J. Raptor Res. 47(2):197–207

E 2013 The Raptor Research Foundation, Inc.

BRIGHTNESS FEATURES OF VISUAL SIGNALING TRAITS IN YOUNG
AND ADULT EURASIAN EAGLE-OWLS
CHIARA BETTEGA
Trento Natural History Museum, via Calepina 14, 38122 Trento, Italy

LETIZIA CAMPIONI
Eco-Ethology Research Unit, ISPA, Lisboa, Portugal

MARÍA DEL MAR DELGADO
Metapopulation Research Group, Department of Biosciences, FI-00014, University of Helsinki, Finland

RUI LOURENÇO
Laboratory of Ornithology, Institute of Mediterranean Agrarian and Environmental Sciences, University of Évora,Núcleo da Mitra, 7002-554 Évora, Portugal

VINCENZO PENTERIANI1
Department of Conservation Biology, Estación Biológica de Doñana, C.S.I.C., c/Americo Vespucio s/n,
41092 Seville, Spain
ABSTRACT.—Recent research has demonstrated the important role of visual communication in nocturnal
birds. Achromatic plumage patches (i.e., pigment-free white feathers) with high levels of contrast against
dark backgrounds are excellent candidates for visual signaling in the dark or at twilight, when differences incolor may be less effective. In this study, with the goal of investigating the signaling role of certain
achromatic plumage patches, we examined the characteristics and patterns of the brightness (i.e., totalreflectance) of Eurasian Eagle-Owl (Bubo bubo) white feather patches for both young birds and adultindividuals. Our results showed that (1) the total reflectance of young birds’ white feathers differedsignificantly from that of adult owls’ white feathers; (2) the brightness differed between the sexes in adults
only, with females showing a significantly higher reflectance than males; (3) the total reflectance of thewhite patch around a young bird’s mouth was positively correlated with brood size; (4) the total reflectanceof the white badge on the throat of adults was positively correlated with their hematocrit values; (5) anassortative mating scenario based on the brightness of an individual’s white badge was deemed possible;and (6) we did not detect any significant relationship in the levels of reflectance for related individuals: theyoung and their parents were not found to be distinguishable based on the brightness of their white feather
patches.
KEY WORDS: Eurasian Eagle-Owl; Bubo bubo; plumage brightness; visual signaling; white feathers.

CARACTERÍSTICAS DEL BRILLO DE LAS SEÑALES VISUALES EN JÓVENES Y ADULTOS DE
BUBO BUBORESUMEN.—Trabajos recientes han demostrado la importancia del papel que las señales visuales desempeñan en la comunicación de las aves nocturnas. Los plumajes acromáticos (es decir, aquellos libres depigmentos) con altos niveles de contraste contra un fondo oscuro son los mejores candidatos para enviar
señales visuales en la noche o en horas crepusculares, cuando las diferencias de color pueden ser menos
efectivas. Con el objetivo de investigar el papel en la señalización de ciertos parches de plumas acromáticas,
estudiamos las caracterı́sticas y los patrones de brillo (es decir, la reflectancia total) de la mancha blancapresente en polluelos y en adultos de Bubo bubo. Los resultados mostraron que (1) la reflectancia total de lamancha blanca de los polluelos difirió significativamente de la de los adultos; (2) en adultos existen
1 Email address: penteriani@ebd.csic.es

