A fine-scale, broadly applicable index of vocal performance: frequency excursion

Abstract

Our understanding of the evolution and function of animal displays has been advanced through studies of vocal performance. A widely used metric of vocal performance, vocal deviation, is limited by being applicable only to vocal trills, and also overlooks certain fine-scale aspects of song structure that might reflect vocal performance. In light of these limitations we here introduce a new index of vocal performance, "frequency excursion". Frequency excursion calculates, for any given song or song segment, the sum of frequency modulations both within and between notes on a per-time basis. We calculated and compared the two performance metrics in three species: chipping, swamp, and song sparrows. The two metrics correlated as expected, yet frequency excursion accounted for subtle variations in performance overlooked by vocal deviation. In swamp sparrows, frequency excursion values varied significantly by song type but not by individual. Moreover, song type performance in swamp sparrows, according to both metrics, varied negatively with the extent to which song types were shared among neighbors. In song sparrows, frequency excursion values of trilled song segments exceeded those of non-trilled song segments, although not to a statistically significant degree. We suggest that application of frequency excursion in birds and other taxa will provide new insights into diverse open questions concerning vocal performance, function, and evolution.

Bioacoustic monitoring of animal vocal behavior for conservation

“…Sexual selection can act strongly on males' songs, since for many species females choose males according to song characteristics (Andersson, 1994). In the song sparrow, Melospiza melodia, for instance, highperformance songs (a measure of frequency bandwidth relative to trill rate; Podos et al, 2016) induce more copulation solicitation displays from females (Ballentine, Hyman, & Nowicki, 2004). Moreover, since singing is physically challenging, higher quality males, or those occupying higher quality territories, should sing songs that are closer to a physiological performance limit (Liu et al, 2017;Nowicki & Searcy, 2005).…”

Rock wrens preferentially use song types that improve long distance signal transmission during natural singing bouts

“…We used this measure because it reflects adjustments within the muscular control of the syrinx during singing (Suthers and Zollinger 2004). Our frequency switch measure also allows us to incorporate all types of sounds made during rock wren songs, including buzzes, which can cause analysis challenges using other performance metrics (Tchernichovski and Mitra 2011, Geberzahn and Aubin 2014, Podos et al 2016. In general, existing measures of song performance consider bandwidth, trill rate, and frequency modulation, often combining them into a single metric (Tchernichovski and Mitra 2011, Geberzahn and Aubin 2014, Podos et al 2016.…”

“…Our frequency switch measure also allows us to incorporate all types of sounds made during rock wren songs, including buzzes, which can cause analysis challenges using other performance metrics (Tchernichovski and Mitra 2011, Geberzahn and Aubin 2014, Podos et al 2016. In general, existing measures of song performance consider bandwidth, trill rate, and frequency modulation, often combining them into a single metric (Tchernichovski and Mitra 2011, Geberzahn and Aubin 2014, Podos et al 2016. By breaking down our performance measure into the three component parts we were able to assess how each of these song type features was used differentially during broadcast song.…”

“…Researchers studying song complexity may quantify song repertoire size, syllable repertoire size, song bandwidth, the number of elements in a song, the structural complexity of a song form and more (Irwin 2000, Drăgănoiu et al 2002, Gil and Gahr 2002, Kagawa et al 2014, Podos et al 2016. Songs with high structural complexity and many frequency switches require precise respiratory and motor control, making them challenging to sing and good candidate indicators of performance ability (Suthers and Zollinger 2004, Taft 2014, Podos et al 2016. In our study we found that rock wrens do not maximize signals of complexity or song performance by preferentially using such songs during broadcast singing bouts.…”

“…1b) are good indicators of individual quality and are challenging to produce, but do not transmit well over long distances (Morton 1975, Podos 1997, Drăgănoiu et al 2002, Ballentine et al 2004, Garamszegi et al 2006, Illes et al 2006, Brumm and Naguib 2009). When songs include many rapid frequency directional switches they are expected to be particularly physiologically difficult to sing, giving them the potential to indicate something about singer quality and energy investment in song (Geberzahn and Aubin 2014, Riede and Goller 2014, Podos et al 2016. Energy investment in each song may also be indicated by a simpler metric: song duration.…”