Clark's nutcracker (Nucifraga columbiana) functions as a keystone seed disperser and ecological mobile link for many western conifers. The bird is the primary seed disperser for limber pine (Pinus flexilis), which is an important seed resource for the bird. In the Southern Rocky Mountains, annual variation in limber pine cone production and growing threats, such as exotic disease, require that nutcrackers rely on additional conifer seed resources. We investigated the utilization of limber pine, ponderosa pine (Pinus ponderosa), and Douglas‐fir (Pseudotsuga menziesii) as seed resources by nutcrackers to determine how temporal variation in seed production and maturation drives nutcracker seed resource and habitat use decisions. Working in Rocky Mountain National Park (RMNP) during three field seasons, we examined (1) cone production in limber pine, ponderosa pine, and Douglas‐fir, and seed energy availability across the RMNP landscape; (2) timing and likelihood of nutcracker seed harvest and caching behavior for each conifer species; and (3) predictors of nutcracker visitation to the different forest stand types. Each year starting in mid‐ to late August, nutcrackers foraged on limber pine seeds. In 2014 and 2015, nutcrackers transitioned from limber pine to harvesting ponderosa pine seeds. In 2016, a year of low ponderosa pine cone production but exceptionally high Douglas‐fir cone production, they transitioned from limber pine to Douglas‐fir seeds. Cone density was a significant predictor of nutcracker counts in every negative binomial regression model in which it was used. However, the best overall model, which included interaction effects, indicated that stand type, year, and month were better predictors. We interpret these three predictors as reflecting temporal variation in patterns of cone production and maturation among the three conifers but also in habitat use by nutcrackers. In all three years of study, limber pine alone did not provide sufficient energy resources for foraging and caching, and nutcrackers used an additional conifer seed resource. With potential future limber pine losses, these other seed resources may be essential to sustain a nutcracker population. We suggest that Douglas‐fir and especially ponderosa pine will serve as increasingly important food resources as limber pine declines across the montane West.

中文翻译：

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克拉克胡桃夹子，针叶林的关键种子分散剂，种子资源利用的时间和能量驱动力

克拉克的胡桃夹子（Nucifraga columbiana）是许多西方针叶树的基石种子分散剂和生态活动的纽带。鸟类是弯角松（Pinus flexilis）的主要种子分散剂，它是鸟类的重要种子资源。在南部的落基山脉，每年松果产量的变化和诸如外来疾病等日益严重的威胁都要求胡桃夹子依赖其他针叶树种子资源。我们调查了边角松，黄松（Pinus tankerosa）和花旗松（Pseudotsuga menziesii）的利用）作为胡桃夹子的种子资源，以确定种子生产和成熟过程中的时间变化如何驱动胡桃夹子种子资源和生境使用决策。我们在三个野外季节在落基山国家公园（RMNP）工作，研究了（1）弯松，黄松和道格拉斯冷杉的视锥细胞产量，以及整个RMNP景观中的种子能量可用性；（2）每种针叶树种胡桃夹子种子收获的时间和可能性以及缓存行为；（3）胡桃夹子造访不同林分类型的预测因子。从每年的八月中下旬开始，胡桃夹子就在松木种子上觅食。在2014年和2015年，胡桃夹子从阔叶松转变为收获美国黄松种子。在2016年，黄松松果产量低，但道格拉斯松果产量异常高，他们从松木过渡到花旗松种子。在每个使用负二项式回归模型中，锥体密度是胡桃夹子数量的重要预测指标。但是，最好的总体模型（包括交互作用）表明，林分类型，年份和月份是更好的预测指标。我们将这三个预测因子解释为反映了三个针叶树的视锥细胞生产和成熟模式的时间变化，以及胡桃夹子在栖息地的使用情况。在所有三年的研究中，仅角柏松并不能提供足够的能源用于觅食和缓存，而胡桃夹子使用了额外的针叶树种子资源。随着未来潜在的松木损失，这些其他种子资源对于维持胡桃夹子种群可能至关重要。