Non-linear phenomena (NLP) in vocalizations may have adaptive functions, such as increasing acoustic impact or conveying fitness or identity information. Yet despite their potential to impart behavioural information, these spectral features have not been studied in most bird species and many genera. This applies to the New Zealand kiwi (Apterygidae), with NLP-like features indicated in one species, but not explored further. Yet as nocturnal and cryptic birds heavily reliant on vocal communication, kiwi are ideal species in which to assess the function of these structures. Furthermore, kiwi are acoustically typical but taxonomically and ecologically unique among birds, so can provide an important reference for determining the behavioural relevance of NLP in other bird species. We have assessed the occurrence of NLP in little spotted kiwi (Apteryx owenii) calls, in the first detailed study of such features in a ratite. NLP in the form of subharmonics are common in this species. We tested for possible adaptive functions of these features by comparing their occurrence with call spectral parameters and conditions. Subharmonics increased with frequency and during calls produced in territorial contexts, indicating that these features may provide acoustic impact, possibly to convey aggression or fitness information. Our results also suggest that NLP are unlikely to provide identification information in this kiwi species.
non-linear phenomena, two-voicing, Apterygidae