On April 29, 1999, Joel Gieger wrote to Oregon Birder's Online (OBOL):

>Another unusual feeder bird is a blond male HOUSE FINCH.
>We are used to seeing one or two apricot-colored males, but
>this one is a real straw-colored blond, every place where
>it should be red. I vaguely recall seeing some mention of such
>birds on OBOL in past weeks ... is this coloration supposed to
>be a dietary effect? The blond finch seems healthy despite its
>wan coloring.

The quick and easy answer that is usually given is: Yes, this is a dietary
effect. But the long answer is: maybe, maybe not.

What we do know is birds do not "make" the pigment, carotenoids, that cause
the red to yellow colors in feathers. They have to get these chemicals from
their diets. We also know that female House Finches prefer to mate with red
males over orange or yellow males. We also know the color differences are
not related to genetic factors: yellow male can molt into red males and vice
versa.

It is generally assumed that yellow House Finches eat a diet that is low in
carotenoids and the red House Finches eat a diet high in carotenoids.
Therefore, the red color is "signal of status" to female House Finches;
redder males are dominant over yellow males and will sire offspring at are
higher in status than yellow males. Or the red males are better at getting
food than the yellow males and will make better providers.

However, these assumptions have been questioned by several researchers. A
competing hypothesis is the color is a signal of male health. Males free of
parasites and disease may be better at converting the dietary carotenoids
into red pigment than unhealthy males. Therefore, the red males are more
"healthy" than yellow males and may have better immune systems.

A recent study, published in the Auk, January 1999 (pages 35-44) gives
support to this second hypothesis. Zahn and Rothstein looked at the
relationship between avian pox and plumage color in male House Finches. A
previous study (Thompson, et al. 1997) found a correlation between high
parasite load during molt and orange to yellow plumage. They also found
that House Finches with avian pox also tended to be orange or yellow. Red
males were relatively parasite and disease free. Zahn and Rothstein looked
at the relationship between avian pox and plumage color using historical
records as well as recent data.

Avian pox has only recently been introduced into the House Finch
population; it was first reported in California in 1972. It is mainly
restricted to the western United States. It is a viral disease that is
transmitted by biting flies; it can also be transmitted from one infected
bird to another via shared perches and feeding areas. Avian pox causes
swelling, particularly around the eyes and on the legs. If the bird
survives the infection, it can be left with missing toes or oddly-shaped
bills which last the rest of the birds life.

Zahn and Rothstein hypothesized that the incidence of avian pox and
yellow-orange coloration would increase following 1972. They looked at
museum skins in various California museum collections and measured the
incidence of avian pox (missing toes or oddly-shaped bills) in two groups of
male House Finches, those collected before 1950 and those collected after
1960. They also classified the plumage color of each bird as either red,
orange, or yellow. Prior to 1950, 76.6 % of males were red; after 1960,
51.3% of males were red. There was also a shift in the non-red birds.
Prior to 1950, only 4.3% of the non-red birds were yellow; after 1960, 26.3%
of the non-red birds were yellow. So the number of non-red birds are
increasing and the color is fading. There has also been an increase in the
incidence of avian pox. The 94 birds collected before 1950 showed no sign
of avian pox; 12 of the 78 birds collected after 1960 showed signs of avian
pox (missing toes). These birds were evenly split between red and
orange-colored birds. However, these birds could have been collected
several molts after the pox infection and could have been healthy (pox-free)
during their last molt.

Zahn and Rothstein also looked at recent populations for different areas.
Avian pox is relatively rare in the eastern United States. Recent banding
studies of House Finches show populations differ in the proportion of red
males. Two populations from California showed very high incidence of orange
and yellow males. In Pasadena, CA, only 12% of the males were red and 16.1%
were yellow; Santa Barbara, 29.4% of males were red and 14.6%. The studies
in the eastern U.S. showed a higher incidence of red males. In two New
Jersey studies, 93% and 87.5% of males were red while only 2.5% and 0.7%
were yellow.

The authors seem to think this correlation between avian pox and plumage
color proves convincing evidence that plumage color is a signal of a male's
health and immunity to disease. However, I still have a few problems with
this study.

First, the evidence is still correlational. Just because avian pox and
plumage color is correlated doesn't mean that avian pox causes the plumage
color. We still don't know the mechanism birds use to convert dietary
carotenoids to red feather pigment and then deposit those pigments into the
feathers. We also don't know how the virus interferes with this process.

Second, the authors acknowledge the shift from red males to more orange
males occurs at the same time bird feeding became more popular. In fact,
their most yellow population is found around a bird feeder. They use this
to support the idea that pox is causing the color shift because feeder
increase the density of House finches and increases the likelihood the
disease would be passed from one bird to another. However, they dismiss the
idea that the shift might be due to a shift in diet from a natural, varied
diet to a diet dominated by sunflower seeds and other seed that might not be
as high in carotenoids. They also assume that birds from the same flock eat
the same food and should show the same plumage color in the absence of
disease. The data from the eastern U.S. (where bird feeders are probably
common) suggests dietary deficiency isn't the cause (since orange-yellow
males are less common there) but doesn't rule it out. I would like to see
an experiment to rule out the dietary deficiency.

