House Finches are not just what they eat: A reply to Hill

Auk, The, Jan 2001 by Zahn, Sherie N, Rothstein, Stephen I

In attacking our paper, Hill not only criticized our methodology, he also touted his methods as superior. Unfortunately, Hill has misrepresented the implications of his scheme for quantification of plumage coloration, and that makes it difficult for others to be certain of the hue of birds he has studied. Hill (1993b) assigns a composite plumage score to each male by summing values based on three characteristics: hue (the red-orange-yellow continuum), chroma (degree of color saturation, such that pink is a low chroma red), and tone (total reflectance). Because those birds vary in all three characteristics (Hill 1998), that method results in one numeric score that represents three distinct variables. Hill equates high composite color scores with increased brightness and redness. Although there may be a correlation here, highly chromatic orange males can have higher scores than low chroma red ones. Furthermore, brightness is a vague term that relates to both chroma, and tone, but not to hue, yet Hill stresses hue when he equates high color scores with red. Hill's (1998) own data on spectrophotometer output show weaknesses in his composite scores based on his visual assessments of hue, chroma and tone (as used in all of his prior papers). Hill (1998) reported that only his hue and chroma values were correlated with readings from the spectrophotometer. Although he pointed out that tone contributes less numerically to his composite color score than do hue and chroma, Hill's (1998) results show that his composite scores have a component-tone-that adds noise and is essentially a random variable. In recognition of that, Hill (1998) stated that "I find it relatively easy to assign a hue score to patches of feathers, more difficult to assign a saturation [chroma] score, and very tough to assign a tone score." Hill's own data and perceptions thus agree with the near unanimity of our 22 judges and because they validate the reliability of color categorizations based on hue, they validate our methodology for categorizing colors.

We chose to focus on only one aspect of coloration-hue-because it is unknown how House Finches integrate hue, chroma, and tone and it is not even clear if those three axes of color variation are completely meaningful to birds given that the Munsell system is based on human perception. In addition, we knew that the historical data on color and recent data collected by other workers, all of which categorized birds as red, orange, or yellow, were based on hue alone. We do not dispute Hill's general findings that female House Finches prefer males with high composite color scores under his scheme, but it is not clear just what is important to females until Hill analyzes the effects of hue, chroma, and tone separately. Thus, conclusions by Hill such as "it appeared to be the red pigmentation of males and not a correlated character that the female House Finches were choosing" (1990) may be invalid. Hill's attempts to determine the cues females use for mate choice are further complicated by the fact that humans and birds perceive color differently and only the latter perceive ultraviolet light, as acknowledged by Hill (1998). Yet even Hill's (1998) spectrophotometric methodology does not involve UV reflectance, which may be a problem because yellow bird plumages often show some reflectance in the UV range (J. Endler pers. comm.). Perceptual differences between humans and birds do not affect our paper (Zahn and Rothstein 1999), as we focussed on differences in plumage reflectance with no assertion as to whether those differences are important to birds.