Leucogeranus leucogeranus
We found that, in general, vocal development in Siberian Crane chicks goes similar to other studied crane species (Niemeier 1979; Klenova et al. 2010, 2014): in autumn, chicks produced mainly calls with the juvenile high funda- mental frequency component only (Fig. 1a; Online Resource 1), in winter or early spring they started to produce calls with both juvenile high and adult low fundamental fre- quency components (Fig. 1b–d; Online Resource 2) and in the end of spring or in summer they produced mainly calls with the adult low fundamental frequency component only (Fig. 1e; Online Resource 3). The ages of onset, culmina- tion and completion of the voice breaking varied greatly for different Siberian Crane chicks, but on the average came to 7.7 ± 1.8, 9.2 ± 1.7 and 12 ± 2 months correspondingly (Table 3). Durations of the voice breaking and its first phase were 5.3 ± 1.8 and 1.7 ± 1.5 months correspondingly. Our sample did not allow us to compare the parameters of voice breaking in males and females statistically due to the small number of females among the studied cranes; however, in general, we did not notice any signs of sexual differences in our study species (Table 3).
Although the chicks’ mean percentages of high and low frequency components changed considerably with age, we did not find significant differences in most comparisons (Mann–Whitney U-test, p > 0.05; Fig. 3c). From 2.5 to 10.5 months, the mean percentage of the high frequency component decreased slowly, while the mean percentage of the low frequency component increased slowly (Fig. 3c). Statistically significant changes occurred only between 10.5 and 12.5 months both for mean percentages of high and low frequency components (70.2 ± 13.4% at 10.5 months, 14.0 ± 9.6% at 12.5 months for Mean_Perc_high and 52.4 ± 12.5% at 10.5 months, 92.7 ± 7.0% at 12.5 months for Mean_Perc_low). From 12.5 to 18 months, the mean per- centage of high frequency component slowly came to zero, while the mean percentage of low frequency—to 100%, which meant the completion of voice breaking in Siberian Cranes (Fig. 3c).
We did not find significant correlation between onset, cul- mination, completion, duration of the voice breaking and its first phase and a chick’s body mass at the age of 2.5, 4, 7.5 and 9.5 months and testosterone level at the age of 2.5, 5, 6.5, 7.5, 9.5 and 10.5 months (Table 2). We found only a weak negative correlation between body mass at 9.5 months and completion of voice breaking, and a weak positive correlation between body mass at 4 months and duration of the first phase of voice breaking (Table 2), but the relationship became insignificant after sequential Bon- ferroni correction. However, we found significant correla- tion between the mean deltas of the high and low frequency components between the closest recording sessions and tes- tosterone level (for ΔMean_Perc_high: r = 0.29, p = 0.028, for ΔMean_Perc_low: r = 0.32, p = 0.016; Spearman corre- lation); these relationships also became insignificant after sequential Bonferroni correction. Similarly, GLMM showed a significant effect of “testosterone” for the mean deltas of each of the components between the closest recording ses- sions (ΔMean_Perc_f0 and ΔMean_Perc_g0) and a signifi- cant effect of “individual” and of conjoint factor “individ- ual with testosterone” for the mean delta of low-frequency component between the closest recording sessions (ΔMean_ Perc_g0; Table 4). A comparison of F-ratios showed that the effect of testosterone on voice-breaking variables was always stronger than the effect of the other factors (Table 4); these relationships remain significant after a sequential Bonferroni correction (Table 4).
Although we failed to find strong statistical relation between voice breaking parameters and body mass in Sibe- rian Cranes through the development, we noticed some tem- poral coincidence between them. Thus, the onset of voice breaking immediately followed the end of active body mass gain that occurred at 7.5 months (Fig. 3b). At the same time, the age of culmination of voice breaking and significant changes in mean percentages of the high and low frequency components showed an obvious relation to the change of testosterone level, which increased significantly between 9.5 and 10.5 months of life on average (Fig. 3a, c). We also observed this pattern at the individual level: for most chicks, significant changes in the mean percentages of the high and low frequency components in calls occurred along with a registered increase in testosterone levels (Fig. 4). [1]