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. 预测栖息地质量是动物选择繁殖斑块（breeding patch）时面临的核心挑战，因其直接影响繁殖成功率。动物的繁殖位点选择可基于过往经验，或是基于物候更早的竞争者的种群密度与繁殖成功率所传递的社会信息。
2. 动物繁殖物候（breeding phenology）的变异通常与栖息地质量的变异呈显著相关。一般而言，优质栖息地可使动物更早达成繁殖所需的营养阈值，或是规避巢捕食风险，因此动物在这类生境中的繁殖时间更早。此外，栖息地质量可能会影响物种间的物候重叠，进而介导种间竞争（interspecific competition）。据此，我们提出假说：迁徙定居者可利用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，我们在研究样地（study plots）尺度上，对两种居留山雀（resident tit species）的孵化物候（hatching phenology）进行了人为提前与延后处理，并研究了迁徙斑鹟（Ficedula hypoleuca，migratory pied flycatchers）的雌雄个体定居模式（settlement patterns）。实验处理在连续两年中随机分配，且在两年均使用的研究样地中，两年的处理组进行了互换。
4. 两年的实验结果均显示，各处理组的雄鸟定居数量无显著差异，但较晚抵达繁殖地的雌鸟会避免与物候延后样地中的雄鸟配对。此外，雄鸟的配对概率（pairing probability）随其抵达繁殖地的时间显著降低。
5. 本研究结果证实，迁徙定居者可利用竞争者的物候来评估栖息地质量，但我们无法明确引发该现象的具体机制（例如资源质量、捕食压力或种间竞争）。
6. 此外，我们的研究表明，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、社会信息利用以及迁徙动物雄性先熟（protandry）的演化这一研究背景，对本研究的发现展开了讨论。