Dark Color of the Coastal Plain swamP sParrow 
(Melospiza georgiana nigrescens) may Be an evolutionary 
resPonse to oCCurrenCe anD aBunDanCe of salt-tolerant 
feather-DegraDing BaCilli in its Plumage
Resumen.?Melospiza georgiana georgiana cr?a en el noreste de Am?rica del Norte en pantanos montanos de agua dulce. Su 
pariente cercano, M. g. nigrescens, cr?a en el noreste de Am?rica del Norte, pero en pantanos costeros de agua salada. M. g. nigrescens 
es m?s oscuro que M. g. georgiana. Las plumas de colores m?s oscuros son m?s resistentes a la degradaci?n bacteriana por bacilos, que 
son inusualmente tolerantes a la sal. Evaluamos si las diferencias en los colores de las plumas del montano y p?lido M. g. georgiana y 
del oscuro M. g. nigrescens pueden ser una respuesta adaptativa a las diferencias en la presencia y actividad de los bacilos en ambientes 
que difieren en salinidad. Los individuos de M. g. georgiana fueron capturados y muestreados en pantanos de ar?ndano agrio en el 
oeste de Maryland, y los de M. g. nigrescens en marismas de la costa oeste del R?o Delaware, justo en donde se ensancha en la Bah?a de 
Delaware. El n?mero de aves que presentaban bacterias que degradan plumas en sus plumajes fue significativamente mayor entre las de 
las marismas que entre las de los pantanos de agua dulce. El n?mero de colonias de bacilos degradadores de plumas por ave fue tambi?n 
mayor en las aves de las marismas que en las de los pantanos de agua dulce. Concluimos que el plumaje oscuro de M. g. nigrescens 
evolucion? para resistir a la degradaci?n de las plumas por parte de los bacilos tolerantes a la sal que aparecen de modo m?s frecuente y 
abundante en su plumaje que en el plumaje p?lido de M. g. georgiana.
?  531  ?
The Auk 126(3):531?535, 2009
? The American Ornithologists? Union, 2009. 
Printed in USA.
The Auk, Vol. 126, Number 3, pages 531?535. ISSN 0004-8038, electronic ISSN 1938-4254. ? 2009 by The American Ornithologists? Union. All rights reserved. Please direct all 
requests for permission to photocopy or reproduce article content through the University of California Press?s Rights and Permissions website, http://www.ucpressjournals.
com/reprintInfo.asp. DOI: 10.1525/auk.2009.08142
El Color Oscuro de Melospiza georgiana nigrescens Puede Ser una Respuesta Evolutiva a la Presencia 
y Abundancia de Bacilos Tolerantes a la Sal y Degradadores de las Plumas en su Plumaje
Ashley M. Peele,1,4 edwArd h. Burtt, Jr.,1,5 MAx r. schroeder,2 
And russell s. GreenBerG3
1Department of Zoology, Ohio Wesleyan University, Delaware, Ohio 43015, USA;
2Department of Botany-Microbiology, Ohio Wesleyan University, Delaware, Ohio 43015, USA; and
3Smithsonian Migratory Bird Center, National Zoological Park, 3001 Connecticut Avenue NW, Washington, D.C. 20008, USA
4Present address: Department of Ecology and Evolutionary Biology, Tulane University, New Orleans, Louisiana 70118, USA.
5Address correspondence to this author. E-mail: ehburtt@owu.edu
Abstract.?The Southern Swamp Sparrow (Melospiza georgiana georgiana) breeds in northeastern North America in montane, 
freshwater marshes and fens. Its close relative, the Coastal Plain Swamp Sparrow (M. g. nigrescens), breeds in northeastern North 
America, but in coastal salt marshes. Coastal Plain Swamp Sparrows are darker than Southern Swamp Sparrows. Darkly colored 
feathers are more resistant to bacterial degradation by bacilli, which are unusually salt-tolerant. We tested whether the difference in 
feather color of the pale montane Southern Swamp Sparrow and the dark Coastal Plain Swamp Sparrow could be an adaptive response 
to differences in the occurrence and activity of bacilli in habitats that differ in salinity. Southern Swamp Sparrows were caught and 
sampled in cranberry fens in western Maryland, whereas Coastal Plain Swamp Sparrows were sampled in salt marshes on the western 
shore of the Delaware River, just where it broadens into Delaware Bay. The number of birds with feather-degrading bacteria in their 
plumage was significantly greater among Swamp Sparrows in salt marshes than among those in freshwater fens. The number of colonies 
of feather-degrading bacilli per bird was also higher for salt-marsh Swamp Sparrows than for those from freshwater fens. We conclude 
that the dark plumage of Coastal Plain Swamp Sparrows evolved to resist feather-degradation by salt-tolerant bacilli that occur more 
frequently and abundantly in their plumage than in the pale plumage of the Southern Swamp Sparrow. Received 25 July 2008, accepted 
