download.file("http://rug.mnhn.fr/seewave/WAV/alauda.wav", destfile="alauda.wav")
alauda<-readWave("alauda.wav")
f <- 22050
spectro(alauda, f=f, ovlp=75, zp=8, palette=rev.gray.colors.1, scale=FALSE)
par(new=TRUE)
dfreq(alauda, f=f, ovlp=50, threshold=6, type="l", col="red", lwd=2)