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diferencias en el brillo de la mancha blanca en función del sexo, siendo las hembras las que tienen valores
más altos de reflectancia; (3) la reflectancia total de la mancha blanca de los polluelos está positivamente
relacionada con el tamaño de nidada; (4) en el caso de los adultos, existe una relación entre la mancha
blanca en la garganta y el valor de hematocrito de los individuos; (5) es posible que la mancha blanca de losbúhos adultos actúe como una señal en un posible escenario de emparejamiento selectivo; y (6) no
detectamos ninguna diferencia en el brillo de la mancha dentro de los núcleos familiares, es decir, losindividuos de cada familia no parecen diferenciarse en función de los rasgos de la mancha blanca.
[Traducción de los autores editada]Plumage coloration is one of the most common
ways birds convey information, including an individual’s quality and social status, to conspecifics. Bothmales and females may also use plumage characteristics in mate choice (Hill and McGraw 2006a,
2006b). Additionally, several studies have confirmed
the role of coloration in parent-offspring communication, with parents responding to nestlings’ skin,plumage, mouth color, and markings (e.g., Jourdie
et al. 2004, Kilner 2006, Wiebe and Slagsvold 2009,
Ligon and Hill 2010, Parejo et al. 2010). In diurnal
birds, this type of information is generally conveyed
through a wide range of carotenoid- and melaninbased colors, as well as structural colors (e.g., blue,
violet, ultraviolet, and white patches; Hill and
McGraw 2006a, 2006b). Males normally have morecolorful and conspicuous plumage than females,with females typically the selective sex. Similar tomales, however, females also may display plumage
patches and conspicuous ornamentation that may
serve a selective signaling function (e.g., Roulin
1999, Amundsen and Pärn 2006, Roulin and Altwegg 2007, Legagneux et al. 2010, Parejo et al.
2011).
Thus far, most studies on the use of plumage
coloration in signaling among animals have focusedon diurnal species, because communication in nocturnal birds was traditionally thought to rely solely
on vocalization (Hill and McGraw 2006a, 2006b).
However, a growing body of literature has recently
demonstrated the role of visual communication in
nocturnal birds (Aragonés et al. 1999, Penteriani
et al. 2006, 2007a, 2007b, Penteriani and Delgado
2009, Parejo et al. 2010, Bortolotti et al. 2011), particularly through achromatic patches (i.e., pigmentfree white feathers). Because visual signals used in
the dark or at twilight are communicated in low
light conditions, achromatic plumage patches withhigh levels of contrast against dark backgrounds are
excellent candidates for signaling when differencesin color may be less effective. White feathers result
from the scattering of light in all directions by unpigmented feather keratin (Prum et al. 1999).

The Eurasian Eagle-Owl (Bubo bubo) has two different, age-dependent patches of white feathers.
The first patch forms as a large white border tracing
the edge of nestlings’ and fledglings’ (hereafter referred to as ‘‘young’’) mouths (Fig. 1A). This patch
begins to become clearly visible at approximately
30–35 d of age and gradually disappears in fullgrown individuals (i.e., before the start of natal dispersal). These white patches appear to play a role inparent-offspring communication and may serve tosignal the quality of the young, to assist parents infeeding, or to increase a fledgling’s visibility in thedark. Feeding parents appear to be able to discriminate among young based on the brightness of theirmouth feathers, which may result in the young withhigher brightness being fed more or with better
food (Penteriani et al. 2007b).
Full-grown individuals develop a second whitebadge visible on the throat only during call displays(Penteriani et al. 2006; Fig. 1B). These displays aremainly performed at dawn, dusk, or in the moonlight, when visual contrast is higher (Penteriani andDelgado 2009, Penteriani et al. 2010). Both males
and females display this badge, and its brightness
appears related to an individual’s physical state(Penteriani et al. 2006) and/or fecundity, withmales showing the highest brightness producing
more young (Penteriani et al. 2007a).
In this study, we examined the characteristics andpatterns of the brightness of eagle-owl white feather
patches. First, we focused on identifying potential
relationships between the brightness of the eagleowl’s two white feather patches and several internal(e.g., an individual’s physical condition) and external (e.g., availability of main prey) factors with thepotential to affect the properties of reflectance. Second, we examined possible brightness differences
between the white feathers of the young and breeders. Because the white plumage of the young birds
and breeders serves different functions, we hypothesized that the reflectance properties of their plumage
also might differ and, for example, show different
patterns of variation between the sexes. Thirdly, weJUNE 2013OWL VISUAL SIGNALING

199

Figure 1. The difference between the white patch around the mouth of a Eurasian Eagle-Owl young bird (A) and thewhite badge on the throat of a full-grown individual (B).

investigated whether breeding pairs mated assortatively based on plumage characteristics. Although assortative pairing can occur for many reasons otherthan mate choice, assortative mating by plumage color may occur when mate choice is directly related tophenotype (Bortolotti et al. 2008). Finally, we tested
whether plumage brightness could be used to identify siblings and their parents by comparing the feather brightness of owls within family groups.
METHODS