The biggest problem I have is with the delay between molt and the time the
females pick a mate. The molt occurs during October, while pairing
generally occurs in the spring. If females are using plumage color is a
signal of the overall health of the male, the plumage color may be
misleading. It is telling the female about the health of the male several
months ago, not during the spring when she is making her choice. Perfectly
healthy males may have yellow feathers because they just happened to have a
bout of pox months ago. A currently infected male (which could infect the
female during courtship) might have beautiful, bright red feathers.
Ideally, the signal should be more current. The only way this delayed
signal would work and be maintained in the population is if the plumage
color was a signal of some long-term, genetic resistance to pox. I would
like to see more research (perhaps in captivity) into whether there is a
genetic resistance to avian pox (and parasites) that can be passed only the
offspring.

So, the red to yellow continuum in plumage color of House Finches may be
caused by a dietary deficiency or by an infection of parasites and disease.
More research is needed into the relationship between diet, disease and
plumage color.

And you thought you asked a simple question.

Even though the initial question was posted on OBOL, I am posting this
message to three different lists (Tweeters, OBOL, and Inland Northwest
Birders) because the issue of yellow House Finches has been discussed on all
three lists. I apologized to those people (including me) who will receive
multiple copies of this post. I also apologize to those on OBOL I may
offend because this is a post on a bird that isn't rare (and, therefore, may
violate the purpose of OBOL). (Please don't start that thread again,
however.)

The full citation for the article I discussed is:
Zahn, S. N., and S. I. Rothstein. 1999. Recent increase in male House
Finch plumage variation and its possible relationship to avian pox disease.
Auk 116(1):35-44.

The article cited above was:
Thompson, C. W., N. Hillgarth, M. Leu, and H.E. McClure. 1997. High
parasite load in House Finches (Carpodacus mexicanus) is correlated with
reduced expression of a sexually selected trait. American Naturalist
19:270-294.

Other articles of interested (cited in the Zahn and Rothstein paper):
Belthoff, J.R., A. M. Dufty, Jr., and S. A. Gautheaux, Jr. 1994. Plumage
variation, plasma steroids and social dominance in male House Finches.
Condor 96:614-625.

Brush, A. H., and D. M. Power. 1976. House Finch pigmentation: carotenoid
metabolism and effect of diet. Auk 93:725-739.

Hamilton, W. D., and M. Zuk. 1982. Heritable true fitness and bright
birds: a role for parasites? Science 218:384-387.

Hill, G. E. 1990. Female House finches prefer colorful males: sexual
selection for a condition-dependent trait. Animal Behavior 40:563-572.

Hill, G. E. 1991. Plumage colouration is a sexually selected indicator of
male quality. Nature 350:337-339.

Hill, G. E. 1992. Proximate basis of variation in carotenoid pigmentation
in male House Finches. Auk 109:1-12.

Hill, G. E. 1993. Male mate choice and the evolution of plumage coloration
in the House Finch. Evolution 47:1515-1525.


Deb Beutler
Dept. of Zoology
Washington State University
Pullman, Whitman Co., WA

dbeutler at wsunix.wsu.edu
-----Original Message-----
From: Joel Geier <jgeier at ibm.net>
To: Multiple recipients of list OBOL <OBOL at MAIL.ORST.EDU>
Date: Monday, April 19, 1999 6:15 AM
Subject: Slate-colored Junco, Blond House Finch, & Yellowthroats


>Hello again,
>
>To add the list of late-staying(?) SLATE-COLORED JUNCOS, a
>female was still visiting a feeder atop our porch roof as
>of 4/18.
>
>Another unusual feeder bird is a blond male HOUSE FINCH.
>We are used to seeing one or two apricot-colored males, but
>this one is a real straw-colored blond, every place where
>it should be red. I vaguely recall seeing some mention of such
>birds on OBOL in past weeks ... is this coloration supposed to
>be a dietary effect? The blond finch seems healthy despite its
>wan coloring. It had me confused the first time I saw it singing
>from atop a tall Douglas fir, as from a distance it looks
>somewhat like a female Purple Finch.
>
>A few of the lingering GOLDEN-CROWNED SPARROWS are very scruffy
>looking, but most are wearing very bright spring plumage.
>
>Our first COMMON YELLOWTHROAT was singing on the evening of Apr 17.
>By Apr 18 at least 3 were present around our yard, along with
>ORANGE-CROWNED WARBLERS and about a dozen YELLOW-RUMPED WARBLERS
>(mostly Audubon's, a few Myrtles), and what seems to be an increased
>number of RUBY-CROWNED KINGLET. The apple, pear, and cherry trees
>are now all in full bloom and the male RUFOUS HUMMERS are displaying
>like mad. We're still waiting for the Vaux's Swifts to return to
>our chimney.
>
>Happy ongoing spring,
>Joel
>--
>Joel Geier
>Suver area, SE Polk Co.
>jgeier at ibm.net
>