Acoustic (dis)similarity, coordinated singing and simulating songs

Marcelo Araya-Salas, PhD

Grace Smith-Vidaurre

2024-12-10

Bioacoustics in R with warbleR

warbleR logo

Bioacoustics research encompasses a wide range of questions, study systems and methods, including the software used for analyses. The warbleR and Rraven packages leverage the flexibility of the R environment to offer a broad and accessible bioinformatics tool set. These packages fundamentally rely upon two types of data to begin bioacoustic analyses in R:

  1. Sound files: Recordings in wav or mp3 format, either from your own research or open-access databases like xeno-canto

  2. Selection tables: Selection tables contain the temporal coordinates (start and end points) of selected acoustic signals within recordings

Package repositories

These packages are both available on CRAN: warbleR, Rraven, as well as on GitHub: warbleR, Rraven. The GitHub repository will always contain the latest functions and updates. You can also check out an article in Methods in Ecology and Evolution documenting the warbleR package [1].

We welcome all users to provide feedback, contribute updates or new functions and report bugs to warbleR’s GitHub repository.

Please note that warbleR and Rraven use functions from the seewave, monitoR, tuneR and dtw packages internally. warbleR and Rraven have been designed to make bioacoustics analyses more accessible to R users, and such analyses would not be possible without the tools provided by the packages above. These packages should be given credit when using warbleR and Rraven by including citations in publications as appropriate (e.g. citation("seewave")).

Parallel processing in warbleR

Parallel processing, or using multiple cores on your machine, can greatly speed up analyses. All iterative warbleR functions now have parallel processing for Linux, Mac and Windows operating systems. These functions also contain progress bars to visualize progress during normal or parallel processing. See [1] for more details about improved running time using parallel processing.

Vignette introduction

In the previous vignette, we performed quality control processing of selected signals and made lexicons for visual classification. Here we continue with the case study of microgeographic vocal variation in long-billed hermit hummingbirds, Phaethornis longirostris [2] (and a short sidenote using Tinamus major for an example of a tonal signal) by:

  1. Detecting signal frequency range

  2. Extracting spectral entropy and frequency contours as time series

  3. Comparing methods for quantitative analysis of signal structure

    • data set of 29 acoustic parameters
    • spectrographic cross-correlation
    • dynamic time warping on frequency contours

  4. Visually inspecting frequency contours

  5. Measuring acoustic parameters as a batch-process across signals

  6. Calculating pairwise acoustic (dis)similarity between signals

  7. Analysis of geographic variation in Phaethornis longirostris songs

We also include some examples at the end of the vignette of how to perform coordinated singing analysis and simulate songs.

This vignette can be run without an advanced understanding of R, as long as you know how to run code in your console. However, knowing more about basic R coding would be very helpful to modify the code for your research questions.

For more details about function arguments, input or output, read the documentation for the function in question (e.g. ?cross_correlation).  

Prepare for analyses

library(warbleR)

# set your working directory appropriately
# setwd("/path/to/working directory")

# run this if you have restarted RStudio between vignettes without saving your workspace
# assumes that you are in your /home/username directory
setwd(file.path(getwd(), "warbleR_example"))

# Check your location
getwd()

This vignette series will not always include all available warbleR functions, as existing functions are updated and new functions are added. To see all functions available in this package:

# The package must be loaded in your working environment
ls("package:warbleR")

Extract acoustic parameters as time series

Detect frequency range

Raven selection tables can return low and high frequencies in your selections (e.g. if all.data or freq.cols in run_raven or imp.raven is TRUE), but the accuracy of these frequency selections depends on how the signals themselves were selected. Below we demonstrate how to visualize and detect the frequency range in your selected signals using the functions freq_range_detec and freq_range, which have options for setting bandpass filters to exclude background noise or other non-target acoustic signals.

freq_range_detec creates a plot that will print to your graphics device, and also outputs a data frame per recording with the frequency range. This data frame can be used in subsequent analyses if saved as an object. freq_range_detec works best with single signals. If used for a whole recording, freq_range_detec will pick up all sounds in the recording.

Finally, although we have been using Phaethornis longirostris vocalizations throughout these vignettes, these signals are harmonically structured. The functions for detecting frequency ranges, freq_range and freq_range_detec work best on tonal signals, so for this example we will use Great Tinamou ( Tinamus major) songs.