Trapping and Collecting Blood from Young Birds
and Breeders. From 2003 to 2010, we measuredfeather brightness in a sample of 89 owls from 25
nesting sites in Sierra Morena in southwestern
Spain. The sample consisted of 30 breeders (21males and 9 females) and 59 young (31 males and
28 females). We captured most of the breeding
males using a taxidermic mount and playback of amale call to simulate a territorial intrusion. A dho
gaza net positioned behind the mount caught any
males responding to the calls. We trapped females
and several other males using bownets placed innests when young were 20–35 d old. At this age,young birds are capable of thermoregulation andnight temperatures are consistently warm in the
study area (approximately 20uC). We placed nestlings in a box covered by a metal grid to ensure they
were visible to their parents, which we then captured when they returned to the nest. A maximumof three trapping nights per breeding season occurred at any given nest. We measured the bodymass of trapped breeders and young (30–35 dold) to the nearest 10 g using a 1-kg Pesola scale.
We used forearm length and mass to calculate body
condition indices (BCI, Table 1; Green 2001) andcollected 2 ml of blood from the brachial vein.
Blood samples were stored in tubes with heparinat 4uC for transport to the laboratory, where wecentrifuged each sample for 10 min at 4000 rpm
to obtain hematocrit (HT) values (i.e., the volume
percentage of red blood cells; Table 1). We used
HT as an indicator of nutritional status (Delgado
et al. 2010, Campioni et al. in press); nutritional
deficiencies result in anemia due to a shortage ofessential amino acids (Costa and Macedo 2006).

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Table 1. Health status, nest-site quality, and brood sizes of Eurasian Eagle-Owl young (n 5 59) and breeders (n 5 30)
included in this study.
YOUNG

BREEDERSVARIABLES

MEAN

SD

RANGE

MEAN

SD

RANGE

Hematocrit
Body condition index
Rabbit biomass in diet (%)
Rabbit latrines per km of transect
Brood size

43.9
0.00
58
23.1
2.7

8.49
0.05
6.2
9.93
0.74

29.1–70.2
20.09–0.08
42–79
7.7–36.3
1–4

50.7
0.00
58
21.2
2.3

6.94
0.04
10.8
11.57
0.99

41.7–66.7
20.05–0.08
42–93
7.7–46
0–4

Despite HT variability (e.g., due to sex, age, season,and nutritional status; Fair et al. 2007), HT may beconsidered an accurate indicator of physical condition when used in conjunction with other parameters, such as body mass (Fair et al. 2007). We aged
the young birds according to Penteriani et al.
(2004) and sexed them using DNA extracted fromthe blood samples (Delgado et al. 2010, Campioni
et al. in press).
Measurement of White Plumage Brightness. We
assessed the total reflectance of the white feathersof young birds and breeders using a Minolta CM2600d portable spectrophotometer (Minolta Co.,
Ltd., Osaka, Japan) with UV (xenon flashlightsource) and visible light (standard illuminant
D65). For each owl, we superimposed four layersof three feathers (12 total) on a dark velvet surface
with 0% reflectance (Quesada and Senar 2006, Penteriani et al. 2006). We estimated brightness as thepercentage of total reflectance data in the interval
360–700 nm. Feathers were plucked from the birds
and analyzed in the laboratory for all individuals,
with the exception of 17 young birds; for these 17
birds, white feathers were assessed during a field experiment rather than in the laboratory (Penteriani
et al. 2007b).
Diet Analysis and Main Prey Availability. Previous
diet analyses showed that rabbits (Oryctolagus cuniculus) are the main prey of our study population(Campioni et al. in press). Therefore, we considered the abundance of nearby rabbits and theamount of rabbit biomass in an individual’s diet
(Table 1) as potentially additive factors affecting
an individual’s physical state and, consequently,
the brightness of its white plumage. We collectedprey remains and pellets during visits to nests, roosting perches, and feeding perches in the breeding
territories, and used them to assess the diet of eagleowls. We identified prey species using bone identification keys and by comparing with a referencecollection at the Laboratory of Archaeo-sciences at
IGESPAR (Instituto de Gestão do Património Arquitectónico e Arqueológico) in Portugal. Using these
data, we determined the minimum number of prey
individuals consumed. We calculated rabbit biomass
percentages using the mean mass estimates obtained from previous studies or bone measurements
to estimate the mass of each individual (Lourenço2006). We used rabbit fecal pellet counts, performed from March to May 2009, to estimate therelative abundance of rabbits at the 25 nesting sites
used in this study (latrine counts; Palomares 2001a,
2001b). To obtain comparable indices of prey abundance (i.e., number of latrines per km of transect,
Table 1), we drew a circular plot around each nest.
The area of the circular plot was equal to the mean
eagle-owl home-range size in our study population(218.9 6 30.8 ha), as calculated by the minimum
convex polygon method (Campioni et al. in press).
Within these plots, we walked 2.2-km transects andrecorded the number of latrines (ntot 5 3440 latrines) found within 4 m of each side of the transects (Campioni et al. in press). In our study area,
rabbit density remains relatively stable over timewithin eagle-owl breeding sites, because frequent
releases of rabbits inside the study area create afavorable and steady prey source.
Statistical Analyses. We first used a t-test to compare the total reflectance of the white feather patches of young birds and breeders. Because the totalreflectance differed between the two groups (see Results), we built two separate models for young andbreeders. We used linear mixed models (Zuur et al.
2009) because we had repeated information forsome nests between and within years. Therefore, we
included year and nest site as first- and second-level
random effects, respectively. When random factors
did not improve the model’s likelihood value, as
was the case for breeders, the final model was a lesscomplex linear model. We used the two age-specificJUNE 2013OWL VISUAL SIGNALING

