How songbirds deal with large amounts of serial information: retrieval rules suggest a hierarchical song memory

Many songbirds develop remarkably large vocal repertoires, and this has pro mpted questions about how birds are able to successfully learn and use the often enormous amounts of information encoded in their various signal patte rns. We have studied these questions in nightingales (Luscinia megarhynchos ), a species that performs more than 200 different types of songs (strophen ), or more than 1000 phonetically different elements composing the songs. I n particular, we investigated whether and how both song repertoires and son g performance rules of nightingales were coded by auditory stimuli presente d in serial learning experiments. Evaluation of singing episodes produced b y our trained birds revealed that nightingales cope well with an exposure t o even long strings of master song-types. They can readily acquire informat ion encoded within and between the different master songs, and they memoriz e, for example, which master song-types they have experienced in the same l earning context. Imitations of such song-types form distinct sequential ass ociations that are termed "context groups". Additionally, nightingales deve lop other song-type associations that are smaller in size and termed "packa ge groups" Package formation results from constraints of the acquisition me chanisms which obviously lead to a segmentation of auditorily perceived mas ter song sequences. Further experimentation validated that the song memory of nightingales is organized in a hierarchical manner and holding informati on about "context groups" composed of packages, "package groups" composed o f songs, and songs composed of song elements. The evidence suggests that im plementation of such a hierarchical organization facilitates a quick retrie val of particular songs, and thereby provides an essential prerequisite for a functionally appropriate use of large vocal repertoire is in songbirds.