Download a tinamou recording from xeno-canto, make selections and visualize/detect frequency ranges.

tin <- query_xc(qword = "Tinamus", download = FALSE)

# select a single recording
tin <- tin[tin$Recordist == "Marcelo Araya-Salas", ]

# download this recording
query_xc(X = tin, download = TRUE)

mp32wav()
# here we will use a data set with sound files that have been already annotated
# read the selections back into the global environment
Tin.sels <- read.csv("manualoc_output.csv")
str(Tin.sels)

# cut the original wave file by selections for freq_range_detec below
writeWave(seewave::cutw(read_sound_file("Tinamus-major-154191.wav"), from = Tin.sels$start[1], to = Tin.sels$end[1], f = 44100, plot = FALSE, output = "Wave"), filename = "Tinamus-major-154191-1.wav")

writeWave(seewave::cutw(read_sound_file("Tinamus-major-154191.wav"), from = Tin.sels$start[2], to = Tin.sels$end[2], f = 44100, plot = FALSE, output = "Wave"), filename = "Tinamus-major-154191-2.wav")
# note that changing the threshold argument in combination with the bandpass argument can improve the detection
freq_range_detec(read_sound_file("Tinamus-major-154191-1.wav"), flim = c(0, 2.5), bp = c(0, 3), threshold = 15, plot = TRUE)
# here, giving a strict bandpass with very low threshold improves freq_range detection
# since the curving end of the tinamou signal is lower amplitude than the rest of the signal
c(read_sound_file("Tinamus-major-154191-1.wav"), flim = c(0, 2.5), bp = c(0, 3), threshold = 1, plot = TRUE)

The function freq_range allows you to simultaneously return the frequency ranges for all signals in a selection table, including the graphical output as freq_range_detec. Check out the resulting image file in your graphics device. In addition to image files, this function returns the original selection table, as a data frame with the newly calculated low and high frequency measurements.

# use arguments from freq_range_detec above
fr <- freq_range(Tin.sels, threshold = 1, res = 100, flim = c(0, 2.5), bp = c(0.5, 2.5))
str(fr)

Extract spectral entropy as a time series

Spectral entropy can be calculated as time series in selected signals and plotted onto image files. Previously, spectral entropy was only available as a sole measurement across a selection, as measured by spectro_analysis. Check out the resulting image files in your working directory.

Phae.hisnrt <- read.csv("Phae_hisnrt.csv", header = TRUE)
str(Phae.hisnrt)

se <- freq_ts(Phae.hisnrt, wl = 300, length.out = 10, threshold = 10, img = TRUE, img.suffix = "entropy_ts", type = "b", ovlp = 90, sp.en.range = c(-25, 10), flim = c(2, 10), picsize = 0.75, title = FALSE, type = "entropy")

str(se)

Visualizing frequency contours with track_freq_contour

The function track_freq_contour allows you to create spectrograms and visualize the accuracy of dominant frequency and fundamental frequency measurements.

Use track_freq_contour on all the recordings for which you want to extract frequency contours as a time series, or later, calculate other frequency measurements. Scroll through all the spectrograms to get a feeling for how well the frequency measurements will be performed across your recordings.

Running track_freq_contour can allow you to decide which frequency parameters to use in subsequent analyses, namely spectro_analysis and dynamic time warping methods. Also, ff the frequency measurements look acceptable with the bandpass setting used in track_freq_contour, use that same bandpass while running spectro_analysis.

# Note that the dominant frequency measurements are almost always more accurate
track_freq_contour(Phae.hisnrt, wl = 300, flim = c(2, 10), bp = c(1, 12), it = "jpeg")

# We can change the lower end of bandpass to make the frequency measurements more precise
track_freq_contour(Phae.hisnrt, wl = 300, flim = c(2, 10), bp = c(2, 12), col = c("purple", "orange"), pch = c(17, 3), res = 100, it = "jpeg", picsize = 0.8)

 

Note that the fundamental frequency measurements are not always very accurate, so we will remove fundamental frequency measurements later on.

Extract fundamental or dominant frequency contours as a time series

These functions return a data frame that contains estimated frequency contours as a time series across each signal in the input data frame. You can also specify if you want to create image files with the estimated frequency contours plotted over spectrograms. You can change argument settings to better visualize the signals or change the estimation of the frequency contour. For instance, the argument threshold (as in auto_detec) controls the amplitude threshold for estimating frequency values at each time point. Note that the fundamental frequency contour estimation can often have errors, and tends to perform best with more tonal signals. The frequency contours are those that can be visualized using track_freq_contour.

