Monthly observations of frequency shift.

<p>Markers show the ratio of measured to ‘true’ frequency of Antarctic blue whale song. Measured frequency and ‘true’ frequency are calculated from the data from (Gavrilov et al. 2012) and monthly means are pooled from 9 years of acoustic observations (blue dots). The Antarctic recording stations Kerguelen (red triangle), and Casey (green circle) comprise 2 and 1 years of acoustic observations. Error bars show the monthly standard deviation. The black line connects the monthly mean of all observations from all of the long-term recording stations. The yellow star shows the mean of the vessel-based measurements with error bars denoting one standard deviation (note that error bars for the vessel-based observations extend well beyond the range of the vertical axis for this figure).