Data from: Incest, extra-pair paternity and territory quality drive divorce in a year-round territorial bird

Divorce can be an important behavioral strategy to improve fitness. This is particularly relevant for species that are territorial year-round with continuous partnerships, where individuals face constraints on partner choice due to limited vacancies and dispersal opportunities. We tested several hypotheses for divorce in such a species, the cooperatively breeding bird Malurus coronatus. Based on nine years of detailed information on dispersal and survival of 317 breeding pairs, we tested if divorce is driven by inbreeding avoidance, by a better partner or territory, or by social variables (number of helpers and fidelity of partners). We found that divorce is important to escape incest: incestuous pairs were substantially more likely to divorce (64%) than non-incestuous pairs (14%). However, incestuous pair bonds lasted up to a year, highlighting constraints on breeder dispersal. Non-incestuous pairs also divorced, but here the only predictor for divorce was the presence of extra-pair offspring in a previous brood. Although reproductive failure did not trigger divorce, and reproductive success did not improve in the year following divorce, females that dispersed after divorce obtained higher quality territories, unlike females that dispersed after their partner died. Thus divorce may be a strategy to improve long-term benefits associated with better territories. Some divorces appeared to be forced evictions by older females, although direct evidence for this is limited. Taken together, our findings demonstrate the complexity of factors that affect the occurrence of divorce when partner choice is constrained.

离异可作为提升演化适合度（fitness）的重要行为策略。这一结论对于全年维持领地性且保持持续伴侣关系的物种尤为适用：此类物种的个体因领地空位有限、扩散机会匮乏，在伴侣选择方面受到严格约束。我们针对这类物种中的合作繁殖鸟类（cooperatively breeding bird）华丽细尾鹩莺（Malurus coronatus），对多项离异相关假说展开了检验。
基于对317个繁殖对长达9年的扩散与存活详细追踪数据，我们验证了离异行为的驱动因素是否包括：近交回避、寻求更优质的伴侣或领地，亦或是社会因素（如协助繁殖的帮手数量与伴侣忠诚度）。
研究结果表明，离异是规避近交的重要策略：近交繁殖对的离异概率（64%）显著高于非近交繁殖对（14%）。不过近交配对的伴侣关系最长可维持一年，这凸显了繁殖者扩散所面临的现实约束。非近交繁殖对同样会发生离异，但在此类情况下，仅先前巢窝中存在婚外后代（extra-pair offspring）这一因素可预测离异行为的发生。
尽管繁殖失败并未触发离异，且离异后的次年繁殖成功率并未得到提升，但离异后扩散的雌性个体能够获得质量更高的领地，这与伴侣死亡后扩散的雌性个体形成鲜明对比。由此可见，离异或可作为一种策略，以获取与优质领地相关的长期收益。
部分离异事件似乎是由年长雌性主动驱逐所致，不过相关直接证据较为有限。
综上，我们的研究结果揭示了当伴侣选择受到约束时，影响离异发生的各类因素所具有的复杂性。