Figure 2.

Song complexity through time including three song lineages—Black, Blue and Red. Complexity scores were computed from three song measures: # themes, # unit types and duration of each song. Box plots represent all songs in each year to show the variability in scores per year. 2004 had two song types present: the Blue song type was recorded at the start of the season (2004a: Blue box plot); all other recordings that year were of the Dark Red revolutionary song type (2004b: Dark Red box plot). Statistically significant (*p < 0.05, **p < 0.01) complexity scores (Kruskal–Wallis with Bonferonni correction for multiple comparisons) not included in the figure for ease of viewing include (electronic supplementary material, table S1): 1998 versus 2002*, 1998 versus 2003**, 1999 versus 2004b**, 1999 versus 2005*, 2000 versus 2003**, 2003 versus 2005** and 2003 versus 2006**.