Recent Increase in Male House Finch Plumage Variation and Its Possible Relationship to Avian Pox Disease

Male House Finches (Carpodacus mexicanus) in southern California show a large range of plumage variation, with individuals varying along a continuum from yellow to orange to red. This extreme variation is largely unrelated to age and is likely to have fitness consequences (e.g. Hill 1990, 1991; Belthoff 1994; Thompson et al. 1997). New analyses based on museum specimens, a literature review, and recently captured individuals indicate that the high level of variation is a new phenomenon because most males were red prior to the 1950s, whereas half or more are currently orange or yellow. The change toward orange and yellow plumage may be related to a high incidence of avian pox that was first noted in the early 1970s but probably began earlier. In addition to a temporal link between pox and plum- age variation in California, there is also a geographic link. Pox is very common in Hawaii, where most males are orange or yellow, but rare or absent on San Nicolas Island off the coast of southern California and in eastern North America. At least 90% of the males are red in each of these two latter areas. The link between pox and plumage color may occur as an effect of physiological condition, pathogen virulence, or host resistance among populations,