Higher song complexity and intruder pressure at dawn in a vocally complex songbird

Dawn is a critical period for vocal displays in songbirds. At dawn, songbirds intensify their vocalisations, for example increasing their singing rates in response to increased intruder pressure at this time, and for mate guarding. However, little is known about diurnal variations in song complexity, a sexually selected vocal trait associated with mate choice and territorial defence. We compared song complexity in 17 territorial male Tui (Prosthemadera novaeseelandiae) between dawn, solar noon and dusk. In addition, we investigated whether song complexity correlated directly with rival intrusion rate. Our results indicated that song complexity and intrusion rate were significantly higher at dawn than dusk. Analyses also suggested that higher entropy (spectral complexity) and syllable rate, in addition to shorter song duration, were associated with reduced intrusion rate into the singer's territory. Males with higher song complexity may be perceived as stronger competitors and therefore would attract lower rival intrusion. Our study presents evidence of both diurnal changes in Tui song complexity and associations between song complexity and intrusion rate. These findings support the theory that dawn is important for songbird vocal communication in terms of territory assertion.