16 January 2009.
Key words: bacilli, color, feather-degrading bacteria, Melospiza georgiana, Swamp Sparrow.
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532 ?  Peele et Al.  ? Auk, Vol. 126
Southern swamp sparrows (Melospiza georgiana geor-
giana), which breed in freshwater marshes of the northeastern 
United States and southern Canada, are predominantly rusty 
brown with a brown or rusty crown. Coastal Plain Swamp Spar-
rows (M. g. nigrescens), which breed in salt marshes along the mid-
Atlantic coast (Greenberg and Droege 1990), have more extensive 
black coloration in the crown and 40% more black feathering on 
the body than Southern Swamp Sparrows (Greenberg et al. 2008). 
The color difference in these two subspecies parallels that be-
tween the subspecies of western Song Sparrows (M. melodia; Zink 
and Remsen 1986), in which darkly colored M. m. morphna of the 
humid Northwest harbor more active feather-degrading bacilli in 
their plumage than pale M. m. fallax of the arid Southwest (Burtt 
and Ichida 2004). The geographic variation in the color of Song 
Sparrows is an example of Gloger?s rule. Humid environments 
such as the Pacific Northwest favor bacterial growth, which may 
select for dark feathers that resist bacterial degradation (Goldstein 
et al. 2004, Gunderson et al. 2008). Because Swamp Sparrows, re-
gardless of color, live in wet habitats, humidity is an unlikely expla-
nation for their color variation. However, feather-degrading bacilli 
are unusually salt-tolerant (Claus and Berkeley 1986). Such toler-
ance could mean that feather-degrading bacilli are more abundant 
in salt-marsh habitat, where their less-tolerant microbial competi-
tors are absent or disadvantaged. If the microbiota of salt-marsh 
habitat is skewed toward feather-degrading, salt-tolerant bacilli, 
selection should favor the evolution of melanic feathers that resist 
bacterial degradation. We hypothesized that the darker Coastal 
Plain Swamp Sparrows, which breed in coastal salt marshes, 
would have more, and more active, feather-degrading bacilli than 
the lighter Southern Swamp Sparrows, which breed in freshwater 
marshes and fens.
Methods
Field sampling and identification of feather-degrading bacilli.?
Individual Southern and Coastal Plain swamp sparrows were cap-
tured with mist nets in late May?June 2006 at breeding sites, and 
their plumage was sampled for microorganisms. Coastal Plain 
Swamp Sparrows were studied in spartina marshes (salinity: 5?10 
ppt) within state refuges near Woodland Beach, Delaware (75.6?W, 
39.4?N), along the western shore of the Delaware River just north 
of where it expands to form Delaware Bay. Southern Swamp Spar-
rows were sampled at an elevation of 800 m on two plots in fresh-
water cranberry fens on the Allegheny Plateau in Garrett County, 
Maryland (79.3?W, 39.6?N).
We removed individuals from mist nets only after washing 
our hands with a quaternary ammonium disinfectant and rubbing 
or shaking them dry. We sampled microorganisms in the plumage 
by pressing a trypticase soy agar (TSA) plate to the back, venter, 
and dorsal surface of the tail. Trypticase soy agar is a rich, non-
specific agar on which most culturable bacteria (e.g., B. licheni-
formis), as well as many fungi and, occasionally, yeast, will grow. 
We pressed a yeast maltose agar (YMA) plate against the ven-
tral feathers after sampling with TSA. Like TSA, YMA is a rich 
medium on which fungi grow particularly well but which also 
captures feather-degrading bacilli. After using TSA on the back, 
we rubbed the dorsal plumage with a mannitol salt agar (MSA) 
plate, which is selective for Staphylococcus spp. We brushed the 
underside of the tail across eosin-methylene blue agar (EMB), 
which is selective for coliforms, and stroked the primaries across 
actinomycetes isolation agar (AIA), which selects for Streptococ-
cus spp. We used TSA first because it does not affect the viabil-
ity of the microorganisms targeted by the more specialized MSA, 
EMB, AIA, and YMA. We sampled several different areas of the 
plumage to ensure a reliable estimate of the presence and abun-
dance of plumage microorganisms.
