Evolutionary history of treecreeper vocalisations (Aves: Certhia)

Since the vocalisations of passerine birds are in general a good means to separate taxa when external morphological differences are few, song and call recordings of 33 treecreeper (Certhia) taxa were sonagraphed and their parameters analysed. The vocalisations show low intra-individual and intra-population variation. Phylogenetic evolutionary units at the population level were delimited by time, frequency and syntax parameters by means of principal-component and discriminant analyses. Traits of territorial song were traced on a phylogenetic tree based on cytochrome b sequences, and a mean acoustic character difference was calculated. All presently recognised nine species could be distinguished clearly from one another by their vocalisations. Subspecies not included in the molecular phylogeny are affiliated with the correct species based on statistical analysis. The obvious subdivision of Certhia species into two groups according to the syntax of their song verses, trill and motif singers, is corroborated by different findings: a high phylogenetic signal in the characters concerned (highest homoplasy index values for trill characters), discriminant analyses for song and call measurements, and a cladistic analysis based on song characters. Innate calls turned out to be less suitable for studies at a low taxonomic level than learnt territorial songs, which require social interactions for their species-specific formation.