Group-living species show a diversity of social organization, from simple mated pairs to complex communities of interdependent individuals performing specialized tasks. The advantages of living in cooperative groups are well understood, but why some species breed in small aggregations while others evolve large, complex groups with clearly divided roles is unclear. We address this problem by reconstructing the evolutionary pathways to cooperative breeding across 4,730 bird species. We show that differences in the way groups form at the origin of cooperative breeding predicts the level of group complexity that emerges. Groups that originate through the retention of offspring have a clear reproductive divide with distinct breeder and helper roles. This is associated with reproductive specialization, where breeders invest more in fecundity and less in care. In contrast, groups formed through the aggregation of unrelated adults are smaller and lack specialization. These results help explain why some species have not transitioned beyond simple groups while others have taken the pathway to increased group complexity.

群居物种表现出多样化的社会组织，从简单的配对到复杂的相互依赖的个体执行专门任务的社区。生活在合作群体中的好处是众所周知的，但为什么一些物种在小群中繁殖，而另一些物种则进化出具有明确划分作用的大型复杂群体，这一点尚不清楚。我们通过重建 4,730 种鸟类合作繁殖的进化途径来解决这个问题。我们表明，在合作育种起源的群体形成方式的差异可以预测出现的群体复杂性水平。通过保留后代而产生的群体具有明显的繁殖鸿沟，具有不同的育种者和帮助者角色。这与生殖专业化有关，育种者在繁殖力上投入更多，在护理上投入更少。相比之下，通过不相关的成年人聚集形成的群体较小，缺乏专业化。这些结果有助于解释为什么有些物种没有超越简单的群体，而另一些物种却采取了增加群体复杂性的途径。