Exposed plates were kept in coolers with ice packs until the 
day?s end, then placed in incubators for 48 h. We incubated YMA 
and AIA at 28?C, and TSA, MSA, and EMB at 37?C. After incu-
bation, we identified and counted all bacterial, fungal, and yeast 
colonies. Most could be identified only to general type (e.g., non-
spore-forming rods), others to a broad taxonomic group (e.g., Ac-
tinomycetes), still others to genus (e.g., Bacillus), and a few to species 
(e.g., Staphylococcus aureus), the latter by using a selective medium. 
The combination of a selective medium and morphology enabled us 
to identify 23 types of culturable microorganisms in the plumage.
We identified Bacillus spp. on the basis of colony morphology, 
as described by Claus and Berkeley (1986). We transferred a repre-
sentative colony of putative Bacillus spp. from each culture plate 
on which it occurred to a test tube of nutrient broth, alkaline salts 
(pH 7.7 and 7.0% NaCl; Burtt and Ichida 1999). These cultures were 
incubated in a shaking incubator at 50?C and 125 rpm for seven 
days. If the broth remained clear, we discarded it and noted that the 
colony was not a species of Bacillis. If the broth became turbid, we 
used a sterile loop to remove a drop of medium and cross-streak it 
on TSA plates to isolate colonies of Bacillus spp. From each plate, 
we subcultured a single colony to a TSA slant, incubated the slant 
at 37?C for 48 h, and stored the slant at 4?C for later use in feather- 
degradation experiments. If the isolated colony was a feather- 
degrader, we considered all colonies of Bacillus on the original 
plate feather-degraders, on the basis of their identical morphology.
Measurement of the rate of feather degradation.?We quanti-
fied in-vitro feather degradation by measuring the concentration 
of oligopeptides released into the feather medium when bacte-
rial keratinase degrades ?-keratin (Goldstein et al. 2004). We per-
formed all bacterial transfers in a laminar flow hood, using aseptic 
technique to prevent contamination. We transferred our isolated 
strains of bacilli from the TSA slants on which they were stored 
to nutrient broth, which was incubated at 37?C and 125 RPM for 
~18 h. This allowed the cells to reach their stationary growth phase 
before use for feather degradation. We transferred 5 mL of each 
liquid culture to a sterile centrifuge tube with a sterile disposable 
pipette. We centrifuged the culture to create a pellet of cells, de-
canted the nutrient broth, resuspended the cells in a small amount 
of sterile saline, and transferred the cell suspension to a sterile dis-
posable glass test tube. We added saline to this suspension until 
the turbidity of the suspension visually matched a 0.5 McFarland 
Standard. This produced a cell density of approximately 1.5 ? 108 
cells mL?1 (Chapin and Lauderdale 2003).
We used 25 ? 150 mm test tubes for the feather-degradation 
analyses. To each test tube, we added 0.06 g of white feather barbs, 
cut from secondary feathers of Domestic Geese (Anser anser). We 
prepared feather medium as described by Williams et al. (1990): 
0.5 g NH4Cl, 0.5 g NaCl, 0.3 g K2HPO4, 0.4 g KH2PO4, 1 g MgCl2, 
6H2O, and 0.1 g of yeast extract per liter of deionized water. 
We added 20 mL of medium to the feather barbs in each tube 
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July 2009 ?  dArk PluMAGe AdAPted to FeAther-deGrAdinG BAcilli  ? 533
before autoclaving the tubes at 121?C and 17 psi for 15 min. After 
the tubes cooled, each was inoculated with 2 drops (0.1 mL) of the 
prepared saline suspension. We incubated the tubes at 37?C and 
125 rpm for five days.
At intervals of 24, 48, 72, 96, and 120 h, we removed a 1-mL 
sample from each of the test tubes to a fresh microcentrifuge tube. 
