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.

配偶离异是提升进化适合度的重要行为策略。这一点对于全年占据领地且维持长期配偶关系的物种尤为关键，这类物种的个体因领地空缺有限、扩散机会匮乏而面临配偶选择受限的困境。
我们以合作繁殖鸟类紫冠细尾鹩莺（Malurus coronatus）为研究对象，针对该类物种的配偶离异现象检验了多项假说。基于9年间对317个繁殖对的扩散与存活情况的详细观测数据，我们验证了三类可能驱动配偶离异的因素：规避近交、寻找更优质的配偶或领地，以及社会变量（协助者数量与配偶忠贞度）。
研究结果表明，配偶离异是规避近交的重要手段：近交繁殖对的离异率（64%）显著高于非近交繁殖对（14%）。但近交配偶关系最长可维持一年，这凸显了繁殖个体扩散所受的限制。
非近交繁殖对同样会发生离异，但此类离异的唯一预测因子是前一繁殖巢中存在配偶外后代。尽管繁殖失败并不会诱发离异，且离异后次年的繁殖成功率也未得到提升，但与因配偶死亡而扩散的雌性个体不同，因离异而扩散的雌性能够获得质量更优的领地。
由此可见，配偶离异或许是一种通过获取更优质领地以提升长期适合度收益的策略。部分离异事件似乎是由年长雌性主动驱逐配偶所致，不过相关直接证据较为有限。
综合来看，当配偶选择受限时，影响配偶离异发生的因素具有复杂性，本研究结果清晰展现了这一点。