Patten Michael A, Shochat Eyal, Reinking Dan L, Wolfe Donald H, Sherrod Steve K
Sutton Avian Research Center, University of Oklahoma, P.O. Box 2007, Bartlesville, Oklahoma 74005, USA.
Ecol Appl. 2006 Apr;16(2):687-95. doi: 10.1890/1051-0761(2006)016[0687:helmar]2.0.co;2.
Bird populations in North America's grasslands have declined sharply in recent decades. These declines are traceable, in large part, to habitat loss, but management of tallgrass prairie also has an impact. An indirect source of decline potentially associated with management is brood parasitism by the Brown-headed Cowbird (Molothrus ater), which has had substantial negative impacts on many passerine hosts. Using a novel application of regression trees, we analyzed an extensive five-year set of nest data to test how management of tallgrass prairie affected rates of brood parasitism. We examined seven landscape features that may have been associated with parasitism: presence of edge, burning, or grazing, and distance of the nest from woody vegetation, water, roads, or fences. All five grassland passerines that we included in the analyses exhibited evidence of an edge effect: the Grasshopper Sparrow (Ammodramus savannarum), Henslow's Sparrow (A. henslowii), Dickcissel (Spiza americana), Red-winged Blackbird (Agelaius phoeniceus), and Eastern Meadowlark (Sturnella magna). The edge was represented by narrow strips of woody vegetation occurring along roadsides cut through tallgrass prairie. The sparrows avoided nesting along these woody edges, whereas the other three species experienced significantly higher (1.9-5.3x) rates of parasitism along edges than in prairie. The edge effect could be related directly to increase in parasitism rate with decreased distance from woody vegetation. After accounting for edge effect in these three species, we found evidence for significantly higher (2.5-10.5x) rates of parasitism in grazed plots, particularly those burned in spring to increase forage, than in undisturbed prairie. Regression tree analysis proved to be an important tool for hierarchically parsing various landscape features that affect parasitism rates. We conclude that, on the Great Plains, rates of brood parasitism are strongly associated with relatively recent road cuts, in that edge effects manifest themselves through the presence of trees, a novel habitat component in much of the tallgrass prairie. Grazing is also a key associate of increased parasitism. Areas managed with prescribed fire, used frequently to increase forage for grazing cattle, may experience higher rates of brood parasitism. Regardless, removing trees and shrubs along roadsides and refraining from planting them along new roads may benefit grassland birds.
近几十年来，北美洲草原上的鸟类数量急剧下降。这些下降在很大程度上可归因于栖息地丧失，但高草草原的管理也有影响。与管理可能相关的一个间接下降原因是褐头牛鹂（Molothrus ater）的巢寄生，它对许多雀形目宿主产生了重大负面影响。我们运用回归树的新应用方法，分析了一组长达五年的广泛巢穴数据，以测试高草草原的管理如何影响巢寄生率。我们研究了七个可能与寄生有关的景观特征：边缘的存在、焚烧或放牧情况，以及巢穴与木本植被、水源、道路或围栏的距离。我们分析中纳入的所有五种草原雀形目鸟类都显示出边缘效应的证据：草鹀（Ammodramus savannarum）、亨氏草鹀（A. henslowii）、迪克西雀（Spiza americana）、红翅黑鹂（Agelaius phoeniceus）和东草地鹨（Sturnella magna）。边缘由沿着穿过高草草原的路边出现的狭窄木本植被带表示。草鹀避免在这些木本边缘筑巢，而其他三个物种在边缘处的寄生率明显高于草原（1.9 - 5.3倍）。边缘效应可能直接与寄生率随着距木本植被距离的减小而增加有关。在考虑了这三个物种的边缘效应后，我们发现有证据表明，与未受干扰的草原相比，放牧地块，特别是那些春季焚烧以增加草料的地块，寄生率显著更高（2.5 - 10.5倍）。回归树分析被证明是一种重要工具，用于分层解析影响寄生率的各种景观特征。我们得出结论，在大平原上，巢寄生率与相对近期的道路砍伐密切相关，因为边缘效应通过树木的存在表现出来，树木是许多高草草原中的一种新栖息地成分。放牧也是寄生率增加的一个关键相关因素。经常用于增加放牧牛群草料的规定火烧管理区域，可能会经历更高的巢寄生率。无论如何，清除路边的树木和灌木并避免在新道路沿线种植它们可能会使草原鸟类受益。
