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

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

The coevolutionary interactions of pathogens and their hosts are likely to be a widespread mechanism that results in the maintenance of genetic variation. Alternatively, highly variable species may be in a transient state, with their variation reflecting directional selection and new selection pressures. With those insights in mind, we set about to study the House Finch (Carpodacus mexicanus), some of whose populations are arguably the most variable among North American birds with regard to plumage coloration in males of the same age. In addition, we were also attracted to House Finches by our observations and those of others (McClure 1989, Power and Human 1976) that this species is highly unusual not only for its color variation, but for its remarkably high incidence of disease, particularly avian pox, which of course raised the question of whether pathogens and plumage color might be related. Lastly the possibility of recent changes in disease incidence was raised by the first published report (Power and Human 1976) of pox disease in mainland populations of this common species, which reported a severe outbreak in 1972. Accordingly we set out to determine whether there is any evidence of a link between plumage color variation and pox and whether extreme variation in color and high pox-incidence might be new conditions.

To address our goals, we collected new data through our own field efforts, studied specimens in museums, reviewed available literature, and amassed data from bird banders over a wide part of the species' current range. Although they do not establish causation, our straightforward data and analyses show strong temporal and spatial links between pox and plumage coloration and add potential new insights to work on this interesting species. We found the following major results: birds in southern mainland California had a much lower incidence of pox disease in the first half of the twentieth century than in the second half; over the same period, red coloration has gone from being characteristic of more than three-fourths of southern California males to a much lower incidence today, with orange and yellow males having become much more common. At present, there are strong macrogeographic associations between high pox incidence and high plumage color variation (Zahn and Rothstein 1999).

Hill's (1990, 1991, 1992, 1993a, b, 1994a, b, c) past work has produced much of what was previously known about plumage color variation and its consequences in this species. Unfortunately Hill's critique of our paper adds confusion, but no new insights, to this interesting system. His frenetic attempt to discredit virtually every aspect of our paper is totally unconvincing, contradicts itself, and highlights serious weaknesses in his own work. The putative major problems that Hill alleges deal with the following: our methodology of representing plumage coloration; our temporal criteria for separating samples in analyzing possible historical changes in pox incidence and plumage coloration; our criteria for analyzing possible patterns in present day macrogeographic variation in plumage color and pox. In addition, Hill misrepresents our paper by alleging that we made statements and conclusions that in fact do not appear in our paper.

We address the misrepresentations first. Hill (2001) repeatedly argues that we concluded that pox disease is the "singular cause" for plumage variation or "the primary or sole source of temporal or geographic variation in male plumage coloration." In fact, we were careful to never state our conclusions in such absolute terms. For example, our abstract states that high incidences of yellow and orange males "may be related to a high incidence of avian pox" (Zahn and Rothstein 1999). No where does our paper state that pox is the only or major determinant of color variation. We would suggest that Hill did nothing further than read the title to our paper were it not for the fact that even it states there is a "possible relationship" between pox and plumage variation.

Another misrepresentation is Hill's supposed confusion over our suggestion that "the high level of variation [in plumage coloration of male House Finches] is a new phenomenon". Hill suggests that maybe we meant that "there were few or no yellow or orange males" (Hill 2001) in the early part of the last century, a suggestion that he then rebuts. This is a pointless discussion by Hill because we never stated that "there were few or no yellow or orange males." Instead, we clearly stressed that early workers, such as Michener and Michener (1931), described the yellow to red range of colors that exists today. Furthermore, our Figure 1 and associated text (Zahn and Rothstein 1999) show that 23.3% of males in a museum sample from the early 1900s were yellow or orange. To further confuse matters, Hill later acknowledges that we recognized that there have always been reports of yellow and orange males and even quotes us as stating "Non-red variants existed historically. . . " So it is unclear why Hill raised that red herring in the first place.