Influence of prescribed fire history on habitat and abundance of passerine birds in northern mixed-grass prairie

Abstract

To more effectively manage remaining native grasslands and declining populations of prairie passerine birds, linkages between disturbance regimes, vegetation, and bird abundance need to be more fully understood. Therefore, we examined bird-habitat relationships on mixed-grass prairie at Lostwood National Wildlife Refuge (NWR) in northwestern North Dakota, where prescribed fire has been used as a habitat management tool since the 1970s. We sampled bird abundance on upland prairie at 310 point count locations during 1993 and 1994 breeding seasons. We also measured vegetation structure and composition at each location. Complete fire histories were available for each point, with over 80% having been burned one to four times in the previous 15 years Post-fire succession generally transformed vegetation structure from short, sparse, and grassy with few forbs and low litter immediately after fire, to increasing and moderate amounts of forbs, litter, and shrubs two to eight years postfire, to tall, dense, shrubby prairie with little forb, grass, or litter understory when fire was absent (> 80 years). Most grassland birds (six of nine species examined) at Lostwood NWR were absent from prairie untreated with fire. Species richness and abundances of Baird's Sparrows (Ammodramus bairdii), Bobolinks (Dolichonyx oryzivorus), Grasshopper Sparrows (A. savannarum), Le Conte's Sparrows (A. leconteii), Sprague's Pipits (Anthus spragueii), and Western Meadowlarks (Sturnella neglecta) were positively related to an index of amount of fire, and these species were absent from unburned units. In contrast, Common Yellowthroats (Geothlypis trichas) and Clay-colored Sparrows (Spizella pallida) both reached highest abundance on unburned prairie. To provide maximum grassland bird diversity, managers of mesic, mixed-grass prairie generally should provide areas with short (2-4 year), moderate (5-7 year), and long (8-10 year, or more) fire return intervals. Because long-term rest may create habitat unfavorable for most species of grassland passerines in mesic, northern mixed prairie, periodic defoliations by disturbances such as fire should be considered essential to restore and maintain native biodiversity.