The difficulty of inferring process from pattern - sex ratio adaptation with helpers at the nest

The possibility that animals can adaptively adjust their offspring sex ratio has intrigued biologists since Darwin. While the population sex ratio is expected to be equal in many species, it was later also hypothesised that parents should adjust their offspring sex ratio based on their situation (e.g. parental condition, temperature, timing within the breeding season, territory quality, cooperative versus non-cooperative breeding systems). Given that so many potential mechanisms for offspring sex ratio manipulation exist, it is important to consider how to unravel the mechanism behind a sex ratio trend by looking at the produced pattern. The Seychelles warbler (Acrocephalus sechellensis) is a textbook example of a bird that is hypothesised to adaptively adjust their offspring sex ratio based on the quality of the territory they reside in. In this study we used evolutionary individual-based simulations to understand what kind of processes could cause a territory quality (or otherwise external) situation-dependent strategy to evolve. We did so by focussing on species with helpers at the nest, and used the Seychelles warbler study system as a case study. We showed that multiple mechanisms can cause the same sex ratio patterns, and hence that it is difficult to disentangle these mechanisms in empirical populations.