Song length competence: evidence for a new fitness indicator in birdsong

In animal displays, motor performance can provide key information to recipients associated with whole-organism condition. It has been argued that biomechanical processes constrain motor displays and performing near the phenotypic boundary can be an honest indicator of quality. Using data from 140 blue tits Cyanistes caeruleus, including male and female songs, we found a new phenotypic boundary defined by a trade-off between song length and note length. Male "song length competence" - a composite metric considering the trade-off with note length - peaked seasonally during the female fertile period and was positively associated with reproductive success. These findings support song length competence as a fitness indicator in blue tits. Recovery competence, a well-known composite metric assessing the efficacy to produce consecutive notes, showed similar seasonal and reproductive variation but with a much smaller impact on reproductive success. We further confirmed previously known trade-offs between 2) frequency modulation vs. note length and 3) frequency jumps vs. inter-note gaps between notes, but these metrics seemed biologically irrelevant with no significant variation in relation to season or individual fitness. We encourage further research on birdsong and other animal displays to shed light on the universality of these principles in the assessment of motor performance.