Grassland bird research in a community pasture

Background: Located northwest of Orangeville, Ontario, the Grey Dufferin Community Pasture is an impressive, nearly 600-acre grassland. Each spring and summer, the pasture supports approximately 600 beef cattle from local farms through rotational grazing. This expansive agricultural grassland also provides a significant amount of wildlife habitat.

We studied three species of grassland birds that nest in the community pasture: Bobolink, Eastern Meadowlark, and Savannah Sparrow. All three species have steeply-declining populations in Ontario and rely on agricultural grasslands, including pastures, for nesting habitat. Unfortunately, grazing during the nesting season can have unintended consequences on ground-nesting birds. Nests can be impacted directly, when trampled by cattle, or indirectly, when the removal of vegetation causes exposure to predators. Grazing can also result in the displacement of birds from their breeding territories if vegetation is no longer suitable for nesting.

Project work: Our objectives were to assess (1) the distribution and abundance of Bobolink, Eastern Meadowlark, and Savannah Sparrow and (2) how cattle grazing impacted abundance and nest success.

In 2019, we used transect surveys to collect preliminary data on grassland bird species distribution and abundance before grazing began and in the middle of the nesting season, once many fields had been grazed. Following the last round of transect surveys in June, we used spot mapping to assess breeding in a sample of Bobolink territories in grazed and ungrazed fields. We also collected vegetation data in June to assess characteristics such as height, density, and ground cover composition to better understand the relationship between vegetation characteristics and field use by the birds before and after grazing occurs.

In 2020, we repeated transect surveys, Bobolink spot mapping, and vegetation surveys. Additionally, we monitored Eastern Meadowlarks and Savannah Sparrows. To monitor Eastern Meadowlarks, we used spot mapping and nest monitoring, visiting each of the ~20 territories across the pasture once per week throughout the breeding season to record behaviour and nesting status. For Savannah Sparrows, we spot mapped a sample of territories in mid-June in grazed and ungrazed fields and monitored nests we located opportunistically.

Overall, in both years, few Bobolinks remained in fields after the first grazing occasion and those that stayed were in fields where vegetation was not grazed as heavily as fields where Bobolinks dispersed. In contrast, the number of Eastern Meadowlark territories in 2020 did not change due to grazing. Nearly all territories remained throughout the breeding season and pairs attempted to renest after nest failures due to grazing or predation. Savannah Sparrows also continued to be abundant across the pasture after grazing occurred.

Conservation implications: Rotational grazing of cattle provides some flexibility in the timing and duration of grazing that may enable strategic management of fields to benefit nesting birds. At the community pasture, Eastern Meadowlark territories were distributed fairly evenly across the pasture; whereas, the density of Bobolink territories was higher in some areas than others. Therefore, conservation actions targeting areas with high Bobolink abundance should benefit both species at risk, along with other species that nest in the pasture (e.g., Savannah Sparrow).

Based on our observations, we recommended three conservation strategies to pasture managers. Providing a nesting refuge where no grazing occurs until ideally after mid-July in one or more fields with high abundance of nesting grassland birds is an effective conservation strategy. This strategy can be implemented by placing these fields last in the order of grazing rotation to delay grazing as long as possible, potentially giving birds time to fledge young from nests before grazing occurs. If grazing cannot be delayed long enough, the next best option may be to lightly graze fields with many nesting grassland birds in spring, leaving enough vegetation for Bobolinks to renest. This second strategy is only effective if light spring grazing is followed by a ~6-week rest period for successful renesting attempts before a second grazing occasion. Lastly, lengthening the rest period between grazing occasions to ≥ 6 weeks in other target fields may enable more successful renesting. This third strategy is more likely to benefit Eastern Meadowlarks and Savannah Sparrows because Bobolinks frequently disperse after the first grazing occasion, unless grazing is light enough to leave sufficient vegetation.

Project dates: 2019 – 2020

Funding: This project has received funding support from the Government of Ontario. Such support does not indicate endorsement by the Government of Ontario of the contents of this material. Additional support provided by Colleges and Institutes Canada, The McLean Foundation, ECO Canada, the Helen McCrea Peacock Foundation, and individual donors.

Campomizzi, A. J., and Z. M. Lebrun-Southcott. 2022. Impact of grazing and conservation opportunities for nesting grassland birds in a community pasture. Canadian Field-Naturalist 136:217–227. https://doi.org/10.22621/cfn.v136i3.2775