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

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

Besides attacking our methodology and advancing the primacy of his own methods, Hill (2001) presented data that he claims are counter to our findings. The data in Hill's Figure 1 have all been presented before. Hill categorizes his study sites as with and without pox, but presents no data on pox. He argues that birds from pox-free sites are not consistently brighter than birds exposed to pox. Because Hill's composite plumage score confounds three variables (whereas we dealt with just one-hue), comparisons between our data and his are difficult to interpret. For similar reasons, Hill's failure to find greater plumage color-score variation at pox sites is not easily interpreted. It is clear that there is a strong association between pox and increased variation when color is represented by hue only, as in our data. We tested for increased variation in recent years in two ways. First, we used a simple hue scoring system with yellow, orange, and red equaling 1, 2, and 3 respectively, which follows the system of Thompson et al. (1997) except that they called the intermediate category "mixed red and yellow/gold" instead of orange. With that system, our pre-1951 museum series for southern California had a mean score of 2.725 and a variance of 0.214. The post-1959 sample had a mean of 2.385 and with a variance of 0.499, was significantly more variable as the F ratio is 2.332 (P

Even if Hill had quantified plumage color in a more interpretable way relative to our assessment of hue, the data in his Figure 1 would have little value in assessing validity of our results. As shown by our data for Santa Barbara County (Zahn and Rothstein 1999), there is considerable spatial and temporal heterogeneity in plumage color. Among our four sites, one had red males at 52.8% (n = 36 males) and another at only 3.2% (n = 31) in 1994. At the latter site in 1995, red males were at 23.1% (n = 13). The data we used in our temporal and spatial assessments came from numerous sites. All samples were collected over two or more years and many involved hundreds or thousands of birds. By contrast, Hill's data for all samples in his Figure 1 other than Michigan were each collected in a single month at a single site (two nearby sites in the case of New York) and had n values of only 7 to 81 males. The bottom line is that one can not use a single small sample collected over a short time period in one season to categorize the coloration of House Finches within a region. Ironically, the charge that Hill applied to our museum data, namely that our samples have little global validity because they are clumped in time and space, applies instead to his own data.

Lastly, regarding Hill's data, we address his suggestion that because males at San Jose (a purported pox area) were as red as ones from the East (where pox is rare or absent), the link between pox and color is weakened. Other problems aside (such as Hill's color scoring scheme and the lack of global validity), we note that our data on temporal trends deal solely with southern California and none of our macrogeographic comparisons involve northern California, where San Jose is located. Because northern and southern California differ in many ways, we cannot assess Hill's data. We do not know the incidence of pox in northern California nor the degree of color variation and Hill's sparse data are of little help here.