Acoustic and physical mate guarding have different effects on intruder behaviour in a duetting songbird

When males compete for mates, they often defend paternity through mate guarding. In addition to physical guarding, in vocal species, especially duetting birds, individuals may duet with their mates in order to guard them. The acoustic mate-guarding hypothesis posits that duetting deters rivals. We experimentally tested the effectiveness of physical and acoustic mate guarding in a duetting songbird, the red-backed fairy-wren, Malurus melanocephalus, using a novel variation of a classic removal experiment. We temporarily removed males, such that females received either no guarding (mate removed), or only acoustic guarding (mate removed, his duet response played from speaker). We found that rival intrusion rates were highest when all guarding was prevented, slightly lower when only acoustic guarding occurred and lowest when pairs were unmanipulated, and both physical and acoustic guarding occurred. This suggests that both guarding techniques deter intruders, but acoustic guarding less so. Intruder display rate during removals was higher than in unmanipulated trials, regardless of acoustic guarding. Results suggest that acoustic guarding may function as a long-range signal that reduces the likelihood of rival intrusion, but physical guarding is necessary to prevent rivals from courting mates. We confirm that physical and acoustic mate guarding serve as important components of intruder deterrence, although they act at different levels. Our study broadens our understanding of multimodal paternity assurance strategies.