The samples were centrifuged at 14,000 rpm for 10 min to remove 
any cells and pieces of feather from the suspension. We transferred 
the supernatant to a fresh microcentrifuge tube and centrifuged 
a second time. We transferred the second supernatant to a fresh 
microcentrifuge tube. We transferred the supernatant to a quartz 
cuvette and measured absorbance at 230 nm with a UV-Vis Spec-
trometer (Beckman DU 530). We used autoclave-sterilized feather 
medium as the blank control and rinsed the cuvette between sam-
ples with autoclave-sterilized Milli-Q water. If needed, we diluted 
the samples with autoclave-sterilized feather medium.
In feather medium that has been inoculated with feather- 
degrading bacilli, degradation of the feather by bacterial enzymes 
produces protein fragments called oligopeptides, and the concen-
tration of these fragments is proportional to absorbance of the 
medium at 230 nm (Goldstein et al. 2004). Greater absorbance in-
dicates that there are more oligopeptides in the medium. Greater 
absorbance and more oligopeptides mean that more feather kera-
tin has been degraded by bacterial enzymes, and the more quickly 
oligopeptides appear in the medium, the more quickly the feather 
is being degraded (Goldstein et al. 2004).
Statistics.?Because we sampled different numbers of indi-
viduals of the two subspecies (Table 1), we used the chi-square test 
for independence to compare the proportion of individuals of each 
subspecies that had one or more feather-degrading bacilli in their 
plumage. From the number of different types of microorganisms 
in the plumage of each Swamp Sparrow, we calculated the average 
species richness of plumage microbes in the two populations. We 
calculated the average Shannon-Weiner diversity index for every 
individual in both populations on the basis of the number of types 
of microbes and abundance of each type. We also calculated the 
mean total number of microbial colonies in the plumage of the two 
populations and the number of colonies of feather-degrading bacilli 
in the plumage of Southern and Coastal Plain swamp sparrows.
We realize that the variances in total abundance of plum-
age microbes and in abundance of feather-degrading bacilli are 
large (Table 1). The variance ratio test (Zar 1984) indicated non-
significant differences when comparing means of total abun-
dance (F = 1.68, df = 39 and 44, P > 0.05) or microbial species 
diversity (F = 1.30, df = 39 and 44, P > 0.05) in the two popula-
tions of Swamp Sparrows. Therefore, we used the t-test for inde-
pendence to compare these means. Variances were significantly 
different for means of species richness (F = 3.68, df = 39 and 44, 
P < 0.001) and abundance of feather-degrading bacilli (F = 8,377, 
df = 39 and 44, P < 0.001) in the plumage. These means were com-
pared with the Mann-Whitney U-test. All analyses were com-
pleted with MINITAB 15 (Minitab, State College, Pennsylvania).
Results
We isolated feather-degrading bacilli from the plumage of 18 of 45 
adult Southern Swamp Sparrows captured in freshwater fens in 
the mountains of Maryland (Table 1). We isolated feather-degrad-
ing bacilli from the plumage of 33 of 40 adult Coastal Plain Swamp 
Sparrows captured in the salt marshes of the lower Delaware River 
(Table 1). Significantly more individuals from the salt marshes had 
feather-degrading bacilli in their plumage than did those from the 
freshwater fens (Table 1).
The abundance of feather-degrading bacilli per individual, 
as estimated from the number of colony-forming units (CFUs) 
counted on all TSA and YMA plates, was significantly higher in 
individuals from salt marshes than in those from freshwater fens 
(Table 1). Several Coastal Plain Swamp Sparrows had almost 500 
CFUs of feather-degrading bacilli per individual (Fig. 1). By contrast, 
one Southern Swamp Sparrow had 27 CFUs, another 26, and the re-
maining 16 that had bacilli in their plumage sample had ?12 CFUs.
Coastal Plain Swamp Sparrows were not only more likely to 
have feather-degrading bacilli in their plumage, and more of them, 
than Southern Swamp Sparrows; they also had significantly higher 
loads of plumage microorganisms overall (Table 1). However, the 
number of types of culturable microorganisms in the plumage was 
similar for the two populations of Swamp Sparrows (Table 1), as 
was the diversity (Table 1).
Maximum and minimum degradation rates of B. licheniformis 
from the plumage of Southern Swamp Sparrows and Coastal Plain 
Swamp Sparrows were almost identical (Fig. 2). Thus, feather- 
degrading bacilli, whether from the plumage of Swamp Sparrows 
in freshwater fens or those in salt marshes, pose the same threat to 
the individual?s feathers. However, a Swamp Sparrow in the Dela-
ware salt marshes was twice as likely to have feather-degrading 
bacilli in its plumage (82% of individuals sampled) as a Swamp 
Sparrow from the Maryland fens (40% of individuals sampled). 
