A new fitness indicator in birdsong: a trade-off between song length and note length

Abstract

In animal displays, motor performance conveys key information on whole-organism condition. Biomechanical constraints can shape the evolution of motor displays and explain their role in animal communication. Using song recordings of 140 colour-ringed male and female blue tits (Cyanistes caeruleus), we describe a trade-off between song length and note length that defines an upper boundary, possibly arising from respiratory constraints. We measured how close each song lies to this boundary and named this metric song effort score. In males, song effort score peaked during the fertile period and was positively associated with clutch size, describing individual difference in vocal performance relevant to fitness. Recovery score, the analogous metric for the trade-off between note length and gap length, also peaked during the fertile period but was not associated with clutch size. Voiced and unvoiced modulation scores, measures of frequency modulation performance in relation to boundary trade-offs (i.e., vocal deviation), declined or did not vary over the season and were unrelated to clutch size. Across behavioural contexts, song effort score was lower during singing duels and higher during alarm, whereas trills were faster in both contexts, supporting trill rate as an indicator of motivation or arousal. Describing how display components constrain each other can improve the quantification of the individual variation on which selection acts. Ultimately, this approach can help unify our understanding of the role of motor performance in animal communication across display modalities and taxa.