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Book preview: Surveys for grassland birds of the Malta Field Office-BLM, including a seven-year study in north Valley County (Volume 2008) by P.(Paul) Hendricks

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Author: P.(Paul) Hendricks
Title: Surveys for grassland birds of the Malta Field Office-BLM, including a seven-year study in north Valley County (Volume 2008)
Publisher: Helena, Mont. : Montana Natural Heritage Program
Subject (keywords, tags): Birds; Bird populations; Bird populations; Bird populations; Grasslands; Rare birds; Habitat surveys


"April 2008"-cover date
Appendix A, Global and state rank definitions -- Appendix B, Summary of point-count surveys in northern Blaine and Phillips counties during 2007 -- Appendix C, Predictive distribution models for grassland birds
Includes bibliographical references (p. 24-26)
Grassland-associated birds have exhibited thesteepest population declines of any suite of bird species in North America over the past several decades, primarily due to loss of habitat resulting from conversion of native prairie to agricultural production. To better understand the relationships of prairie vegetation structure with presence and relative abundance of native prairie bird species, fixed-radius point counts were randomly placed across BLM lands in north Valley County in areas with native grassland cover. Our objective was to gather habitat information to help guide management of grasslands for a variety of species, including a suite of grassland birds that are of conservation concern. The project evolved into a multi-year inventory (2001-2007). No other project focused on grassland birds in Montana has gathered consistent data at the same locations for this length of time. More than 75 species of birds were recorded on 1410 point-counts (189 - 207 points each year) in north Valley County. Twenty species were recorded on at least one point count every year, nine of which are Montana Species of Concern (SOC): Long-billed Curlew (Numenius americanus), Sprague?s Pipit (Anthus spragueii), Brewer?s Sparrow (Spizella breweri), Lark Bunting (Calamospiza melanocorys), Grasshopper Sparrow (Ammodramus savannarum), Baird?s Sparrow (Ammodramus bairdii), McCown?s Longspur (Calcarius mccownii), Chestnut-collared Longspur (Calcarius ornatus), and Bobolink (Dolichonyx oryzivorous). Chestnut-collared Longspur occurred on the greatest percentage (79.7-89.9%) of our point counts. Next in order of relative abundance were Horned Lark, Western Meadowlark, Sprague?s Pipit, and Baird?s Sparrow. Each of these species was detected every year on at least 30% of our count points. The results of our seven-year study revealed considerable variation in precipitation and vegetation conditions in north Valley County that influenced where birds settled during the breeding season. For the seven years combined, Long-billed Curlew and McCown?s Longspur occupied sites with shorter and less dense grasslands, while Grasshopper Sparrow, Baird?s Sparrow, and Bobolink favored taller and denser grassland patches. Chestnut-collared Longspur and Sprague?s Pipit occupied points with intermediate vegetation height and density near-the ground. Lark Bunting tended to occur on sites where vegetation was moderately taller but of equally low near-ground density as Long-billed Curlew and McCown?s Longspur. Brewer?s Sparrow was most dissimilar from the other species, occupying points with tall vegetation but relatively low near-ground density. April-May precipitation varied among the seven years nearly five-fold, with extremes in 2002 and 2007, respectively, and mean vegetation height and density tended to correspond accordingly. April-May precipitation accounted for 77% of variation in mean vegetation height among years on our points. Likewise, we found April and May precipitation a good predictor of the percentage of point counts on which some SOC birds would occur. Greater numbers of Baird?s Sparrow, Chestnut-collared Longspur, and Bobolink tended to occur in years with greater spring precipitation, while Long-billed Curlew showed a negative response to increased spring precipitation, an expected pattern because this species selects sites with low-stature grass for nesting. Within-year differences in vegetation structure between occupied and unoccupied points were not consistently strong across years for any species except Baird?s Sparrow, which occupied points with taller and denser near-ground vegetation than was available where it was absent. Grasshopper Sparrow occupied points each year with taller or denser vegetation, but not always both. Each of the other SOC birds occupied points in some years that did not differ from unoccupied points in either measure of vegetation structure. That all of these bird species co-occurred every year, sometimes in substantial numbers and despite large differences in preferred vegetation structure, indicates that current management is maintaining a mosaic of vegetation conditions at large enough landscape scales to retain what appears to be a complete or nearly complete mixed-grass prairie bird community. Surveys conducted during 2007 in nearby northern Blaine and Phillips counties found the same SOC birds in roughly the same proportions, with the exception of Baird?s Sparrow, which was less abundant, and McCown?s Longspur, which was more abundant (reflecting shorter and less-dense vegetation in Blaine and Phillips counties). This suggests that a large region of north-central Montana continues to support a diverse grassland b

Contributor: Montana State Library
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