Data from: Winter food limits timing of pre-alternate molt in a short-distance migratory bird

1. Molt is critical for fitness for many organisms for several reasons: it allows growth and maintains the function of the integument for protection, thermoregulation and communication. 2. Feather molt in birds is costly and therefore typically does not overlap with migration or reproduction. In spring, the rapid succession of pre-alternate molt, migration (if a migrant), and breeding suggests that timing of molt could constrain the initiation of breeding. A tradeoff between time spent molting and breeding might also limit molt quality. 3. The proximate basis for the timing of pre-alternate molt initiation is not well known, though it typically occurs during a resource poor time of year. Food limitation combined with fitness consequences of molting earlier suggests that plasticity in timing of pre-alternate molt in response to food abundance should be advantageous. 4. We experimentally tested, for the first time, if food abundance influences the timing of molt in the wild. We conducted a controlled food supplementation experiment on free-living swamp sparrows (Melospiza georgiana) preceding and during the time of natural pre-alternate molt (January–March 2009, 2010). 5. Supplemented birds began molting the body, face, and crown earlier than control birds (11, 14, and 8 days earlier, respectively) indicating that food abundance limits the initiation of molt. Along with interannual variation, these results indicate that photoperiod is not the sole cue for initiation of molt. 6. Both control and supplemented birds molted in sequence, starting with the body, followed by the crown 9 days later, and the face 11 days after the body. The presence of a sequence further suggests energetic limitation of molt or possibly a strategy to molt specific regions first to ensure completion or at an optimal time. 7. This study provides novel experimental evidence that food abundance can: i) limit pre-alternate molt timing and ii) limit molt timing in the wild. Food limitation of molt timing could allow earlier breeding or production of higher quality feathers, and thus cascade through other life history stages and affect reproductive success. These results indicate that food availability is a cue for molt initiation, possibly acting secondarily to photostimulation.1. 换羽（molt）对多数生物的适合度至关重要，原因涵盖多个方面：它支持机体生长，并维持体被（integument）的功能，从而实现防御、体温调节与信号交流。 2. 鸟类的羽毛换羽成本高昂，因此通常不会与迁徙或繁殖活动重叠。春季时，前交替换羽（pre-alternate molt）、迁徙（若为迁徙物种）与繁殖的时序紧密衔接，这表明换羽的时间可能会限制繁殖的启动。此外，换羽与繁殖所花费的时间之间存在权衡，这也可能制约换羽的质量。 3. 尽管前交替换羽的启动时间通常出现在一年中资源匮乏的时段，但目前人们对其启动时间的近因机制（proximate basis）仍知之甚少。食物限制加上提前换羽带来的适合度后果，意味着根据食物丰度调整前交替换羽时间的可塑性，应是具有进化优势的。 4. 我们首次通过实验检验了野生鸟类的换羽时间是否受食物丰度的影响。我们于2009年至2010年的1月至3月，也就是野生沼泽麻雀（Melospiza georgiana）自然发生前交替换羽的时段前后，对其开展了控制性食物补充实验。 5. 相较于对照组个体，食物补充组的个体开始换羽体区、面部与头顶的时间分别提前了11天、14天与8天，这表明食物丰度会限制换羽的启动时间。结合年际间的差异，上述结果说明光周期（photoperiod）并非调控换羽启动的唯一线索。 6. 对照组与食物补充组的个体均遵循统一的换羽时序：先换羽体区，9天后换羽头顶，换羽体区11天后再换羽面部。这种固定的换羽序列进一步表明，换羽过程存在能量限制，或是一种优先换羽特定体区的策略，以确保换羽完成或在最优时段完成。 7. 本研究提供了全新的实验证据，证明食物丰度可以：① 调控前交替换羽的时间；② 限制野生鸟类的换羽时间。食物对换羽时间的限制，可能使鸟类得以提前启动繁殖，或产生质量更高的羽毛，进而通过级联效应影响其他生活史阶段，并最终影响繁殖成功率。上述结果表明，食物可利用率是调控换羽启动的线索，其作用可能次要于光刺激。