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Nest Success and Cause-Specific Nest Failure of Grassland Passerines Breeding in Prairie Grazed by Livestock

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  • Livestock grazing is a widespread source of habitat modification, and may affect populations of ground-nesting grassland birds by influencing rates of nest failure. Nesting attempts can fail for various reasons, and determining risk of failure from specific causes associated with livestock grazing would enhance development of range management practices in areas managed for threatened grassland bird populations. Domestic livestock may influence nest failure by affecting vegetation structure, numerical or functional responses of predators, or directly by trampling nests. We hypothesized stocking rate may influence nest fate because it affects the amount and distribution of remaining vegetation, and the number of large herbivores to which nests are exposed. In 2007 and 2008, we evaluated nest fates for savannah sparrows and horned larks under 4 stocking rates experimentally applied in 40-ha paddocks in northeastern Oregon, USA. In addition to stocking rate, we evaluated variables such as vegetation structure and predator abundance and activity to help clarify mechanisms responsible for nest failure. We used a discrete competing risks framework to estimate daily probability of nest survival and failure from specific causes. These algorithms, implemented in a stand-alone graphical user interface-driven model, allow incorporation of covariates within an information theoretic approach to model inference. Although stocking rate influenced vegetation structure, the only nest failures related to stocking rate were from trampling. Trampling events were too infrequent to test for treatment effects (only 1 nest of each species), but occurred in the moderate and high stocking treatments. Additional variables were related to variation in nest failure from predation, but we found no support for the hypothesis that these causes of failure were affected by stocking rate. For savannah sparrows, daily probability of nest success (95% CI) = 0.97 (0.96–0.98); predation = 0.018 (0.008–0.028); and trampling = 0.001 (0.000–0.004). For horned larks, daily probability of nest success = 0.96 (0.95–0.98); predation = 0.029 (0.012–0.045); and trampling = 0.003 (0.000–0.007). Our results suggest grasslands managed for livestock may generally be compatible with grassland songbird conservation, at least for the species and stocking rates examined here. The most effective conservation strategies for improving nest success will involve decreasing risk of nest predation. However, we found no evidence that management of stocking rate is an effective method for doing so.
  • This is the publisher’s final pdf. The article is copyrighted by The Wildlife Society and published by John Wiley & Sons, Inc. It can be found at: http://onlinelibrary.wiley.com/journal/10.1002/%28ISSN%291937-2817
  • Keywords: savannah sparrow, stocking rate, grazing intensity, predation risk, cause-specific nest failure, rangeland management, avian demography, nest success, Pacific Northwest Bunchgrass Prairie, horned lark
  • Keywords: savannah sparrow, stocking rate, grazing intensity, predation risk, cause-specific nest failure, rangeland management, avian demography, nest success, Pacific Northwest Bunchgrass Prairie, horned lark
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  • Johnson, T. N., Kennedy, P. L. and Etterson, M. A. (2012), Nest success and cause-specific nest failure of grassland passerines breeding in prairie grazed by livestock. The Journal of Wildlife Management, 76: 1607–1616. doi: 10.1002/jwmg.437
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  • 76
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  • 8
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  • Support for this work was provided by the National Research Initiative of the USDA Cooperative State Research, Education, and Extension Service, grant 2006-35101-16572; The Nature Conservancy; Oregon State University; The Oregon Zoo; Prairie Biotic Research, Inc.; The Cornell Lab of Ornithology; The Garden Club of America; Sigma-Xi Grants-in-Aid of Research; and The Wildlife Society, Oregon Chapter.
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