Sierro, J. | bioRxiv Science

bioRxiv2025

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.

animal behavior and cognition↗

bioRxiv2021

Frequent female song in blue tits: behavioural context suggests a role in intrasexual competition

The blue tit (Cyanistes caeruleus) is an important avian model in evolutionary ecology (> 20,000 published scientific studies). Song, like in other songbird species, is generally described as a male trait and plays an important role in mate attraction and territory defence. Over the decades, there have been recurring reports of anecdotal female song but these have not led to any quantitative study of female song in blue tits. Our systematic sampling over three years revealed prolific female singing in a northern population of colour ringed blue tits. Daytime singing of females occurred throughout the breeding season during agonistic interactions, solo songs and alarm situations, and these contexts are similar to male song. Notably, female song was absent during the dawn chorus; the period around sunrise when males sing intensively just before mating. Female and male song overlapped substantially in acoustic structure (i.e. same song types, peak frequency or trill rates) but there were also significant differences in that females had smaller repertoires, shorter trills and lower vocal consistency. Differential selections pressures related with contextual (functional) differences in the role of male and female song could explain the observed differences in acoustic structure. The new finding of prolific female singing in such a well-studied species suggests we ought to revise our understanding of male and female vocal communication in this (and probably other) species. Identifying the selection pressures associated to the convergence versus divergence of male and female song may provide important insight in understanding birdsong evolution. Lay summaryFemale song has been anecdotally reported in blue tits but there are no quantitative studies of singing behaviour, song structure or context. Here, we report frequent female singing in blue tits, associated mostly with agonistic interactions and alarm situations. Importantly, female song was not observed during dawn chorus, the period around sunrise when males sing intensively just before mating. Female and male song overlapped substantially in acoustic structure (i.e. same song types) but there were also significant differences (i.e. females sang with lower vocal consistency). We speculate that differences in context (function) of male and female song could explain the observed differences in acoustic structure. The new finding of prolific female singing in such a well-studied species suggests we ought to revise our understanding of male and female vocal communication.

animal behavior and cognition↗