Furthermore, the number of colonies of feather-degrading bacilli 
tABle 1. Comparison of the microbiota of the plumage of Southern and Coastal Plain swamp sparrows, with emphasis on feather-degrading bacilli.
Southern 
Swamp Sparrow 
in freshwater fens 
(mean ? SD)
Coastal Plain 
Swamp Sparrow 
in salt marshes 
(mean ? SD)
Statistic and probability that 
the means are equal
Proportion of sparrows with feather-degrading bacilli 18/45 (40%) 33/40 (82%) ?2 = 3.96 df = 1 P < 0.05
Number of types of plumage microorganisms individual?1 9.20 ? 1.84 9.46 ? 0.71 U = 1,499.5 df = 37 and 40 P = 0.94
Mean species diversity of plumage microorganisms 1.58 ? 0.33 1.58 ? 0.29 t = 0.03 df = 75 P = 0.97
Mean abundance of all types of microorganisms in plumage 152.08 ? 178.37 371.51 ? 230.51 t = ?4.21 df = 75 P < 0.001
Mean abundance of feather-degrading bacilli in individuals 
 with one or more feather-degrading bacilli
2.00 ? 0.86 97.89 ? 78.20 U = 41 df = 37 and 40 P = 0.0025
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534 ?  Peele et Al.  ? Auk, Vol. 126
exposure to feather-degrading bacilli. Unlike in Song Sparrows, 
the feather-degrading bacilli from the plumage of dark and light 
Swamp Sparrows degrade feathers at similar rates. However, indi-
viduals of the dark subspecies are significantly more likely to have 
feather-degrading bacilli in their plumage and have significantly 
more bacilli.
In both Song Sparrows and Swamp Sparrows, the subspe-
cies with darker plumage faces a greater threat of bacterial deg-
radation of its feathers. Melanin, which darkens the feathers, also 
retards bacterial degradation (Goldstein et al. 2004, Gunderson 
et al. 2008). Thus, subspeciation in Song and Swamp sparrows may 
be driven, in part, by the greater risk of bacterial degradation to 
the plumage of the darker subspecies. The darker Coastal Plain 
Swamp Sparrows were twice as likely to have feather-degrading 
bacilli in their plumage as the paler Southern Swamp Sparrows, 
and the bacilli were one to two orders of magnitude more abun-
dant in their plumage. Such a difference would seem to constitute 
a considerable selection pressure for dark plumage that can resist 
bacterial degradation.
In the Song Sparrow, dark coloration and increased bacterial 
activity are correlated with the humidity of the habitat (Burtt and 
Ichida 2004). The dark color of populations in humid habitats is 
widespread and well documented (Zink and Remsen 1986, Burtt 
and Ichida 2004). The correlation is known as ?Gloger?s rule.? Sev-
eral explanations have been offered for Gloger?s rule (see review in 
Burtt and Ichida 2004), all of them mutually compatible, but none 
explains the color difference observed in Swamp Sparrows. Both 
light and dark populations of Swamp Sparrow occupy wet habitats 
in which all the suggested explanations for the color difference 
described in Gloger?s rule would apply to both populations. How-
ever, 40% of light Southern Swamp Sparrows from the freshwater 
fens have feather-degrading bacilli in their plumage, whereas 82% 
of dark Coastal Plain Swamp Sparrows have feather-degrading 
bacilli in their plumage and, if there, the bacilli are vastly more 
abundant (97.89 ? 78.20 CFUs) than in the plumage of Southern 
Swamp Sparrows (2.00 ? 0.86 CFUs).
The fens of western Maryland are cooler than the salt marshes 
of upper Delaware Bay. The heat of the salt marshes and the high 
humidity that goes with such heat promote bacterial growth. The 
difference in heat and humidity may account for the greater over-
all microbial load in the plumage of Coastal Plain Swamp Spar-
rows than in the plumage of Southern Swamp Sparrows of the 
mountain fens (Table 1), but this does not explain the difference 
of almost two orders of magnitude in the abundance of feather-
degrading bacilli.
The cranberry fens of western Maryland have a far lower 
salinity than the salt marshes at the head of Delaware Bay (0 vs. 
