Goguen Christopher B, Mathews Nancy E
Department of Wildlife Ecology, University of Wisconsin, 1630 Linden Drive, Madison, WI 53706-1598, U.S.A.
Conserv Biol. 2000 Dec 18;14(6):1862-1869. doi: 10.1111/j.1523-1739.2000.99313.x.
We studied local patterns of Brown-headed Cowbird (Molothrus ater) abundance, parasitism rates, and nest success of a common host, the Plumbeous Vireo (Vireo plumbeus), in relation to the distribution of livestock grazing in an undeveloped region of northeastern New Mexico, 1992-1997. We predicted that both cowbird abundance and parasitism rates of vireo nests would decrease with increasing distance from active livestock grazing, and that the nesting success of vireos would increase. We measured cowbird abundance and host density and located and monitored vireo nests in pinyon-juniper and mixed-conifer habitats that ranged from actively grazed to isolated from livestock grazing by up to 12 km. Cowbird abundance declined with distance from active livestock grazing and was not related to host density or habitat type. Brood parasitism levels of vireo nests (n = 182) decreased from> 80% in actively grazed habitats to 33% in habitats that were 8-12 km from active grazing but did not vary by habitat type or distance to forest edge. Vireo nesting success was higher in mixed-conifer habitat than in pinyon-juniper but was unrelated to distance from active livestock grazing. Nest losses due to parasitism declined with distance from active livestock grazing. Our results suggest that cowbird abundance and parasitism rates of hosts may be distributed as a declining gradient based on distance from cowbird feeding sites and that isolation from feeding sites can reduce the effects of parasitism on host populations. These findings provide support for management techniques that propose to reduce local cowbird numbers and parasitism levels by manipulating the distribution of cowbird feeding sites. The presence of parasitized nests> 8 km from active livestock grazing suggests that, in some regions, management efforts may need to occur at larger scales than previously realized.
1992年至1997年，我们研究了新墨西哥州东北部一个未开发地区褐头牛鹂（Molothrus ater）的局部数量模式、寄生率以及普通宿主铅色绿鹃（Vireo plumbeus）的筑巢成功率，这些都与牲畜放牧分布有关。我们预测，随着与活跃牲畜放牧距离的增加，牛鹂数量和绿鹃巢的寄生率都会下降，而绿鹃的筑巢成功率会增加。我们测量了牛鹂数量和宿主密度，并在从积极放牧到与牲畜放牧隔离达12公里的矮松-杜松和针叶混交栖息地中定位并监测了绿鹃巢。牛鹂数量随着与活跃牲畜放牧距离的增加而下降，且与宿主密度或栖息地类型无关。绿鹃巢（n = 182）的巢寄生水平从积极放牧栖息地的>80%降至距离活跃放牧8 - 12公里栖息地的33%，但不随栖息地类型或距森林边缘的距离而变化。针叶混交栖息地中绿鹃的筑巢成功率高于矮松-杜松栖息地，但与距活跃牲畜放牧的距离无关。因寄生导致的巢损失随着与活跃牲畜放牧距离的增加而下降。我们的结果表明，基于与牛鹂觅食地的距离，牛鹂数量和宿主的寄生率可能呈递减梯度分布，且与觅食地隔离可减少寄生对宿主种群的影响。这些发现为通过控制牛鹂觅食地分布来减少当地牛鹂数量和寄生水平的管理技术提供了支持。在距离活跃牲畜放牧>8公里处存在被寄生的巢，这表明在某些地区，管理工作可能需要在比之前意识到的更大尺度上进行。
