We recorded mimicry of 19 male bower-owning spotted bowerbirds in Taunton National Park (23.3° S, 149.1° E), Queensland, Australia, during 2007 and 2008. Individuals were identified by a unique series of colour bands on both legs. Males were recorded at their bowers using a Sennheiser ME66/K6 microphone and power supply onto a Sony TCD-D8 DAT recorder at a sampling rate of 44.1 kHz. Sampling effort was evenly distributed among bowers (time per bower per year 16.5 ± 1.5 h, mean ± s.e.). Interbower distances were measured using GPS coordinates, converted into kilometres and rounded to the nearest 10 m.
Recordings were converted into spectrograms using Raven
v. 1.3 (Charif et al. 2004
) using a Hann window and a 512 pt. fast Fourier transform. Mimicry was identified using a CD-ROM of bird vocalizations (Simpson & Day 1999
) alongside our field recordings of model vocalizations, and an experienced birder corroborated our identifications. None of our model sounds resembled those of other species found on the park. Mimicry usually comprised one repeated note that could be easily identified after a single note. When a phrase was composed of more than one note (a
), we analysed only those recordings in which the entire phrase was present.
Figure 2. (a) Top to bottom: a pied butcherbird with spectrogram of its species-specific vocalization (one phrase denoted by A, three phrases shown on spectrogram), a male spotted bowerbird with spectrograms of butcherbird and kite mimicry; a whistling kite with (more ...)
Song sharing between males was calculated using the number of model species a focal male shared with a particular bower owner, expressed as a proportion of the focal male's total repertoire. Mantel tests were used to test correlations between the proportion of repertoire shared and interbower distance in 2007 and 2008 separately, using 10 000 iterations on a full matrix without diagonals (Liedloff 1999
We used recordings of butcherbird and kite mimicry to investigate individual differences in production of mimetic vocalizations. We recorded from five to 10 recordings of butcherbird mimicry from each of 10 males and from two to 47 recordings of kite mimicry from each of five males. Spectrograms of these recordings were measured for start frequency, end frequency, minimum frequency, maximum frequency, peak frequency, duration and the time to maximum, minimum and peak frequency. We calculated the proportion of time to the minimum, maximum and peak frequencies, the ratios of the maximum frequency to peak frequency and frequency range, the ratios of the start frequency to end, peak and maximum frequency, the ratio of the peak frequency to frequency range and, for butcherbird mimicry, expressed the duration of the upward sweep at the end as a proportion of overall duration. Measurements that were highly correlated with other variables (r > 0.7) were dropped from further analyses.
Analyses of butcherbird and kite mimicry were carried out separately and discriminant function analysis was used to identify temporal or frequency measurements that classified individual bowerbird mimicry of each model. Means for each individual in canonical space were represented by group centroids and the squared Mahalanobis distances between each group centroid were used as a measurement of acoustic similarity between individuals. Matrices of these values and interbower distances were then used in a Mantel test to assess whether the mimicry of individuals with bowers closer together shared greater structural similarity than did individuals with bowers further apart. The repeatability (intraclass correlation coefficient) of the parameter that accounted for the most variation in mimetic production for each species was calculated based on among and within male variance components derived from a one-way ANOVA (Lessells & Boag 1987
). Unless stated otherwise, all analyses were carried out using JMP (v. 7).