5?10 ppt). Few bacteria are as salt-tolerant as B. licheniformis 
(Claus and Berkeley 1986) and related bacilli. Indeed, we isolate 
them from other bacteria by growing them in a 7.7% salt solution 
(Burtt and Ichida 1999). The salt tolerance of bacilli may account 
for their abundance in the plumage of Coastal Plain Swamp Spar-
rows. With such bacilli abundant in the plumage and surrounded 
by a damp habitat in which the feathers are often wet, selection 
for resistance to bacterial degradation of the feathers could be in-
tense. Melanization of the feathers, as has happened in the Coastal 
Plain Swamp Sparrow, increases their resistance to bacterial deg-
radation (Goldstein et al. 2004, Gunderson et al. 2008).
in the plumage of salt-marsh Swamp Sparrows was 50? greater 
than in the plumage of Swamp Sparrows in freshwater fens.
discussion
Both Song Sparrows (Arcese et al. 2002) and Swamp Sparrows 
(Mowbray 2000) have dark and light subspecies that occur in dif-
ferent habitats. Like Song Sparrows (Burtt and Ichida 2004), the 
differently colored subspecies of Swamp Sparrow face different 
FiG. 1. A plot of the number of Swamp Sparrows with one or more colony- 
forming units of feather-degrading bacilli in their plumage shows that 
more salt-marsh Swamp Sparrows have feather-degrading bacilli in their 
plumage and that they have many more bacilli than Swamp Sparrows 
from freshwater fens.
FiG. 2. The increasing absorbance of the supernatant from the feather 
medium indicates the increasing degradation of the feather. The maxi-
mum (filled symbols) and minimum (open symbols) mean absorbance 
for supernatant from cultures of bacilli isolated from Southern Swamp 
Sparrows (squares) and from Coastal Plain Swamp Sparrows (triangles) 
indicate that the rate of feather degradation is the same, regardless of 
whether the bacilli come from Swamp Sparrows living in salt marshes or 
in freshwater fens.
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The marked color difference between salt-marsh and inland 
populations of Swamp Sparrows despite their genetic similarity 
prompted Greenberg et. al. (1998) to suggest that either (1) there 
is strong selection and sufficient heritable variation or (2) the 
color differences are environmentally induced (Von Bloeker 1932, 
James 1983, Zink and Remsen 1986). We suggest that the ?strong 
selection? hypothesized by Greenberg et al. (1998) is provided by 
the high probability of acquiring feather-degrading bacilli and 
the abundance of such bacilli in the plumage of affected Coastal 
Plain Swamp Sparrows (M. g. nigrescens). Furthermore, the occur-
rence and distribution of eumelanin (gray to black), as opposed 
to pheomelanin (buff to rust), in the plumage appear to be under 
strong genetic control in birds (Smyth 1990). This would account 
for the ?heritable? aspect of Greenberg et al.?s (1998) hypothesis. 
That the color difference between subspecies of Swamp Sparrows 
is substantially less than the difference between the subspecies of 
Song Sparrows (Burtt and Ichida 2004) may be the result of the 
recent expansion of Swamp Sparrows into habitat that was tundra 
as recently as 15,000 years ago, during the Pleistocene glaciation 
(Jacobsen et al. 1987, Cox and Moore 2005). The strength of selec-
tion for melanic colors that resist bacterial degradation and the 
recency of the Swamp Sparrow?s evolutionary response to such se-
lection are indirectly supported by the frequent grayish or black-
ish color of tidal populations of other terrestrial vertebrates?the 
?salt marsh melanism? noted by Grinnell (1913) and Von Bloeker 
(1932).
AcknowledgMents
Our work was supported by a grant from the Smithsonian Insti-
tution to A.M.P., by grants from the National Science Founda-
tion (BIR-998805 and DEB-0543360) to E.H.B., and by an REU 
(Research Experiences for Undergraduates) fellowship to M.R.S. 
Pacific Coast Feather donated Domestic Goose feathers for our 
laboratory experiments. We thank A. L. Downing for her advice 
on the calculation of species diversity and S. Lender and A. J. Gatz 
for their advice on our statistical analysis. We also thank J. Gold-
stein, S. Rohwer, and two anonymous reviewers for suggesting 
improvements in our final draft.
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Associate Editor: S. Rohwer
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