The Wilson Journal of Ornithology 124(1): 139 145. 2012 
FLANGE COLOR DIFFERENCES OF BROOD PARASITIC 
BROWN-HEADED COWBIRDS FROM NESTS OF TWO HOST SPECIES 
REBECCA CROSTON.' 7 K CHRISTOPHER M. TONRA, 2-5 
SACHA K. HEATH, 2,36 AND MARK E. HAUBER 14 
ABSTRACT—We compared the red. green, and blue color values from digital photographs of the rictal flanges of 
nestling Brown-headed Cow-birds (Molothrus ater). a generalist obligate brood parasite, in sympatric Yellow Warbler 
iSetopliagapetechia) and Song Sparrow (Meloxpizu mclojial nests at Mono Lake. C ulifornia. USA. We detected significant 
differences in all three color components across nestlings of different species (R: P < U.0001: G: P < 0.00131. B . P < 
0.0001). hut differences among cowbird nestlings from the nests of these two hosts were not significant (R: P — 0.543. G. 
P = 0.737: B: P = 0.319). Principal components results were mixed: Principal Component I described brightness and 
accounted for 84% of the variance. It did not differ among cowbird nestlings front nests of different hosts (P — 0.319). 
Principal Component IT described chromaiicity and accounted for \4% of the variance, which differed significantly among 
cowbird nestlings from the two different hosts' nests I P = 0.026). Color differences between cowbird nestlings from nests 
of different host species may result from selective parasitism by female parasites based on host nestling flange morphology, 
or ontogenetic effects on cowbird nestlings reared by different host species. Received 25 January 2011. Accepted 21 July 
2011 . 
Evidence of recognition and discrimination of 
parasitic nestlings is relatively rare among hosts 
of avian brood parasite species (Redondo 1993; 
Grim ct al. 2003; Langmore et al. 2003. 2009; 
Schuetz 2005b; Sato et al. 2010; Shizuka and 
Lyon 2010). Patterns of parasite chick's visual 
and/or acoustic similarity of host nestlings in a 
handful of brood parasite lineages ( Anderson et al. 
2009, Sato et al, 2010, Langmore cl al. 2011) 
imply mimicry to avoid rejection (Langmore ct al. 
2003, Payne 2005, Tokue and Ueda 2010, 
Langmore et al. 2011). Hosts may discriminate 
not only by directly rejecting foreign nestlings, 
but by providing belter care or higher quality prey 
(Schuetz 2005a. Solcr 2008) for nestlings with 
particular attributes (Rothstein 1978, Lichtenstein 
2001. Dugas 2009), resulting in variation among 
nestlings in growth rale and condition (Hauher 
Graduate Program in Biology. Ecology. Evolutionary 
biology and Behavior Subprogram, Graduate Center. City 
University of New York, NY 10016. USA 
Department of Wildlife. Humboldt Slate University, 
Areata. CA 95521. USA. 
PR BO Conservation Science. Petaluma, CA 94954, 
USA. 
'Department of Psychology, Hunter College. City 
University of New York, NY 10065, USA. 
Current address: Smithsonian Conservation Biology 
Institute. National Zoological Park. P. O. Box 37012-MRC 
5503, Washington, D C. 20013. USA. 
Current address: Department of Wildlife. Humboldt 
Stale University. Areata. C'A 95521. L SA. 
Department of Psychology. Hunter College. 695 Park 
Avenue. New York. NY 10065. USA. 
Corresponding author; e-mail: RCroston@gc.cuny.edu 
and Kilner 2007). Nestling discrimination among 
cowbird hosts has been documented for Rufous- 
bellied Thrushes ( Turdus rufiventris ) parasitized 
by the non-evicting generalist Shiny Cowbird 
(Molothrus bonuriensis ) (Lichtenstein 2001), but 
is not yet known to occur in any hosts of the 
Brown-headed Cowbird (M. titer). 
Hosts may recognize parasitic nestlings using 
variation in size, color, vocalization, brood size, 
and length of time before fledging (Langmore 
et al. 2003. Schuetz 2005b, Grim 2007). Variable 
coloration of both gapes and rictal flanges may 
have a signaling function, conveying nestling 
identity, need, health or other indicators of quality 
(Thorogood et al. 2008, Dugas 2010), which 
would need to be matched by parasitic nestlings 
(Nicolai 1974, Payne 2005, Hauber and Kilner 
2007). Flange color is typically monomorphic 
within species, but Brown-headed Cowbird nest¬ 
lings appear polymorphic across the species so 
that a nestling has either distinctly yellow or white 
flanges with lew intermediates (Rothstein 1978). 
Polymorphic flange color occurs in only two 
phylogenetically distant New World oscine gen¬ 
era. Geospiza and Molothrus. It is plausible this 
polymorphism in cowbirds is the outcome ot 
selection for preferential parasitism of certain host 
species by female cowbirds to match the host- 
specific flange color by the parasitic nestling 
(Ellison et al. 2007). 
Alternatively, the differences in human-per¬ 
ceived flange phenotype ol cowbird nestlings 
may not result from genetic polymorphism, but 
from differences in carotenoid consumption and 
139 
