The relative importance of predators and resources (i.e. food) for the dynamics of migratory bird populations is poorly known. Resource availability may be more likely in resource poor environments, but given that nest failure in most systems is due mainly to predation, predator effects may predominate. We document a rapid decline of an isolated eastern kingbird Tyrannus tyrannus population breeding in the Great Basin Desert of eastern Oregon, USA, and evaluate whether it was driven by limited food resources (water availability ~ food), nest predation, or first‐year or adult return rate (RR_J and RR_A, respectively) that reflect nonbreeding season events. Most nests failed (~68% of nests) due mainly to nest predation (> 90% of failures); nestling starvation was rare. Bioyear precipitation (October–April), breeding season precipitation, and river flow all varied widely but none could account for annual variation in either nest success (NS) or fledging success of successful nest (FSSN). Neither RR_J nor RR_A varied with year, any measure of water resources, or reproductive success. Annual population growth rate (ln[N_t+1/N_t]) was independent of year, all measures of water availability, RR_J and RR_A, and NS of replacement nests, but was high following years of high NS of initial nests; FSSN was also higher in years of high NS. High rates of nest predation, due likely to the expansion of the local American crow Corvus brachyrhynchos population, appeared to drive the population decline. Empirical data and population simulations indicated that replacement nests contributed little to λ, while simulations revealed that, given site‐specific vital rates, the decline would have been even greater without immigration (λ = 0.812) than the observed (λ = 0.914). Long distance dispersal may thus be critical for persistence of kingbirds in the naturally highly fragmented habitat that it occupies in the Great Basin Desert.

中文翻译：

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长途候鸟雀形目的种群数量在其范围的边缘下降：筑巢捕食，筑巢替换和迁徙

捕食者和资源（即食物）对候鸟种群动态的相对重要性知之甚少。资源可用性在资源贫乏的环境中更有可能出现，但是鉴于大多数系统中的嵌套故障主要是由于掠食造成的，因此掠食者的影响可能占主导地位。我们记录一个孤立的东部必胜鸟的迅速下降泰兰努斯泰兰努斯人口在东部俄勒冈州，美国的大盆地沙漠繁殖，并评估是否由有限的食物资源（水供应〜食物），巢捕食，或第一年或驱动成人回报率（RR _J和RR _A，分别反映非育种季节的事件。大多数巢失败（约占巢的68％），主要是由于巢捕食（失败的占90％）；雏鸟饥饿很少见。生物年降水（10月至4月），繁殖季节降水和河流流量变化很大，但没有一个可以解释成功成巢（NS）或成功成巢成功（FSSN）的年度变化。RR _J和RR _A都不会随年份，水资源的任何度量或生殖成功而变化。年人口增长率（ln [N _{t + 1} / N _t ]）与年份无关，所有可用水量度，RR _J和RR _A，和替换巢的NS，但在最初的巢的高NS数年之后，该值较高；在高NS的年份中，FSSN也较高。巢穴捕食率高，可能是由于当地美洲乌鸦乌鸦座短尾hy种群的增加，似乎推动了种群的减少。经验数据和种群模拟表明，替代巢对λ的贡献很小，而模拟显示，在给定地点的特定生命率的情况下，如果没有移民（λ= 0.812），则下降幅度甚至比观察到的（λ= 0.914）更大。因此，长距离散布对于必胜鸟在大盆地沙漠中自然高度分散的栖息地的持续生存可能至关重要。