201

Figure 2. Sex-specific variation (males 5 stippled bar; females 5 open bar) in the total reflectance of white feather
patches in Eurasian Eagle-Owl young birds and adult breeders.

models to explore relationships between the totalreflectance of eagle-owls’ white patches and the following variables (i.e., fixed factors): (1) individual
traits, including sex, age, hatching order (only foryoung birds), and the two indicators of an individual’s health status (i.e., HT and BCI); (2) nest-sitequality, expressed as the percentage of rabbit biomass in the diet of a family in a given territory andas the availability of rabbits in the vicinity of the nest
site; and (3) brood size (Table 1). For young birds,
we also included a binomial variable representing
whether brightness was measured in the laboratory
or in the field to eliminate any possible
effects of the physical location in which we used
the spectrophotometer to analyze characteristics ofthe feathers.
We performed all statistical analyses using the R
2.10.1 statistical software (R Development CoreTeam 2009) with the nlme package (Pinheiro
et al. 2009). We first explored the data to ensurethat the assumptions of normality, homogeneous
variance, and independence of observations weremet, and found that the residuals of the dependent
variables were all homogeneous, independent, andnormally distributed. We derived simplified models
using backward selection of variables from the fullmodel, and compared candidate models using likelihood ratio tests. For each analysis, we used a specific subsample of individuals for which we had dataon all variables included within the specified model.
We tested the possibility of assortative matingbased on badge brightness using an ANOVA to compare brightness variability among and within breeding pairs. We used a linear mixed model to test forfamily (n 5 12 owl families, corresponding to 73
individuals) resemblance in brightness (i.e., similar values of reflectance among siblings and theirparents).
All tests were two-tailed and statistical significancewas set at a , 0.05.

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Table 2. Final models showing relationships between the total reflectance of Eurasian Eagle-Owl white feather patches
and individual traits, nest-site quality, and brood size.
BIRD AGE

VARIABLE

ESTIMATE

SE

Adult breedersa

Intercept
Hematocrit
Sex
Intercept
Brood size

620.1
14.0
213.7
1122.3
111.9

389.3
7.0
100.7
133.9
49.2

Youngb

t

P

2.0
2.1

0.050
0.045

2.3

0.032

a Total reflectance of white throat badge evaluated with linear model fit by maximum likelihood.
b Total reflectance of white patch around mouth evaluated with linear mixed (including nested random effects of Year and Nest Site)model fit by maximum likelihood.

RESULTS

Brightness of White Feather Patches: General
Patterns. The total reflectance of white feather
patches averaged significantly lower in young(mean 6 SD 5 1371 6 293.4 nm, n 5 59) thanin adults (1625 6 282.4 nm, n 5 30; t87 5 23.90, P
5 0.0001; Fig. 2). The only significant relationshiprevealed by the linear mixed model developed foryoung birds was that the total reflectance of whitefeather patches varied significantly among broodsizes (Table 2, Fig. 2, Fig. 3). The only significant
relationships revealed by the linear model developed for breeders were that the total reflectanceof the white feather badges was positively correlated with HT values (Table 2, Fig. 4), and females(1785 6 162.5 nm, n 5 9) showed higher brightness levels than males (1556 6 297.4 nm,
n 5 21; Table 2, Fig. 2, Fig. 5).
Assortative Mating by Badge Brightness. Although
the available sample was small (n 5 6 pairs), variability in the total reflectance of white feather badges was higher among breeding pairs than withinpairs (F5,6 5 5.4, P 5 0.032; Fig. 5).
Variation in Badge Brightness Within and Among
Families. We did not detect any significant variationin feather brightness between young and their parents or siblings, and differences in feather brightness among families were not enough to identify
individual families of owls (F10,62 5 1.4, P 5 0.22).
DISCUSSION

