Behavioral Ecology Vol. 12 No. 2: 164-170
© 2001 International Society for Behavioral Ecology
Is male plumage reflectance correlated with paternal care in bluethroats?
a Department of Zoology, Norwegian University of Science and Technology, N-7491 Trondheim, Norway b Göteborg University, Department of Zoology, Section of Animal Ecology, Box 463, S-405 30 Göteborg, Sweden
Address correspondence to P.T. Smiseth, School of Biological Sciences, 3.614 Stopford Building, University of Manchester, Oxford Road, Manchester M13 9PT, UK. E-mail: per.t.smiseth{at}man.ac.uk .
Received 14 April 2000; revised 10 July 2000; accepted 15 July 2000.
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ABSTRACT |
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Although it is now well established that the conspicuous male plumage colors of many birds have been subject to sexual selection by female choice, it is still debated whether females mate with colorful males to obtain direct or indirect benefits. In species where males provide substantial parental care, females may obtain direct benefits from mating with the males that are best at providing care. The good parent hypothesis suggests that male plumage coloration signals a male's ability to provide parental care. Alternatively, the differential-allocation hypothesis suggests that colorful males reduce their care in response to increased investment by females mated to attractive males. We tested these hypotheses on the bluethroat (Luscinia s. svecica), a socially monogamous, sexually dichromatic bird, in which males have a colorful throat patch consisting of a structurally derived blue area surrounding a melanin-based chestnut spot. Male plumage coloration was objectively quantified by use of reflectance spectrometry. We found no evidence of a relationship between male coloration of either the blue patch or the chestnut spot and the level of paternal care. Nor were there any correlations between male coloration and body size or body condition. Thus, our study does not support the hypothesis that male coloration signals male parental quality (the good parent hypothesis) or the hypothesis that colorful males reduce their care in response to increased investment by females (the differential-allocation hypothesis).
Key words: direct benefits, female choice, Luscinia s. svecica, parental care, plumage coloration, sexual selection, spectral reflectance.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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There is now good evidence for Darwin's (1871
) suggestion that the
conspicuous male plumage colors of many birds have evolved through sexual
selection by female choice (reviewed in
Andersson, 1994). Since female
choosiness is likely to be costly in terms of time, energy or predation risk
(e.g., Pomiankowski, 1987),
females are expected to benefit from mating with colorful males, but the exact
nature of these benefits is still debated. Because males provide substantial
amounts of care in most species of birds
(Lack, 1968), direct benefits
might be more important to female birds than indirect benefits
(Kirkpatrick and Barton,
1997). Hence, as suggested by the good parent hypothesis, male
coloration may function as a signal of a male's ability to provide care
(Heywood, 1989;
Hoelzer, 1989). On the other
hand, females of many socially monogamous birds engage in extra-pair
copulations, suggesting that indirect benefits, such as inheritance of the
male's attractiveness (Fisher,
1930) or viability
(Fitzpatrick, 1994;
Hamilton and Zuk, 1982;
Zahavi, 1975), might also be
important (Birkhead and Møller,
1992). If male coloration acts as an indicator of indirect
benefits, females mated to colorful males may increase their parental effort
because they value the current reproductive episode highly. This may allow
attractive males to reduce their parental effort, and a negative relationship
between male coloration and paternal care is expected (the
differential-allocation hypothesis,
Burley, 1986;
Møller and Thornhill,
1998). Thus, there are two hypotheses for the relationship between
male coloration and paternal care: (1) the good parent hypothesis which
predicts a positive relationship, and (2) the differential-allocation
hypothesis which predicts a negative relationship.
With the exception of a recent study by Keyser and Hill
(2000), previous studies on
the benefits of female choice have used methods based on human vision to
quantify male coloration. Human-subjective methods have, however, been
criticized because human color vision is different from that of other species
including birds (Bennett et al.,
1994; Endler,
1990). Birds have four classes of cones involved in color
perception (humans have only three), and perceive ultraviolet (UV) light to
which humans are blind (Bennett and
Cuthill, 1994; Cuthill et al.,
2000). Spectrometry is now available as an objective method for
measuring coloration. The method has been successfully used in some recent
studies of mate choice in birds (e.g.,
Andersson and Amundsen, 1997;
Andersson et al., 1998;
Bennett et al., 1996;
Cuthill et al., 1999;
Johnsen et al., 1998).
