). Forestry for Maine Birds: A Guidebook for Foresters Managing Woodlots “With Birds in Mind.” Maine Audubon.
Lambert, J.D., Hannon, S.J. (2000). Short-Term Effects of Timber Harvest on Abundance, Territory Characteristics, and Pairing Success of Ovenbirds in Riparian Bugger Strips. The Auk, 117(3), 687-698. https://doi.org/10.1093/auk/117.3.687.
Perry, R.W., Jenkins J.M.A., Thill, R.E., Thompson, F.R. (2018). Long-term effects of different forest regeneration methods on mature forest birds. Forest Ecology and Management, 408, 183-194. https://doi.org/10.1016/j.foreco.2017.10.051.
Haché, S., Cameron, R., Villard M.A., Bayne E.M., MacLean, D.A. (2016). Demographic response of a neotropical migrant songbird to forest management and climate change scenarios. Forest Ecology and Management, 359, 309-320. https://doi.org/10.1016/j.foreco.2015.10.002.
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