Our results were consistent with experimental
evidence from previous studies on the eagle-owl,
indicating that white plumage in both young andbreeders plays an important role in animal communication among crepuscular and nocturnal species(Penteriani et al. 2007a, 2007b, 2009, 2010).
In breeders, the brightness of the white throat
badge was positively correlated with hematocrit

values. Hematocrit may be considered an indicatorof phenotypic condition, with high hematocrit values associated with a better nutritional state andlower levels of infection (Costa and Macedo 2006,
Fair et al. 2007). Males with the highest levels ofbrightness fledge more young, an indication of their
quality as mates (Penteriani et al. 2007a). Signals
may be reliable indicators of individual quality ifthey are costly (Zahavi 1975, Grafen 1990, Iwasa etal. 1991). Following Kose and Møller (1999), we cansuggest at least two potential costs associated withusing white plumage for signaling. First, melanization strengthens feathers; non-melanized feathers
are more likely to break due to their higher levelsof structural weakness (Burtt 1986, Bonser 1995,
Kose and Møller 1999). Second, if feathers without
melanin are particularly susceptible to breakage, it
is also possible that feather parasites may display apreference for the melanin-free parts of feathers(Kose and Møller 1999). Diet quality also may affectwhite plumage patterns in birds (McGlothlin et al.
2007). Although the costs associated with the brightness of white feathers is a topic in need of further
research (Hill 2006, Prum 2006), our results suggest
a definite relationship between bright, white plumage and an individual’s physical state. This relationship is a prerequisite for the use of white markings
as a signal for the quality of an individual, supporting previous evidence from Gustafsson et al. (1995)
and McGlothlin et al. (2007).
Because the white badges of eagle-owls play a rolein determining the intensity of territorial responses
to intruders (Penteriani et al. 2007a), our results
supporting evidence that the badge may be a signalof individual quality are in agreement with the assertion of Johnstone and Norris (1993), who statedthat badges used to settle conflicts should also be
accurate indicators of an individual’s condition.
The costs imposed by social interactions may ensure

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Figure 3. Illustration of the significant relationship between the total reflectance of the white feather patch aroundyoung eagle-owls’ mouths and brood size.

honesty in signaling; only high-quality, dominant
individuals would be capable of withstanding thecosts of aggression, minimizing the benefits ofcheating (e.g., Møller 1987).
Previous analyses of museum specimens showed
higher levels of throat-patch brightness in females
than in males (Penteriani et al. 2006). This finding
may have some importance during mate choice(Amundsen and Pärn 2006 and references therein),given that in some monogamous species with biparental care, males appear to choose their mates inthe same way as females (e.g., males may rely on afemale’s white feathers to assess her quality; Hanssen et al. 2009, Parejo et al. 2011). In several speciesof birds, both sexes have a level of parental investment; therefore, both males and females should beselective when it comes to partners (Trivers 1972). It
is now clear that female coloring has a number offunctions, including the communication of a variety
of individual qualities (e.g., Amundsen and Pärn
2006, Roulin et al. 2001, 2003, Roulin and Altwegg

2007, Roulin 2009). An alternate and not mutually
exclusive explanation is that plumage coloration
plays a role in female-female competition for bothmates and territories, although female owls generally show low levels of intrasexual aggression toward
intruding females (Penteriani et al. 2007a).
Although the available sample was small, the potential assortative mating pattern found in this study
also may support the role of white feathers as asignal of individual quality. Assortative mating,which may occur through a variety of behavioral
mechanisms (Bitton et al. 2008, Bortolotti et al.
2008), has been observed in both structurally colored species (Andersson et al. 1998, Komdeur et al.
2005, Bitton et al. 2008) and species showing whitemarks (Bortolotti et al. 2008). For example, assortative mating may be determined by individuals’ mutual preference for similar phenotypes (e.g., Burley
1983, Johnstone et al. 1996) and/or by intrasexualcompetition for nest sites. This competition may
result in high-quality individuals gaining access to204