# Fundamental frequency contour
ff_df <- freq_ts(Phae.hisnrt, wl = 300, length.out = 20, threshold = 15, img = TRUE, img.suffix = "ff", type = "p", ovlp = 70, clip.edges = FALSE, leglab = "freq_ts", ff.method = "tuneR")

str(ff_df)
# Dominant frequency contour

# Uses seewave function dfreq by default
df_df <- freq_ts(Phae.hisnrt, wl = 300, length.out = 20, threshold = 15, img = TRUE, img.suffix = "ff", type = "p", ovlp = 70, clip.edges = FALSE, leglab = "freq_ts", fsmooth = 0.2)

str(df_df)

Manually tailor frequency contours with tailor_sels

The functions above to track/extract dominant and fundamental frequency contours perform best with more tonal signals. The frequency measurements for signals with harmonic structure tend to jump around, and might not always match your own visual tracking of frequency contours. If this is the case, you can use the function tailor_sels to fix frequency contours where individual frequency measurements are clearly far off from the frequency contour detected by the human eye. The tailor_sels function is the same used in vignette 2 in this series, but by changing a few arguments, you can use tailor_sels to fix frequency contours.

Note that manually fixing frequency contours might not make sense, depending on your question and/or the contour in question. For instance, a dominant frequency contour for a harmonic signal that jumps around and does not form a smooth contour may in fact be the truth, rather than mis-estimation of the contour. On the other hand, fundamental frequencies can be more easily traced by the human eye across a signal, so using tailor_sels to fix a frequency contour that jumps around the signal makes more sense.

When the new graphics window for tailor_sels appears, it will show spectrograms as we saw in vignette 2, but with frequency contours plotted as points over each spectrogram. To fix the frequency contour, click near the malaligned points to place them over the frequency contour that you detect by eye. tailor_sels makes a new .csv file in your working directory that merges your original data frame (below, Phae.hisnrt) with the modified frequency time series (below, ff_df with any modified frequency values). To check that your manual tracing improved frequency contours, you can use track_freq_contour to make spectrograms with your new frequency contours plotted as custom contours.

# Use the original data frame of songs for the main tailor_sels dataset
# the data frame with the fundamental frequency contours is provided for manual tracing
tailor_sels(Phae.hisnrt,
  wl = 300, flim = c(2, 10), wn = "hanning", mar = 0.1,
  osci = TRUE, title = c("sound.files", "selec"), auto.contour = TRUE, ts.df = ff_df, col = "red", alpha = 0.6
)

# rename your tailor_sels output csv as desired, then read it back into R
mff <- read.csv("seltailor_output_mff.csv")
str(mff)

track_freq_contour(Phae.hisnrt, wl = 300, flim = c(2, 10), bp = c(1, 12), it = "jpeg", custom.contour = mff)

Count inflections across frequency contours

This function calculates the modulation index for any frequency contour or time series. The function spectro_analysis (see below) also calculates a modulation index for signals, but as a single value across the length of the signal.

df_inf <- inflections(X = df_df, pb = TRUE)
str(df_inf)

##Quantitative measurements of acoustic (dis)similarity

Compare methods for quantitative analysis of signal structure

Bioacoustic research relies on quantifying the structure of acoustic signals and comparing that structure across behavioral/ecological contexts, groups or species. However, measuring signal structure in a way that fully accounts for the variation in the signals could be a tricky task. Some of the differences that are apparent by visual inspection of spectrograms might not be picked up by some analyses. Hence, choosing the most appropriate analytical approach is a critical step.

The warbleR function compare_methods attempts to facilitate method selection. This function produces graphs (as image files in the working directory) with spectrograms from 4 signals that allow visual inspection of the performance of acoustic analysis methods at comparing those signals. The signals are randomly picked up from the provided data frame (X argument), and the function compares 2 warbleRmethods at a time. The methods available are: * cross-correlation by warbleR function cross_correlation * dynamic time warping on dominant or fundamental frequency contours with freq_DTW * spectral parameters with spectro_analysis

In the last vignette, we tailored selections of Phaethornis longirostris songs that were originally downloaded from xeno-canto, detected by auto_detecand filtered by signal-to-noise ratio (SNR). Here we will pick up the workflow with these filtered and tailored selections, using the data frame Phae.hisnrt.

