# fetch file
download.file("http://rug.mnhn.fr/seewave/WAV/alauda2.wav", destfile="alauda2.wav")
# load file
s <- readWave("alauda2.wav")
# generate a wide png file
png("skylark_horiz.png", width=32000, height=4000, point=120)
par(lwd=6)
spectro(s, ovlp=75, scale=F)
dev.off()

The image is quite heavy, download it here if you want to have a look at the result.