Comparison of categorical color perception in two Estrildid finches

Sensory systems are predicted to be adapted to the perception of important stimuli, such as signals used in communication. Prior work has shown that female zebra finches perceive the carotenoid-based orange-red coloration of male beaks—a mate choice signal—categorically. Specifically, females exhibited an increased ability to discriminate between colors from opposite sides of a perceptual category boundary than equally-different colors from the same side of the boundary. The Bengalese finch, an Estrildid finch related to the zebra finch, is black, brown and white, lacking carotenoid coloration. To explore the relationship between categorical color perception and signal use, we tested Bengalese finches using the same orange-red continuum as in zebra finches, and also tested how both species discriminated among colors differing systematically in hue and brightness. Unlike in zebra finches, we found no evidence of categorical perception of an orange-red continuum in Bengalese finches. Instead, we found that the combination of chromatic distance (hue difference) and Michelson contrast (difference in brightness) strongly correlated with color discrimination ability on all tested color pairs in Bengalese finches. The pattern was different in zebra finches: this strong correlation held only when discriminating between colors from different categories, but not when discriminating between colors from within the same category. These experiments suggest that categorical perception is not a universal feature of avian, or even Estrildid finch, vision. Our findings also provide further insights into the mechanism underlying categorical perception and are consistent with the hypothesis that categorical perception is adapted for signal perception.

Methods
These behavioral data were collected by EMC, PAG, and DB. The data are from a behavioral assay in which birds were provided with a foraging grid with 12 wells. On top of six of the wells, we placed colored circular discs that were either the same color on both halves (solid) or different colors on each half (bicolor): two bicolor discs were used in each trial alongside two solid discs of each of the colors comprising the bicolor discs. We trained birds to search for food rewards beneath discs that they perceived as bicolor. A bird passed a trial if it flipped both bicolor discs before flipping any of the solid discs. The data archived here have been processed to show the mean "pass frequency," i.e. the proportion of trials out of 10 in which birds flipped both bicolor discs before flipping any solid discs. Some of the data files show pass frequency for each bird for each color combination; others show the mean pass frequency across birds (i.e. all zebra finches averaged together) for a given color combination.

The data also contain  the bird ID (a unique identifier for each bird), the number ID of the colors on each half of the disc (col1 and col2), and the chromatic distance and contrast for the color combination in that trial. Chromatic distance was calculated using the Receptor Noise-Limited model (see main text for details). Contrast is Michelson contrast (see main text for details).