Examining the origins of vocal learning using the oldest passerine: fundamental studies on the vocal communication of the titipounamu

Abstract

Vocal communication is an essential behaviour for many animals, allowing them to recognise, assess and coordinate with one another. Some social animals also benefit from vocal learning, which is an ability to develop and shape vocal repertoires based on auditory templates and acoustic environments. This ability enhances communication by enabling individuals to adapt and fine-tune their vocal signals. Hummingbirds (order: Apodiformes, family: Trochilidae), parrots (order: Psittaciformes), and oscine songbirds (order: Passeriformes, suborder: Passeri), were thought to have each independently evolved this ability, due to their distant phylogenetic relationship. Among these birds, Passeriformes have the most diverse life history traits and vocal repertoires, providing us with an excellent opportunity to understand how and why vocal learning evolved. However, the avian phylogeny has recently been dramatically reassembled; and the new phylogeny has the potential to transform our current understanding of vocal learning. In the new phylogeny, the New Zealand wrens (Acanthisitti) are now the oldest passerines, and a crucial link between parrots and songbirds, which includes both oscines (canonical learners) and suboscines (assumed non-learners). Thus, a thorough examination of New Zealand wren vocal abilities may provide critical insights into whether vocal learning is a binary (yes/no) trait and whether this ability actually evolved in the common ancestor of parrots and songbirds. Here, I undertook this research from four angles.

First, I conducted a global literature review of vocal development studies in birds across the avian phylogeny and vocal learning abilities. Vocal development is considered a key feature for classifying taxa in the classical binary division: learners display pronounced developmental stages and prolonged development, while non-learners display no clear stages and have rapid vocal development. However, I found that the empirical data on which these classifications are narrowly focused on a handful of model species, on songs rather than calls, and exclude females almost entirely. This uneven documentation creates a skewed view of how vocal development patterns reflect vocal learning, and even among the existing data the patterns are not what the binary hypothesis would predict. In the remaining chapters, I addressed this gap by assessing the vocal communication system of one of the two living species of New Zealand wrens, the titipounamu or rifleman (Acanthisitta chloris granti). I quantified their vocal repertoire and described nine adult call types and three juvenile call types. I also found contextual variations in the acoustic parameters of some call types and discuss their social relevance and functions in communication. I then investigated titipounamu vocal development and found that the onset timings of juvenile call types follow systematic stages. In addition, the period of transition of one juvenile call to the adult form extends beyond fledging and dispersal. These patterns indicate that titipounamu vocal development are more similar to learning oscine songbirds than non-learning species. Finally, I tested for plasticity in adult vocalisations to determine if birds are adjusting their vocalisations – an ability usually attributed to learning. I found that some adult contact calls have more similar structures between partners than strangers, and individuals show differences across years. These results support the idea that these birds are capable of vocal plasticity. This thesis highlights the importance of fundamental studies on the repertoire, development, and variation of vocalisations of underrepresented species to further advance our understanding of vocal learning and its many forms.

Taken together, my findings on the protracted vocal development and plastic nature of adult contact calls indicate that rudimentary vocal learning is likely present in the titipounamu. Given that New Zealand wrens have a simple syrinx (vocal organ), distinct from that of a vocal learner, these living fossils offer a new perspective on vocal learning in its rudimentary form. These results undermine the longstanding paradigm that used a simple binary classification of learner versus non-learner and suggests that rudimentary vocal learning is much more widespread, and evolved much earlier, than previously thought. Thus, these findings support the new hypotheses arguing for vocal learning as a continuum, spectrum, or collection of modular abilities.