Figure 3.

Beak size variation in samples of more than 100 G. fortis. Shown are frequency histograms of PC1 (larger values correspond to larger beaks), with the black portions of bars representing mature males and the grey portions representing all other birds. Panels are labelled according to sampling site (Academy Bay, AB; El Garrapatero, EG; Borrero Bay, BB) and year. The 1973 Borrero Bay histogram is not shown here because of limited space, but is provided in electronic supplementary material D. Arrows show discontinuities between the small beak and large beak modes in the samples statistically confirmed to have strong bimodality (see figure 4). x-axes are not comparable across panels because PC1 was calculated within each sample.