Phae.hisnrt <- read.csv("Phae_hisnrt.csv", header = TRUE)

compare_methods(
  X = Phae.hisnrt, flim = c(0, 10), bp = c(0, 10),
  wl = 300, n = 10, methods = c("XCORR", "dfDTW")
)

compare_methods will produce 10 image files in the working directory (since we specified n = 10) that look like this:

 

In this graphic, the acoustic pairwise distance between signals is shown next to the arrows linking them. The font color of a distance value corresponds to the font color of the method that generated it, as shown in the scatterplots (in this case black font represents XCORR distances). Distances are standardized, with 0 being the distance of a signal to itself and 1 the farthest pairwise distance in the pool of signals. Principal Component Analysis (princomp function) is applied to calculate distances when using spectral parameters (SP). In this case, the first 2 PC’s are used. Classical Multidimensional Scaling (also known as Principal Coordinates Analysis, cmdscale function) is used for all other methods. The image file name contains the methods being compared and the row number of the selections. This function internally uses a modified version of the spectro function from the seewave package to create spectrograms. Note that the spectrograms are all plotted with the same frequency and time scales.

Also note that the graphs contain 2 scatterplots (1 per method) of the acoustic space of all signals in the input data frame X. The position of the 4 signals in the spectrograms is highlighted in the acoustic space scatterplot. These graphics allow you to directly assess if the distances between signals in the acoustic space accurately represent the spectrographic similarity (e.g. how similar their acoustic structure looks in the spectrograms).

You can run compare_methods for any combination of the quantitative methods for assessing acoustic (dis)similarity mentioned above. Importantly, to include the SP method (spectral parameters measured by the function spectro_analysis), you need a large enough dataset, as the PCA that summarizes the spectral parameters needs more units (rows) that variables (columns).

Measure acoustic parameters with spectro_analysis

We can now perform acoustic measurements with the function spectro_analysis. This function relies on the temporal coordinates in selection tables to measure 29 parameters across selections. spectro_analysis is a batch process that is faster than calculating measurements manually, e.g. one selection and recording at a time. spectro_analysis uses and customizes several functions available in the seewave package.

Use the bandpass filter to your advantage here, to filter out low or high background noise before performing measurements. Also note that changing the amplitude threshold will change the amplitude at which noises (including non-target signals) are detected for measurements.

params <- spectro_analysis(Phae.hisnrt, bp = c(2, 10), threshold = 15)
write.csv(params, "acoustic_parameters.csv", row.names = FALSE)

Remove parameters derived from fundamental frequency (based on track_freq_contour results).

params <- params[, grep("fun|peakf", colnames(params), invert = TRUE)]

Calculate acoustic parameters by song type

In addition to calculating acoustic parameters per individual signals using spectro_analysis, you can also calculate these acoustic parameters by song type (average, minimum and maximum values per song type group).

data(list = c("Phae.long1", "Phae.long2", "Phae.long3", "Phae.long4", "lbh_selec_table"))
writeWave(Phae.long1, "Phae.long1.wav")
writeWave(Phae.long2, "Phae.long2.wav")
writeWave(Phae.long3, "Phae.long3.wav")
writeWave(Phae.long4, "Phae.long4.wav")

# Add a 'song' column
lbh_selec_table$song <- rep(1:4, each = 3)[1:11]

# Measure acoustic parameters
sp <- spectro_analysis(lbh_selec_table, bp = c(1, 11), 300, fast = TRUE)

# Add song data
sp <- merge(sp, lbh_selec_table, by = c("sound.files", "selec"))

# Caculate song-level parameters for all numeric parameters
sng <- song_analysis(X = sp, song_colm = "song", parallel = 1, pb = TRUE)
str(sng)

Dynamic time warping of frequency contours

The dynamic time warping methods in warbleR all rely on functions from the dtw package, and are available for both dominant and fundamental frequencies. df_DTW and ff_DTW calculate the dominant and fundamental frequency contours, respectively, of each signal and compares using dynamic time warping. You can interpolate measurements across the frequency time series using the length.out argument.

