23.2 Comparing two shearwater species (Audubon and Galápagos)

Figure 23.1 is an overview comparing two species on each of five variables. Numeric codes have been replaced with text labels and variable categories ordered for better interpretation. Vertical scales are the same, aiding comparison across graphics as well as within.

Figure 23.1: Barcharts of five variables comparing two species, with Audubon’s shearwater in brown and Galápagos shearwater in dark blue

The data are fairly balanced on sex. eyebrows are a distinguishing feature, with Galápagos shearwater having pronounced or very pronounced eyebrows. collar is also a distinguishing feature with only a few Audubon’s shearwater having no collar. The variable undertail gives mixed results, although the single Galápagos shearwater with a Black & WHITE undertail looks suspicious. border is similar for the two species with most birds having no border.

A way of looking at several variables together is a parallel coordinate plot, as in Figure 23.2. The variable border has been left out as it does not distinguish the species. The variable undertail has been reversed to reduce line crossings between the species. The separation of Audubon and Galápagos by collar and eyebrows is clear. The single unusual Galápagos case is emphasised, partly because it turns out to be one of the few birds with very pronounced eyebrows.

Figure 23.2: Parallel coordinate plot for Audubon and Galápagos shearwaters with three descriptive variables

A comparison of the species using collar and eyebrows is shown in Figure 23.3. Using both variables together distinguishes the two species except for one bird, an Audubon’s shearwater with no collar and pronounced eyebrows.

Figure 23.3: A comparison plot of Audubon and Galápagos shearwaters, using barcharts of collar faceted by eyebrows