Resource allocation effects on the timing of reproduction in an avian habitat specialist

Variation in nutrient allocation can influence the timing of breeding and
ultimately reproductive output. Time and space constraints might exist,
however, if fewer food resources are available to meet the costs of
reproduction early during the reproductive season. Here, for the first
time, we test whether nutrient allocation strategies for reproduction in a
shrub-dependent avian species differs with timing of breeding in different
ecoregions: a high-elevation landscape, containing spatially complex
vegetation (Rocky Mountains) versus a low-elevation, more homogenous
landscape (Great Plains). We analyzed data collected from radio-telemetry
and stable isotopes to assess the degree to which endogenous (body)
reserves are used for reproduction and whether variation in allocation
strategies was associated with time of year, ecoregion, habitat quality
(including sagebrush type and plant greenness), or maternal
characteristics. Using a Bayesian statistical framework, we found that
females relied on a similar amount of endogenous reserves for reproduction
in first nesting and renesting attempts. Additionally, endogenous
contributions declined more rapidly throughout the nesting season in the
Rocky Mountains than in the Great Plains. Individuals in high- and
intermediate-elevation sagebrush types in the Rocky Mountains used similar
amounts of endogenous reserves, whereas females nesting in low-elevation
sagebrush used less. Females nesting at intermediate elevations, which
experience the greatest flush of new green vegetation during the nesting
season, switched their reliance from endogenous to exogenous sources for
reproduction as green vegetation became available during spring. Our study
highlights adaptations of a nutrient-allocation strategy across areas with
varying levels of resources in time and space in a habitat specialist
bird. Nutrient allocation by individuals residing in high-elevation areas
favors a strategy that mainly uses nutrients gained from wintering
habitats, whereas individuals residing in low-elevation areas mainly use
exogenous sources for reproduction.

营养分配的差异会影响繁殖时机，并最终影响繁殖产出。然而，若繁殖季节早期可用于承担繁殖成本的食物资源较少，则可能存在时间与空间上的限制。本研究首次检验了依赖灌木生存的鸟类在不同生态区（高海拔、植被空间结构复杂的落基山脉 vs 低海拔、景观更均一的大平原）中，其繁殖营养分配策略是否随繁殖时机而变化。我们分析了无线电遥测（radio-telemetry）和稳定同位素（stable isotopes）数据，以评估内源性（身体）储备用于繁殖的程度，以及分配策略的差异是否与季节、生态区、栖息地质量（包括蒿属灌木类型和植物绿度）或母体特征相关。通过贝叶斯统计框架（Bayesian statistical framework），我们发现雌性在首次筑巢和重新筑巢尝试中，用于繁殖的内源性储备量相似。此外，整个筑巢季节内，落基山脉中内源性贡献的下降速度快于大平原。落基山脉中高海拔和中海拔蒿属灌木类型生境中的个体使用的内源性储备量相似，而在低海拔蒿属灌木生境中筑巢的雌性使用量更少。在中海拔筑巢的雌性（筑巢季节新绿植被生长最旺盛），随着春季绿植被的出现，其繁殖依赖从内源性转向外源性资源。本研究揭示了栖息地专性鸟类（habitat specialist bird）在资源时空分布异质的区域中，其营养分配策略的适应性。高海拔地区个体的营养分配倾向于主要利用越冬栖息地获取的营养，而低海拔地区个体则主要使用外源性资源进行繁殖。