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

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

Hill refers to our Discussion section as "Perhaps the weakest part of the paper . . . " In that section, we argued that differential diet uptake of carotenoids is unlikely as a complete explanation for plumage color variation in House Finches. Instead, we recognized that carotenoid pigments in birds, or the precursors of those pigments, must come from the diet but suggested that pigments are not likely to be limiting in nature. We argued that variation in color is more likely to be related to ability to use ingested carotenoids. We further suggested that pox, either through direct effects on uptake of carotenoids (such as through pathogenic effects on the intestine), and general factors that reduce a bird's condition (e.g. diseases, ectoparasites) are the primary factors responsible for a bird's failure to become red. Hill strongly attacked our suggestions and in doing so gave insufficient weight to mounting evidence concerning carotenoid metabolism in birds (Olson and Owens 1998) and undue importance to his own feeding experiments with captive birds (Hill 1992).

We need not review the evidence concerning carotenoid metabolism, other than to state that it is widely recognized that carotenoids are naturally abundant in plants and that there are links between an animal's ability to use carotenoids and its condition (Hudon 1994, Olson and Owens 1998). For House Finches in particular, Thompson et al. (1997) showed that birds afflicted with pox during molt are more likely to grow non-red feathers than birds not afflicted. The latter study is clearly applicable to our findings indicating spatial and temporal links between pox and was done in our primary area of focus, southern California. Besides pox, Thompson et al. (1997) found that intense mite infestations during molt were also related to a decreased likelihood of growing red plumage. Hill briefly acknowledges that Thompson et al. (1997) showed that pox affects color, but, instead of admitting that this supports the conclusions in our paper, he argues that that effect does not mean that pox "is the primary or sole source of temporal or geographic variation" in plumage coloration. Of course, we never argued that it was. We merely argued that a bird's condition is likely to influence its coloration and that pox is one of a number of things that can depress condition.

In his early work (e.g. Hill 1992), Hill attributed all plumage color variation in House Finches to diet and differential foraging ability. Evidence for that viewpoint seems to come from three sources. As in a previous rebuttal (Hill 1994) to a critique of his differential foraging ability hypothesis by Hudon (1994), Hill (2001) cites the same single study (Slagsvold and Lifjeld 1985) showing that carotenoids are limiting for birds. But the species in that study is mainly a carnivore, whereas the House Finch is primarily herbivorous, and carotenoids are so widespread in plant matter that they may be limiting only for animals that are primarily carnivorous (Hudon 1994, Olson and Owens 1998). In a second line of putative evidence, bright male House Finches (under Hill's composite scoring scheme) provided more food for their offspring and therefore seemed to be better foragers than dull-colored males (Hill 1991). However, that result is consistent with both our condition hypothesis and Hill's foraging-ability hypothesis because birds in the best condition are likely to be the ones best able to feed both themselves and their offspring. In arguing for the importance of diet, Hill (2001) states that Hill and Montgomery (1994) "provided evidence that there are differences among males in access to nutritional resources during molt." The latter paper showed that bright males grow feathers more quickly and begin to molt earlier than dull males. Although that result is consistent with bright males being better foragers for all aspects of food, including carotenoids, it is also consistent with such males simply being in better condition as regards all factors affecting condition, including disease. In that paper, Hill and Montgomery (1994) stated that "reduced plumage brightness of males in the drab Alviso population is a result either of reduced access to carotenoid pigments or of reduced ability to metabolize carotenoids (e.g. due to parasites or poor health)." Hill and Montgomery's suggestion that disease may be important in limiting a finch's ability to metabolize carotenoids agrees with our general conclusion yet strangely it is not acknowledged in Hill's (2001) critique.