| Popis: |
Grassland birds, including Bobolinks (Dolichonyx oryzivorus), have experienced steep population declines across their breeding range in North America. Because of these declines, Bobolinks were listed as threatened by the Ontario and Canadian governments. We assessed nest survival across land-cover types and uses to identify pertinent factors important for conservation of this species-at-risk. We analyzed nest survival for 463 Bobolink nests monitored over 6 years from 3 regions across southern and eastern Ontario, Canada. We used RMark to analyze daily survival rate of nests by assessing 53 models that incorporated temporal, local, and landscape variables. Daily survival rate of Bobolink nests was strongly associated with cattle (Bos taurus) stocking rate (cattle-days/ha) and date. Daily survival rate decreased across the nesting season from 0.98 (CI 95% = 0.97-0.99) to 0.92 (CI 95% = 0.87-0.95). Stocking rate was the only important local variable and was negatively associated with daily survival rate, which decreased from 0.96 (CI 95% = 0.96-0.97) to 0.69 (CI 95% = 0.51-0.82) as stocking rate increased from 0 to 243 cattle-days/ha. Landscape variables, including region, percent forest, cropland, and grassland within 2, 5, and 10 km, were not in competitive models. Our results suggest that all field uses we studied have potential for conservation actions because daily survival rate of nests did not vary significantly across late-cut hayfields, fallow fields, restored grasslands, or pastures; however, stocking rate is important in pastures. In grazed pastures where Bobolink conservation is a priority, we recommend keeping stocking rates low (≤ 40 cattle-days/ha) when most Bobolink nests are active, e.g., 21 May (earliest date of incubation) through 30 June (one week after median date of fledging for first nesting attempts) in our study area. |
