Nest Success of the Yellow Warblers (Dendroica petechia) and Willow Flycatchers (Epidonax traillii) in Relation to Predator Activity in Montane Meadows of the Central Sierra Nevada, CA.

MASTERS THESIS: Willow flycatchers (Empidonax traillii) and yellow warblers (Dendroica petechia) are both riparian-dependent neotropical migrant songbirds that have declined throughout much of their former range in California. Population declines have been primarily associated with the loss of riparian breeding habitat; increases in brood parasitism and nest predation have also contributed to declines and may be limiting population growth. IN this study, I determined the effect of potential nest predator activity on the nest success of both species, and identified potential willow flycatcher nest predators using yellow warbler nests as a surrogate. I used track plate transects, point counts, and time-constrained visual encounter surveys to assess potential mammalian, avian, and reptilian nest predator activity, respectively. I used simple linear regression and multiple linear regression analyses to determine the relationship between nest predator activity and nest success. Nest predation was the major cause of nest failure in both species. Only a small subset of the potential nest predator species detected in the meadows had activity indidices that were associated with the nest success of either species and include short-tailed weasel, Douglas squirrel, brown-headed cowbird, Clark’s nutcracker, and Cooper’s hawk. The magnitude of the effects of the different predator species on nest success varied depending on the prey species and the stage in the nesting cycle. Nest location relative to the forest edge and the amount of the meadow covered with water also influenced nest success. Short-tailed weasel, Douglas squirrel, lodgepole chipmunk, deer mouse and unidentified chipmunk species were photographed depredating yellow warbler nests baited with zebra finch eggs. Management recommendations include the active flooding of portions of the meadows to restrict meadow access to forest edge associated nest predators and habitat restoration in areas of meadows that avoid the creation of ‘ecological traps’ should be employed to insure the persistence of high quality breeding habitat for willow flycatchers and yellow warblers in the central Sierra Nevada.

硕士学位论文：柳纹霸鹟（Willow flycatcher）与黄林莺（Yellow warbler）均为依赖河岸生境的新热带迁徙鸣禽，二者在加利福尼亚州的大部分原有分布区内种群数量均出现下降。种群衰退主要与河岸繁殖栖息地丧失相关；巢寄生率上升以及巢捕食事件增加同样加剧了种群下降，并可能限制种群增长。本研究旨在明确潜在巢捕食者的活动对两种鸟类巢成功率的影响，并以黄林莺的巢作为替代样本，识别柳纹霸鹟的潜在巢捕食者。研究分别采用轨迹板踏查样带、点计数法以及受时间限制的视觉偶遇调查法，依次评估潜在的哺乳类、鸟类与爬行类巢捕食者的活动情况。本研究通过简单线性回归与多元线性回归分析，探究巢捕食者活动与巢成功率之间的关联。巢捕食是两种鸟类巢失败的主要原因。本次研究在草甸中检测到的潜在巢捕食者物种中，仅有一小部分的活动指数与任一鸟类的巢成功率存在关联，包括短尾鼬（short-tailed weasel）、道格拉斯松鼠（Douglas squirrel）、棕头牛鹂（brown-headed cowbird）、克拉克星鸦（Clark's nutcracker）以及库珀鹰（Cooper's hawk）。不同捕食者物种对巢成功率的影响强度因猎物物种以及筑巢周期的阶段而异。巢相对于林缘的位置以及草甸的积水面积同样会对巢成功率产生影响。研究拍摄到短尾鼬、道格拉斯松鼠、黑松花鼠（lodgepole chipmunk）、鹿鼠（deer mouse）以及未明确分类的花鼠物种盗食了植入斑胸草雀（zebra finch）卵的黄林莺巢。本研究提出的管理建议包括：对部分草甸进行主动淹水，以限制与林缘相关的巢捕食者进入草甸；同时应在草甸区域开展栖息地修复工作，避免形成‘生态陷阱’，以保障内华达山脉中部地区柳纹霸鹟与黄林莺高质量繁殖栖息地的存续。