FIGURE 2. Distribution of beak shapes in passerine birds, illustrating the tremendous diversification of morphology in Hawaiian honeycreepers.

a, Graph showing variation in beak morphology in a sample of passerine birds worldwide. Variation is represented by scores on the first two axes of a principal components analysis (which derives uncorrelated axes of variation as linear combinations of the original variables). Towards the bottom right, beaks are short and stout; towards the top left, they are long and slender. Mainland cardueline finches have diversified primarily in bill width and depth, retaining the basic finch-like beak shape, whereas the Hawaiian honeycreepers have also diversified in bill length. (Data from ref. 35.) b, Adaptive radiation in Hawaiian honeycreepers, showing how beak shapes are adapted for particular food types. (Panel b reproduced, with permission, from ref. 74; courtesy of D. Pratt (North Carolina Museum of Natural Sciences, Raleigh).)