The aim of this study was to take advantage of this new approach in testing
the two hypotheses on the relationship between male coloration and parental
care: the good parent hypothesis and the differential-allocation hypothesis.
The bluethroat (Luscinia s. svecica) is a suitable model species for
such a study. It is a brilliantly colored, sexually dichromatic bird in which
males have a metallic blue throat patch surrounding a central chestnut spot
(Amundsen et al., 1997;
Cramp, 1988). The throat patch
is displayed to females during courtship
(Peiponen, 1960), suggesting a
function in female choice. The blue patch is a structural color with a strong
UV reflectance component (Andersson and
Amundsen, 1997; Johnsen et
al., 1998), while the chestnut spot, as judged from its spectral
reflectance curve, is melanin-based.
Structural and melanin-based colors have been suggested to be poor
candidates as reliable signals of phenotypic quality
(Gray, 1996). However, recent
theoretical (Andersson, 1999;
Fitzpatrick, 1998) and
empirical studies (Keyser and Hill,
1999) suggest that structural colors might be informative of
phenotypic quality. For bluethroats, both aviary and field experiments suggest
that females use plumage reflectance of the blue patch as a cue in mate choice
(Andersson and Amundsen, 1997;
Johnsen et al., 1998).
Moreover, plumage reflectance of the blue patch differs between first-year and
older males and may thus be an indicator of male phenotypic quality
(Andersson and Amundsen, 1997;
Andersson S, unpublished data; but see
Johnsen et al., 1998).
Finally, bluethroats are predominantly socially monogamous
(Johnsen and Lifjeld, 1995),
and males provide substantial amounts of care
(Rohde et al., 1999;
Smiseth et al., 1998;
Smiseth and Amundsen, 2000).
Females are likely to benefit from mating with males that provide good quality
care. Male care has a strong effect on the production of recruits (the growth
rate of nestlings produced by females that receive little or no male care is
severely retarded; Rohde,
1996), and the feeding rates of individual males vary profoundly
(from around five to around 20 visits/h in the later part of the nestling
period; Smiseth and Amundsen,
2000). A previous study on bluethroats, using traditional
human-subjective methods for measuring coloration, found no significant
relationships between male coloration and paternal care
(Reinsborg, 1995). However,
the lack of significant results in this study may be due to the inadequate
methods used to measure coloration. A recent study, in which male
attractiveness was manipulated by use of leg bands matching the blue and
chestnut elements of the throat patch, found no evidence of an effect of male
attractiveness on male or female care
(Rohde et al., 1999). However,
this finding should not be taken as evidence against the
differential-allocation hypothesis as a recent study found no evidence of
females preferring males with leg bands matching the colors of the throat
patch (Johnsen et al., 2000).
In this study, we focus explicitly on the reflectance properties of the
plumage as measured by use of spectrometry.
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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We did the fieldwork in Øvre Heimdalen (61°25' N, 8°52' E, 1100 m above sea level), southern Norway, during June-July 1997. Bluethroats arrive at the area in mid-May, and males establish territories immediately. Both parents feed the nestlings at similar rates (e.g., Smiseth and Amundsen, 2000
; Smiseth et al.,
1998) except for the first days after hatching when females are
engaged in brooding (Arheimer,
1982; Reinsborg,
1995). Feeding rates have not been found to vary with the time of
day (Arheimer, 1982;
Reinsborg, 1995).
We obtained data on plumage coloration and morphometrics for 32 males. For
24 of these, we knew the exact hatching date for the first-hatched nestling.
For the rest, we estimated hatching date on the basis of nestling masses,
assuming normal growth of the nestlings
(Rangbru, 1994). We used
hatching date to estimate time of pair bonding since the actual time of pair
bonding is difficult to observe due to cryptic female behavior (see
Smiseth and Amundsen, 1995).
Hatching date and pairing date are probably correlated in migratory birds
where breeding activities start immediately after female settlement
(Alatalo et al., 1984; see also
Johnsen et al., 1998). Since
parents may adjust their feeding rate to the number of nestlings in the brood
(e.g., Wright and Cuthill,
1990), we standardized brood size to six nestlings. This was done
on days four through six after hatching by adding or removing
intermediate-sized nestlings so that the natural size-difference within the
brood was not altered. The mean ± SE clutch size in the year of study
was 6.0 ±.1 eggs (range, five-seven eggs, n = 24).
