Repertoire‐based individual acoustic monitoring of a migratory passerine bird with complex song as an efficient tool for tracking territorial dynamics and annual return rates

Abstract

1. In field ecological and behavioural studies, it is often necessary to identify specific individuals. In birds, colour rings are frequently used to mark individuals; however, rings are often difficult to observe, especially in small species and dense habitats. Acoustic-based monitoring detecting individuals by their characteristic vocalization is a potentially suitable alternative, but this approach is challenging in species with complex songs. 2. On the example of the Tree Pipit (Anthus trivialis), a small migratory passerine often singing in flight or from perches obscured by foliage, we demonstrate that acoustic monitoring based on the syllable repertoire can be very efficient tool for individual recognition. During a 3-year study, we obtained over 500 recordings from males from one study population (a number of them returning after winter). Males banded with colour rings were repeatedly recorded throughout the seasons, and syllable repertoires were determined from spectrograms for each recording. 3. The repertoire of each unambiguously identified male was distinct and stable within as well as between seasons; and males with similar syllable repertoires differed in syntax. Based on the congruence between identification based solely on spectrogram assessment, and that based on observation of colour rings, we inferred that reliable identification of singing males (including non-ringed ones) was possible in the studied population from assessing a repertoire and song syntax of <5-min recording (containing 20-30 songs). The acoustic-based data: (i) increased the overall estimated number of territorial males at the study locality (from 49 ringed to 61), and improved the estimates of the period of their presence; (ii) revealed dynamic within-season changes in territory occupancy that would otherwise be missed; and (iii) allowed identification of returning birds (including nonringed ones and those actively avoiding approaching humans). 4. Our results suggest that some commonly used methods may substantially underestimate return rates of migratory bird species. Individual acoustic monitoring should be applicable on various bird species with complex song and stable repertoires, and may be particularly useful for those living in dense habitat or sensitive to handling.

Cues to individual identity in songs of songbirds: testing general song characteristics in Chiffchaffs Phylloscopus collybita

“…Repertoire size cumulative curves in closely related Willow Warblers (Phylloscopus trochilus) show clear repertoire size asymptotes after 100 songs have been recorded (Gil and Slater 2000). A much more limited number of songs is also sufficient for good repertoire estimates in other species, even when they have much more complex repertoires (Kipper et al 2006;Hesler et al 2010;Mamede and Mota 2012;Petrusková et al 2016). Estimating repertoire size is not a trivial matter, and recently, new approaches have been discussed for this (Garamszegi et al 2004;Peshek and Blumstein 2011;Kershenbaum et al 2015).…”

“…Individual recognition based on a unique repertoire (Thompson 1970;Petrusková et al 2016) is highly unlikely in Chiffchaffs. Though individuals might have unique repertoires (cf.…”

“…Scientists have focused their attention on individual variability in bird vocalizations for decades, and many studies show individual variation in vocalizations in birds (Hutchison et al 1968;Thompson 1970;White et al 1970;Peake et al 1998;Lengagne 2001;Petrusková et al 2016). Studies on how individual identity is expressed in vocalizations are crucial for understanding the individual recognition process, but they are also important for practical situations, for example, this type of information might be eventually used as an alternative or complementary method for individual monitoring (Terry et al 2005;Laiolo et al 2007;Mennill 2011).…”

“…For example, birds may develop individually specific repertoires of songs, song types or syllables (Thompson 1970;Cicero and Benowitz-Fredericks 2000;Petrusková et al 2016). Further, only subunits within the repertoire were suggested to serve as individual signatures.…”

Avian SDMs: current state, challenges, and opportunities

“…After removing false-positives in such a data set (Campos-Cerquiera and Aide 2016), bioacoustic monitoring can deliver very accurate presence-absence information that could also be used for SDM approaches in rare, cryptic, nocturnal or otherwise hard to find bird species. Also, the identification of individuals in species with a complex song (Petrusková et al 2015) might allow study of speciesenvironment relationships in SDMs at the individual level to e.g. link individual performance to predicted populationor species-level habitat suitability.…”