Vocal and non-vocal behavior interact differently in territorial strategies of two sympatric Rallidae species

Territorial interactions between animals involve correlated signaling and direct actions, yet different species vary in how they utilize each component. In theory, opponents should balance costs and benefits of territorial interactions, and restrict their conflicts to signaling when physical interactions are likely to escalate to serious injuries. We tested these predictions by simulating territorial intrusions in two sympatric non-passerine bird species: the Water Rail (Rallus aquaticus) and Little Crake (Zapornia parva). These species differ physically and behaviorally, with the former being larger and more aggressive, and known to cause serious or fatal injury to other birds. We measured vocal signals and approach behavior of each species towards conspecific and heterospecific playbacks (Little GrebeTachybaptus ruficollis). Both species increased their calling rate in response to their conspecific treatments; however, Water Rails produced louder call variants, decreased the fundamental frequency of their calls, and produced more duets. In contrast, Little Crakes did not modify the acoustic structure of their calls and rarely participated in duetting. In addition to differences in vocal behavior, Water Rails approached the speaker exceptionally, whereas Little Crakes did it regularly. We conclude that while settling territorial conflicts, Water Rails utilized a purely signaling strategy involving reliable vocal signals and thus the avoidance of direct actions, whereas Little Crakes relied primarily on direct actions.