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

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

The third apparent reason for Hill's defense of the importance of diet deals with his feeding experiments, which do indeed demonstrate a clear effect of diet on the coloration of captive House Finches. In those experiments, Hill showed that birds fed a special diet deficient in carotenoids molted into dull plumage. Birds fed the same diet but given a red carotenoid, canthaxanthin, grew bright red plumage. The first of those results was completely predictable, because all workers agree that carotenoid pigments in animals must come from dietary intake (see discussions in Hudon 1994 and Zahn and Rothstein 1999). The second result has no bearing on what occurs in nature as canthaxanthin is not present in the finches' diets nor is it the pigment responsible for their red color. Those results show only that people can control bird coloration by feeding them unnatural diets, a trick long known to zoo keepers. Those results do not show that the range of naturally occurring diets controls or even influences coloration. Even the first workers to use diet to manipulate coloration in captive House Finches, concluded that the dietary intake of carotenoids is necessary but "not completely sufficient to explain color variation in native birds" (Brush and Power 1976).

In questioning the primacy of diet, we noted that there were no major plant perturbations in California in the mid-1900s when House Finch coloration apparently shifted. Hill (2001) retorted that there were "massive changes in the biota starting around the turn of the century." In fact, the major changes to California's flora that have displaced native plants that dominate the open habitats used by House Finches took place by the mid-1800s (Mensing 1998). Indeed, changes to open habitats occurred so early after the European colonization that there is even considerable controversy concerning the original nature of those habitats (Hamilton 1997).

Remarkably, after criticizing us for doubting his early diet-as-key-factor hypothesis, Hill's (2001) critique acknowledges "that a variety of factors combine to determine expression of carotenoid-based coloration" and that "degree of parasitism" is one of these factors. So Hill is free to modify his hypothesis, but without admitting that his initial diet hypothesis was overly simplistic, whereas we are not. Furthermore, the bottom line of Hill's critique is that after all the disparagement of our methods, logic, and data, he comes to the same general conclusion we reached, namely there is a "possible relationship" between pox and coloration after all.

Acknowledgments.-We thank John Endler and Kathleen Whitney for their valuable comments on this manuscript.

LITERATURE CITED

BRUSH, A. H., AND D. M. POWER. 1976. House Finch pigmentation: Carotenoid metabolism and the effect of diet. Auk 93:725-739.

GRINNELL, J. 1911. The Linnet of the Hawaiian Islands: A problem in speciation. University of California Publications in Zoology 7:179-195.

HAMILTON, J. G. 1997. Changing perceptions of preEuropean grasslands in California. Madrono 44: 311-333.