Understanding how spring phenology influences early life can provide important insights into drivers of future development and survival. We combined unique, long-term data from a bighorn sheep population and satellite-derived phenology indices to quantify the relative importance of maternal and environmental influences on milk composition and lamb overwinter survival. Based on 216 milk samples from 34 females monitored over 6 years, we found that longer snow-free and vegetation growing seasons increased milk fatty acid, iron and lactose concentrations. Structural equation modelling revealed no causality between milk energy content, lamb weaning mass and lamb overwinter survival. Our results suggest that spring conditions can affect milk energy content, but we did not detect any effect on lamb overwinter survival either directly or indirectly through lamb weaning mass. The effect of green-up date on milk composition and energy content suggests that herbivores living in seasonal environments, such as the bighorn sheep, might rely on a strategy intermediate between ‘capital’ and ‘income’ breeding when energy demands are high.

了解春季物候如何影响早期生活，可以为未来发展和生存的驱动力提供重要的见解。我们结合了大角羊种群的独特，长期数据和卫星衍生的物候指标，以量化母体和环境影响对牛奶成分和羔羊越冬存活率的相对重要性。根据在6年中监测的来自34位雌性的216份牛奶样本，我们发现更长的无雪期和植被生长期会增加牛奶中的脂肪酸，铁和乳糖浓度。结构方程模型表明，牛奶能量含量，羔羊断奶量和羔羊越冬存活率之间没有因果关系。我们的结果表明，春季条件会影响牛奶的能量含量，但是我们没有发现直接或间接通过断奶羔羊对羔羊越冬存活率的任何影响。绿化期对牛奶成分和能量含量的影响表明，生活在季节性环境中的食草动物，例如大角羊，在能量需求较高时可能依赖于“资本”繁殖与“收入”繁殖之间的策略。