These functions return a matrix of pairwise acoustic dissimilarity (e.g. acoustic “distance”) measurements that can be used in analyses of acoustic similarity, as well as image files with the frequency contours plotted over the spectrograms. If you require only the time series without the dynamic time warping analysis for either the dominant or fundamental frequency, check out the functions freq_tsand freq_ts.

Note that as the freq_range and freq_range_detec functions, the dynamic time warping functions tend to work best on more tonal signals. Check out the resulting image files in your working directory.

# Harmonic Phaethornis signals
dm <- freq_DTW(Phae.hisnrt, length.out = 30, flim = c(2, 10), bp = c(2, 9), wl = 300, img = TRUE)

str(dm)
# Tonal Tinamou signals
Tin.sels <- read.csv("Tinamus-major-154191_sels.csv", header = TRUE)

dm <- freq_DTW(Tin.sels, length.out = 30, flim = c(0, 2.5), bp = c(0.5, 2.5), wl = 512, img = TRUE)
str(dm)

Spectrographic cross-correlation

Cross-correlation calculates the pairwise similarity of multiple signals by spectrogram cross-correlation. cross_correlation calculates a correlation of the amplitude values at each step by sliding one spectrogram over the other. This function is a modified version of the corMatch and makeTemplate functions from the package monitoR. cross_correlation runs over multiple signals and returns a list of output, including:

cross_correlation requires margins half the duration of the signal on either side of the signal (e.g. before and after the start and end coordinates). Signals that are very close to either the start or end of a recording may throw errors. Additionally, when sound files have been modified by bandpass filters, some pairwise cross-correlations may not work and the correlation matrix may contain NA values. cross_correlation now contains an argument to remove columns of the matrix that contain NA values, which will reduce the data in the matrix but also allow the matrix to be used in subsequent analyses.

Here our output is a matrix of peak correlation per pairwise comparison, with the recording-selection names as the matrix dimension names.

xc <- cross_correlation(Phae.hisnrt, wl = 300, na.rm = FALSE)
str(xc)

Analysis of geographic variation using spectro_analysis measurements

We can evaluate whether or not the observed variation in song structure is reflected by the meters we just measured. For this we will conduct a Principal Component Analysis on scaled (z-transformed) meters and look at the grouping of songs (data points) in the scatter plot.

# Run the PCA with only numeric variables of params
pca <- prcomp(x = params[, sapply(params, is.numeric)], scale. = TRUE)

# Check loadings
summary(pca)
Importance of components:
                         PC1   PC2   PC3    PC4    PC5    PC6    PC7    PC8    PC9    PC10    PC11    PC12    PC13   PC14    PC15    PC16    PC17
Standard deviation     3.799 2.647 1.768 1.0113 0.9368 0.8842 0.8449 0.7261 0.6307 0.52078 0.44692 0.43914 0.41945 0.3552 0.23923 0.18924 0.15921
Proportion of Variance 0.481 0.233 0.104 0.0341 0.0293 0.0261 0.0238 0.0176 0.0133 0.00904 0.00666 0.00643 0.00586 0.0042 0.00191 0.00119 0.00084
Cumulative Proportion  0.481 0.714 0.819 0.8528 0.8821 0.9081 0.9319 0.9495 0.9627 0.97178 0.97844 0.98487 0.99073 0.9949 0.99685 0.99804 0.99888
                          PC18    PC19    PC20    PC21    PC22     PC23
Standard deviation     0.15130 0.07228 0.05806 0.03699 0.02449 2.21e-15
Proportion of Variance 0.00076 0.00017 0.00011 0.00005 0.00002 0.00e+00
Cumulative Proportion  0.99965 0.99982 0.99993 0.99998 1.00000 1.00e+00
# Extract PCA scores
pcascor <- as.data.frame(pca[[5]])

# Plot the 2 first PCs
plot(pcascor[, 1], pcascor[, 2],
  col = as.numeric(as.factor(params$sound.files)), pch = 20,
  cex = 1, xlab = "PC1", ylab = "PC2"
)

# Add recordings/individuals labels
x <- tapply(pcascor[, 1], params$sound.files, mean)
y <- tapply(pcascor[, 2], params$sound.files, mean)

labs <- gsub(".wav", "", unique(sapply(as.character(params$sound.files), function(x) {
  strsplit(x, split = "-", fixed = TRUE)[[1]][3]
}, USE.NAMES = FALSE)))

text(x, y, labs, cex = 0.75)

Songs are grouped by sound file. As each sound file represents a single individual, this suggests that songs have individual signatures. Let’s look at the song type level. First, we need to classify the songs by song type. We can check the spectrograms we previously created to do this.

