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inl uence plumage colors, but, because color was not quantii ed objectively (instead assessed by eye)
and because the authors recognized that one of the carotenoids they used (canthaxanthin) is likely
not a part of the natural diet of house i nches, additional work was clearly needed. In interpreting
their results, Brush and Power (1976) gave equal attention to carotenoid metabolism in this process
and surmised that red pigments in house i nch feathers are synthesized from important dietary pre-
cursors that are lacking in the typical seed diet (see more below).
Hill (1992, 1993) then performed numerous, extensive follow-up studies on house i nch diet and
coloration, concurrent to his work on the mate-choice function of this plumage trait (Hill 1990,
1991, 1994). Male house i nches exhibit extensive variation in the size and color intensity of caro-
tenoid-based plumage patches among populations, and one of his new aims was to understand the
basis for such geographic variability. Interestingly, in captive diet experiments using canthaxanthin
supplementation, males from various populations (e.g., Michigan, New York, California, Hawaii)
converged on a similar color appearance (based on subjective quantitative assessments of color
when matched to standardized color chips) when fed the same diet, suggesting that variations in
dietary carotenoid intake among populations explains their different color expressions (Hill 1993).
The size of colorful patches in some populations, however, remained unchanged from their previ-
ous condition, which is consistent with the idea that the area of coloration is not as environmentally
sensitive and instead under stronger genetic control. Hill (1992) also examined the degree to which
factors like carotenoid storage or age were related to color expression. Regardless of whether birds
were captured from the wild just prior to molt or were fed a carotenoid-dei cient (plain seed) diet for
months prior to molt, male house i nches consistently grew drab plumage when fed a carotenoid-
dei cient diet at the time they were growing their feathers (Hill 1992); this stands as one of the better
experimental tests of the idea that carotenoid storage in internal tissues, such as liver or adipose,
contributes minimally to proximate color acquisition (but see McGraw et al. (2006b) and more
below). Hill (1992) also observed that i rst-year males in the wild tended to have less red plumage
than older adults, and this may be associated with a poor diet among young birds, though in this
correlational study other factors like health (see more below in this section) could not be ruled out.
With one or two exceptions (e.g., Slagsvold and Lifjeld 1985), this compilation of excellent stud-
ies stood for several years as our only means of understanding carotenoid color variation in non-
domestic birds (until work by Bortolotti and colleagues on American kestrels in the mid-1990s;
Bortolotti et al. 1996), but even so it still lacked a clear naturalistic context. Dietary carotenoids in
wild birds had never been studied, until Hill and colleagues undertook such an investigation of male
house i nches from California and Mexico (Hill et al. 2002). Naturally foraging male house i nches
that were in the process of growing their colorful feathers were captured, euthanized, and their gut
contents extracted and analyzed for total carotenoid content (HPLC was not used to identify indi-
vidual compounds). Hill et al. (2002) found that, regardless of age, there was a signii cant positive
correlation between the plumage redness of males from California and gut carotenoid concentration
(Figure 23.1). This relationship between diet and color had been previously established in guppies
(
Poecilia reticulata
; Grether et al. 1999), but it was an essential validation of theories of dietary
control for coloration in a wild bird.
One additional piece of the puzzle that was of interest to Hill and colleagues studying house
i nches was whether particular carotenoid types in the diets of house i nches were valuable for
acquiring red coloration. Inouye et al. (2001) used HPLC to describe the diversity of types of carote-
noids responsible for yellow and red coloration in house i nch feathers, and used likely biochemical
product-precursor relationships to hypothesize that a special pathway for acquiring red coloration
involved the metabolism of dietary b-cryptoxanthin (thought to be a rare dietary compound) into
red feather ketocarotenoids like 3-hydroxy-echinenone. For the i rst time in a lab experiment on
house i nches, Hill (2000) administered supplemental dietary b-cryptoxanthin (in the form of tan-
gerine juice) during molt and found that males occasionally developed red feathers. This hinted at
a very specii c, natural dietary control agent for coloration in this species. Since then, i eld studies
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