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Figure 4. Illustration of the significant relationship between the total reflectance of the white feather badges on
breeding Eurasian Eagle-Owls and their hematocrit values.

the best territories and pairing with high-quality
mates (Creighton 2001, Ferrer and Penteriani2003).
The fact that there were no differences in thebrightness of white feathers between male and female young is noteworthy, especially when considering that, in some owl species, young brothers andsisters differ in color (e.g., Gasparini et al. 2009,
Roulin et al. 2012). Possibly, differences in thebrightness of the white signal may only appear in
full-grown birds, because it is only after dispersal
that these individuals need to signal their state toconspecifics (e.g., during territorial confrontationor mating). Because we found no sex differencesin feather brightness of the young, our results suggest that parents do not discriminate between thesexes of their offspring using the brightness of whiteplumage. To our knowledge, there is only one reported case in which nestling males differ from nestling females in mouth coloration (Barn Swallow,

Hirundo rustica; Saino et al. 2008). Even with thedifference in mouth coloration, there was no direct
evidence of sex discrimination by Barn Swallow parents. In a previous experimental study, Penteriani
et al. (2007b) hypothesized that, within the samebrood, parental care is apportioned based on thebrightness of the owlets’ white markings aroundthe mouth. In a subsequent study, Parejo et al.
(2010) demonstrated the role of visual communication between parents and offspring for a nocturnal
species, the Common Scops-Owl (Otus scops). In
feeding behavior, parents favored lighter young thatshowed lower ultraviolet reflectance of the cere. Cumulatively, this evidence supports the possibility
that a nestling’s white feathers could be a signal ofoffspring state, thereby determining the amount of
parental care given to each young. That is, parents
may decide to feed their young in a way that maximizes their own reproductive success (Godfray
1995, Mock and Parker 1997).

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205

Figure 5. Variation in the total reflectance of the white feather badges within (males 5 stippled bar; females 5 openbar) and among breeding pairs of eagle-owls, illustrating greater variation among pairs than within pairs, thereby
suggesting possible assortative mating based on feather brightness.

In this study, we found new evidence supporting
the possibility that the brightness of a young bird’s
white feathers signals its quality. We detected a positive correlation between brightness and brood size,
which may indicate that because larger broods areassociated with better nesting sites (e.g., sites rich inhigh-quality food sources) and/or better parents,
feather brightness may also be a consequence ofthe quality of the nest’s surrounding environment
and/or physical characteristics of the parents. However, we did not find any correlation between plumage coloration and the amount of rabbit biomass inthe parents’ diet or rabbit availability in the breeders’ home ranges.
This study contributes new information to thegrowing field of visual communication in nocturnal
animals. Evidence has clearly shown that visual communication is critical to diurnal birds and that, fordecades, we have overlooked more sophisticatedstrategies of animal communication. Birds may not

be the only group of species in which nocturnal
animals communicate through visual signals. Research on crepuscular and nocturnal species with
achromatic patches and patterns on their fur or skin
may reveal previously unsuspected ways to send information in the dark of night.
ACKNOWLEDGMENTS
This work is dedicated to the memory of our beloved
friend Gary Bortolotti, with whom we spent many unforgettable hours talking and speculating on the intriguing field ofvisual communication in nocturnal animals. Comments by
Alexandre Roulin, Jeff P. Smith, and two anonymous referees greatly improved the manuscript. The work was fundedby one research project of the Spanish Ministry of Science and Innovation (CGL2012-33240; with FEDER cofinancing), two research projects through the Spanish
Ministry of Science and Innovation (CGL2004-02780/BOS
and CGL2008-02871/BOS; with FEDER co-financing) andby the Junta of Andalucı́a (Excellence Project, RNM-5090).
We manipulated and marked owls under the Junta de Andalucı́a, Consejerı́a de Medio Ambiente authorizations

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No. SCFFS-AFR/GGG RS-260/02 and SCFFS-AFR/CMM RS1904/02.
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Received 22 June 2012; accepted 11 January 2013
Associate Editor: Jeff P. Smith