 

Songs from sound files 154070 and 154072 seem to belong to the same song type. Sound files 154129 and 154161 represent a different song type. Finally, the songs from each of the other 2 sound files have a unique structure, so each one represents a different song type. Sound file 154123 is represented here too, but in vignette 1 we decided against using signals from this recording. We can add this information to the plot by using symbols to represent song types.

# Create a song type variable

# First, extract recording ID
songtype <- gsub(".wav", "", sapply(as.character(params$sound.files), function(x) {
  strsplit(x, split = "-", fixed = TRUE)[[1]][3]
}, USE.NAMES = FALSE))

# Now change IDs for letters representing song types
songtype <- gsub("154070|154072", "A", songtype)
songtype <- gsub("154129|154161", "B", songtype)
songtype <- gsub("154138", "C", songtype)

# Add song type as a variable representing symbol type
plot(pcascor[, 1], pcascor[, 2],
  col = as.numeric(as.factor(params$sound.files)),
  pch = as.numeric(as.factor(songtype)),
  cex = 1, xlab = "PC1", ylab = "PC2"
)

# Add song type labels
x <- tapply(pcascor[, 1], songtype, mean)
y <- tapply(pcascor[, 2], songtype, mean)

text(x, y, unique(songtype), cex = 1)

Songs of the same song type are more similar (they cluster together). This PCA confirms that the visually obvious differences in the song structures are well described by the meters measured in warbleR. Likewise, it also confirms that we can detect biologically relevant differences from sound files that have undergone mp3 compression and conversion back to wav format (see also [2]). Importantly, if this analysis were for publication we would have to run other filtering steps on the acoustic parameters generated by spectro_analysis, including removing any collinear variables.

Coordinated singing analysis

warbleR contains functions to visualize and test whether or not singing bouts between individuals can be considered coordinated singing. The function plot_coordination plots signals as polygons to better visualize overlap between these signals.

The function test_coordination uses a Monte Carlo randomization test to determine whether or not the probability of finding overlapping songs in a singing event is less than or more than what you would expect by chance. test_coordination can be run simultaneously for many singing events, if all are contained in a single data frame (see test_coordination documentation for more information).

data(sim_coor_sing)
str(sim_coor_sing)

The sim_coor_sing dataset contains three types of singing bouts:

# save plots in a list
g <- plot_coordination(sim_coor_sing, it = "jpeg", img = FALSE, res = 300)

# print list of plots to graphics device
g

We can test for overlapping or alternating coordinated singing using the less.than.chance argument in test_coordination. Here we will test for alternating coordinated singing, since less.than.chance is set to TRUE.

cs <- test_coordination(sim_coor_sing, iterations = 1000, less.than.chance = TRUE, cutoff = 10)
str(cs)

Simulating songs

You can simulate songs using warbleR. Depending on your question, you can simulate songs with different numbers of subunits, subunit durations, numbers of harmonics, amplitudes, gaps between subunits, amplitude fading of subunits, among other options. Songs are simulated under Brownian motion frequency drift.

# simulate a song with 3 tonal elements
ss <- simulate_songs(n = 3, harms = 1)

# plot the simulated song
# seewave::spectro(ss)

# simulate a song with 3 harmonic elements of differing amplitude
ss <- simulate_songs(n = 3, harms = 3)

# plot the simulated song
seewave::spectro(ss)

 

This vignette concludes the warbleR vignette series. After running the code in this third vignette, you should now have an idea of how to:

In these vignettes we presented examples of how the warbleR functions can be used in a streamlined workflow for flexible and rigorous bioacoustics analyses. The output of warbleR functions can be used in other packages for statistical analyses, such as machine learning.

Citation

Please cite warbleR when you use the package:

Araya-Salas, M. and Smith-Vidaurre, G. (2017), warbleR: an R package to streamline analysis of animal acoustic signals. Methods Ecol Evol. 8, 184-191.

Reporting bugs

Please report any bugs here.  

References

  1. Araya-Salas, M. and G. Smith-Vidaurre. 2016. warbleR: an R package to streamline analysis of animal acoustic signals. Methods in Ecology and Evolution. doi: 10.1111/2041-210X.12624