The relationships between avian brood parasites and their hosts are widely recognised as model systems for studying coevolution. Through the examination of tractable interactions between one species of brood parasite and one species of host, important insights have been gained on the processes that shape and regulate the world’s biodiversity. While research has favoured the examination of simpler pairwise interactions, the examination of multispecies interactions remains rare despite most brood parasites being known to parasitise multiple species of host and hosts often subject to parasitism by multiple brood parasite species. With little attention paid to these more complex interactions, important opportunities to understand multispecies interactions go unrealised.

In this thesis, I begin by establishing the extent of the opportunities offered by avian brood parasitism as a system for studying multispecies interactions. By compiling data on all known brood parasite–host relationships, I demonstrate that complex interactions are the global norm but most research has focused on pairwise interactions, especially in regions with low brood parasite–host network complexity. I argue that despite brood parasitism’s rich research pedigree, the study of multispecies interactions represents an almost entirely new frontier for the examination of the ecology and evolution of multispecies interactions. Through the following two chapters, I demonstrate how brood parasitism can address questions on multispecies interactions which have important implications for our understanding of the natural world.

To understand why some species are targeted by brood parasites while others are not, I assemble a dataset on phenotype and life-history information for all species belonging to the passerine superradiation to examine the behavioural and ecological characteristics associated with host status. I find that brood parasites target species that, in addition to being biologically compatible, are the most conspicuous members of the community and discuss how evolutionary pressures may have facilitated convergence on the same host species by sympatric brood parasites contributing to increased network complexity.

Next, I investigate the role and evolutionary origins of the whining vocalisation which is produced in response to observing a brood parasite by a group of distantly related host species found around the world. I find that the whining vocalisation plays an important role in communicating the threat of brood parasitism between species and that the pressure imposed on hosts by multiple species of brood parasites has selected for global convergence on a functionally referential vocalisation that sympatric and allopatric hosts respond to innately but are only able to produce once learned from others. Together, these findings highlight the important role that brood parasite–host networks have had in shaping host phenotypes.

Overall, this thesis reveals avian brood parasitism to be a complex global network of interactions rather than a series of pairwise relationships as it has been traditionally treated. Furthermore, by showcasing its applications for addressing longstanding questions in evolutionary ecology, I make the case for avian brood parasitism to be looked at from new perspectives in recognition of its suitability as a model system for studying multispecies interactions.