Analysis of North Island robin (petroica australis longipes) and North Island tomtit (petroica macrocephala toitoi) vocalisations from Pureora Forest Park, Central North Island

Abstract

North Island robin (Petroica australis longipes) and North Island tomtit (P. macrocephala toitoi) vocalisations were recorded from three study areas at Pureora Forest Park, Central North Island between November 1996 and January 1997. Song and other vocalisations were analysed from 52 robin and 20 tomtit individuals. Song activity was measured for these two species and a clear 'dawn chorus' was found during the first three months of the study. Song production was a significant part of the bird's daily activity. N.I. robins exhibited a marked annual song cycle correlated with changing day-length. Song production was found to be greater in bachelor robins than in paired males. This suggests the main function of song in N.I. robins is mate attraction. Both robins and tomtits give a 'downscale'- type call. Downscaling in robins increased during the post-breeding moult, almost to the exclusion of full song. It is likely that this call is territorial. N.I. robin repertoires (number of different syllable types) varied between 11 and 32. A five-minute song sample was sufficient to establish much of the repertoire in a N.I. robin. A total of 99 different syllable types were identified from all robins studied. Robin songs were compared using the Similarity Index method, and found to become more similar as the season progressed. However, no correlation was found between song similarity and distance between birds. Early in the breeding season (November) repertoire sizes and syllable output rates were greater for bachelor birds than for paired males. Later (January), the reverse was true. Possible reasons for this are discussed. Using the SI method, an individual's song was found to change significantly between months for robins. A high degree of ordering of syllable types was revealed within robin songs, the vocal behaviour conformed to a third-order Markov chain model. N.I. tomtit song is less varied and much shorter than the robin's, usually consisting of a single repeated song type between one and two seconds in length. Tomtit songs were compared using the Difference Index method. Using this method, a continuous geographic variation in song was revealed between tomtit individuals within study areas. Between study areas, greater differences in mean song length were found between more distant males. Tomtit songs appear highly stereotyped, with often only one song type uttered by an individual. However, using the DI method, individual tomtit song was found to change significantly between months. These changes represent a 'drift' in song frequency or length. Deletions of end sections of song were also common, and individuals singing two song types were occasionally heard. Calls were documented for both species according to behavioural context and comparisons were made between species. Although considerable similarities were found in call types between species, species-specific differences in call frequency structure were found.