Data from: Competitor phenology as a social cue in breeding site selection

1. Predicting habitat quality is a major challenge for animals selecting a breeding patch, because it affects reproductive success. Breeding site selection may be based on previous experience, or on social information from the density and success of competitors with an earlier phenology.
2. Variation in animal breeding phenology is often correlated with variation in habitat quality. Generally, animals breed earlier in high quality habitats that allow them to reach a nutritional threshold required for breeding earlier or avoid nest predation. In addition, habitat quality may affect phenological overlap between species and thereby interspecific competition. Therefore, we hypothesized that competitor breeding phenology can be used as social cue by settling migrants to locate high quality breeding sites.
3. To test this hypothesis, we experimentally advanced and delayed hatching phenology of two resident tit species on the level of study plots and studied male and female settlement patterns of migratory pied flycatchers Ficedula hypoleuca. The manipulations were assigned at random in two consecutive years, and treatments were swapped between years in sites that were used in both years.
4. In both years, males settled in equal numbers across treatments, but later arriving females avoided pairing with males in delayed phenology plots. Moreover, male pairing probability declined strongly with arrival date on the breeding grounds.
5. Our results demonstrate that competitor phenology may be used to assess habitat quality by settling migrants, but we cannot pinpoint the exact mechanism (e.g. resource quality, predation pressure, or competition) that has given rise to this pattern.
6. In addition, we show that opposing selection pressures for arrival timing may give rise to different social information availabilities between sexes. We discuss our findings in the context of climate warming, social information use, and the evolution of protandry in migratory animals.

1. 对于选择繁殖斑块（breeding patch）的动物而言，预测栖息地质量是一项核心挑战，因为栖息地质量直接影响繁殖成功率（reproductive success）。动物的繁殖位点选择可基于自身过往经验，或是基于物候更早的竞争者的种群密度与繁殖成果所传递的社会信息（social information）。
2. 动物繁殖物候（breeding phenology）的变异通常与栖息地质量的变异相关联。一般而言，动物会在优质栖息地中提前繁殖——这类栖息地可帮助动物更早达到繁殖所需的营养阈值，或是规避巢捕食（nest predation）风险。此外，栖息地质量可能会影响物种间的物候重叠（phenological overlap），进而影响种间竞争（interspecific competition）。据此，我们提出假说：前来定居的迁徙者可将竞争者的繁殖物候作为社会线索（social cue），以此定位优质繁殖位点。
3. 为验证该假说，我们在样地（study plot）尺度上对两种定居性山雀物种（resident tit species）的孵化物候（hatching phenology）进行了提前与延后的实验操控，并研究了迁徙性斑姬鹟（Ficedula hypoleuca）的雌雄定居模式。实验处理在连续两年间随机分配，且在两年均使用的研究样地中，处理组在两年间进行了轮换。
4. 两年间，各处理组的雄性定居数量均无显著差异，但晚到达的雌性会避免与孵化物候延后样地中的雄性配对。此外，雄性的配对概率会随着其抵达繁殖地的日期显著下降。
5. 我们的研究结果证实，前来定居的迁徙者可借助竞争者的物候来评估栖息地质量，但我们无法明确催生该观测模式的确切机制（例如资源质量、捕食压力或种间竞争）。
6. 此外，我们发现，针对抵达时间的反向选择压力可能会导致雌雄个体间可获取的社会信息存在差异。我们结合气候变暖（climate warming）、社会信息利用以及迁徙动物雄性先熟（protandry）的演化这一背景，对本研究的发现展开了讨论。