The authors were recording birds whose territories were certainly too distant from each other to have birds changing locations. They showed, that an individual identification should in principle be possible, but did not perform it in a given region with several neighbouring territories, nor over several years.
In a more recent work (published after the development of the technique described here), C. Walcott  et al. used a commercially available program for analysing bird calls based on "Canary 1.2" and a multivariate factor analysis running on Macintosh and applied it on banded common loons. They were mainly able to recognise the loons and to follow them when they changed their territory, however, the program also produced some uncertainties  due to its concept (it automatically looks for empirically determined characteristic frequencies of the introduction, of lengths of tacts, etc., but would not be able to analyse unison calls where two birds are calling simultaneously).
Ein Teil des notwendigen Equipments