Female zebra finches perceive male beak colours in just two discrete colour categories, red and orange, according to a study published online in Nature this week. The findings may represent the first known demonstration of categorical colour perception in a bird, adding to our understanding of avian colour perception and signal evolution more generally.
Categorical perception is a mechanism by which a receiver’s perceptual system processes continuously varying stimuli into discrete categories, discriminating between variants on different sides of a perceptual boundary but not between those within the same category. In male zebra finches, beak colours range from light orange to dark red. Females show a mating preference for males with red versus orange beaks, and beak redness is positively correlated with variation in cell-mediated immunity. It has remained uncertain, however, whether females perceive this variation continuously or exhibit categorical perception.
Stephen Nowicki and colleagues used a food-reward protocol to test for categorization and discrimination of eight stimuli spanning the orange-red colour spectrum in female zebra finches. The authors created paper discs comprising either a single, solid colour or two halves, each containing a different colour. Female birds were trained to flip bicolour discs first before any solid discs, and they learned to recognize bicolour versus solid discs rather than particular colour combinations. The findings suggest that although birds did not perceive variants within a colour category as identical, discrimination increased most sharply across a category boundary. The authors also show that this categorization could not be explained by stimulus brightness alone, and that the categories probably did not arise as a result of avian photoreceptor sensitivity.