An experimental test of duet function in a fairy-wren (Malurus) with moderate cuckoldry rates

Individuals within social groups commonly combine vocal signals, forming duets. Although these displays have been described across taxa, their function is not fully understood. In birds, territory defense is a well-supported function of duetting, but additional functions are likely. Extrapair paternity (EPP) is common in birds, and males at risk of cuckoldry are expected to guard paternity in many ways, possibly including acoustic guarding with duets. Theoretical models predict species with moderate-to-high EPP rates will invest most in paternity-guarding, yet few studies have investigated duet function in such species. We conducted a dual-speaker playback experiment to test duet function in the red-backed fairy-wren, Malurus melanocephalus, which has moderately high EPP rates. Breeding groups were presented with 3 playback treatments: male solo, female solo, and duet. The territory defense hypothesis predicts pairs will respond aggressively and sing duets during duet playback in early breeding stages but predicts no same-sex bias in response to male/female songs. Acoustic paternity-guarding predicts males will duet with mates during male playback and in their mate's receptive stage, and they will bias aggression toward the speaker playing male song. Focal pairs duetted more during playback than controls and most during pre-breeding. Males and females responded more strongly to duet than solo playback, especially during pre-breeding, but neither showed same-sex response bias. Results most strongly supported the territory defense hypothesis, providing evidence that, despite cuckoldry risk, vocal duets were not used primarily for paternity-guarding. This helps us toward understanding the function of vocal displays across different mating systems.