Differential effects of policy-based management on obligate and facultative grassland birds.
Policy-driven conservation on private lands, such as the U.S. Farm Bill's Conservation Reserve Program (CRP), provides crucial habitat for consistently declining populations of grassland birds. In 2004, new CRP enrollments were required to be managed to floristically diversify old grass-dominated fields and provide early-successional habitat. Obligate and facultative grassland birds of varying conservation importance, along with the entire avian community, has received limited research attention within this applied management context. Moreover, multiple parameters (e.g. nest survival and adult density) - opposed to single parameter studies - need to be investigated simultaneously to better understand drivers of avian population dynamics associated with management. Using a combination of graphical and model selection approaches we addressed these gaps focusing on four main questions: (1) How does abundance and richness of various grassland bird community designations respond to broad (e.g. grass type) and specific (control, disking, herbicidal spray, and spray/interseeding) management treatments?; (2) How do grassland birds of conservation concern and economic importance collectively respond to management?; (3) Are various response parameters, including nesting parameters, consistent or decoupled across facultative and obligate species?; and (4) Is there a best management treatment to recommend? Overall, compared to non-native grass (smooth brome [Bromus inermis]) dominated fields, native fields had a greater number of avian species and number of individuals with higher and lower cumulative management, respectively. Facultative grassland species were mostly driving this trend and likely utilizing native fields that tended to have more vertical vegetation structure and forb diversity compared to brome fields. Grassland obligates with higher Conservation Values, such as bobolinks (Dolichonyx oryzivorus), however, were most abundant and species-rich on unmanaged brome fields compared to native fields. Response parameter decoupling was not readily apparent from our comprehensive modeling analysis of both adult survey and nesting data; however, nest survival was generally higher on native fields for eastern meadowlarks (Sturnella magna), an obligate, yet adult densities for meadowlarks were greater on unmanaged brome fields. Overall, pinpointing a specific mid-contract management practice for CRP that has benefits for all grassland birds remains challenging due to species-specific habitat requirements. However, it appeared the moderate effects of light strip-disking on vegetation may present a good management option for landowners because this treatment may be beneficial across multiple species of grassland birds and would also limit the impact of management to obligates utilizing unmanaged brome fields while also improving the avian Conservation Value of native fields.