On days seven (n = 26) and eight (n = 5) after hatching,
we video filmed parental care of males and females for 3 h (median starting
time: 0723 h, range: 0647-1318 h). One male that was filmed on day nine after
hatching was later excluded from further analyses on feeding rates. We placed
tripods approximately 1 m from the nests 1 day before recording in order to
habituate the parents to their presence. There were no indications that the
video equipment disturbed the birds as they returned to the nest with food on
average 3 min 8 s (range, 0 min 18 s-11 min 11 s) after the person starting
the camera had left the nest site. Later the same day, males were caught in
mist nets at the nest site and weighed (to the nearest.1 g using a Pesola
spring balance). We also recorded tarsus length (to the nearest.1 mm using
slide calipers) and age (first-year or older;
Svensson, 1992). The males
were then brought to a field laboratory situated in the center of the study
area where the plumage reflectance of the blue patch and the central chestnut
spot was measured on live birds held in the hand by use of a PS1000 diode
array spectrometer (Ocean Optics, Dunedin, USA). Illumination in the 380-700
nm range was provided from a tungsten-halogen lamp using a bifurcated quartz
fiber optic probe held perpendicularly against a 5 mm wide measuring spot. The
probe was mounted inside a matt black plastic tube insuring the same distance
and angle relative to the measured patch between each scan and individual.
Using the CSPEC software (Ancal, Las Vegas, Nevada, USA), we measured spectral
reflectance in relation to a Spectralon (LabSphere, North Sutton, USA) white
standard (with 98-99% reflectance from 300 to 700 nm) and the software setting
"number of scans" set to one. For all males, we obtained five
repeated measures of the blue patch and three of the chestnut spot. The probe
was lifted and replaced between each repeated measure. In order to generate
reflectance curves for individual males, we calculated mean reflectance values
at every.5 nm wavelength between 380 and 700 nm. These values were adjusted by
use of running average calculated from 25 measuring points on either side of
the focal point to minimize the effect of potential measurement errors on the
slope of the curves. For the wavelengths near the end of our measurement range
(i.e., near 380 and 700 nm), running averages were calculated using the set of
available data points (e.g., for the measuring point at 380.5 nm, running
average was calculated from only one measuring point on either side).
Brightness (overall intensity) was calculated as the reflectance sum over
the 380-700 nm range (R380-R700). In calculating chroma
(spectral purity), we used information on visual pigment and oil droplet
absorption spectra for European starlings
(Hart et al., 1998) to
approximate avian color perception (bluethroats are likely to have a visual
system similar to starlings). For each of the plumage color elements, we first
decided which of the four cones was the most sensitive to the peak in
reflectance. For the blue patch, the average reflectance peak was near the
cross-over between the absorption spectra of the UV and shortwave-sensitive
cones and, therefore, we considered both cones to be sensitive to wavelengths
near the reflectance peak. For the chestnut spot, the peak reflectance was
within the range of the longwave-sensitive cone. We then found the range of
wavelengths to which the above-mentioned cones were the most sensitive. The
border of this range was set at the wavelength corresponding to the mid-point
between the peak sensitivities of the neighboring cones, and their associated
oil droplets, at either side of the peak reflectance range (the mid-point was
498 nm for the blue patch and 576 nm for the chestnut spot, respectively).
Chroma was then calculated as the reflectance sum over the peak reflectance
range divided by the total reflectance sum. Thus, chroma was calculated as
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max) and (2) as the wavelength for the maximum negative
slope (slope). Since our reflectance curves were truncated
at 380 nm (Figure 1a), and the
reflectance peak occurs near this wavelength
(Johnsen et al., 1998),
max could not be measured directly. Instead, we estimated
max based on a regression between max
and the slope of the reflectance curve in the 400-405 nm range derived from
complete reflectance curves for 60 male bluethroats measured in 1998
(R2 =.62, F2,57 = 46.75, p
<.0001, y = 22466582x2 + 61539x + 403.7). We included
slope as an alternative estimate since it has been found to
correlate strongly with human-perceived hue (CIELAB hue colorimetrics) for
other structural colors (Andersson et al.,
1998). Since the reflectance curve for the chestnut spot increased
monotonically (Figure 1b), we
calculated hue as the wavelength that corresponded to median reflectance
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This estimate was likewise highly correlated with human-perceived hue (CIELAB hue colorimetrics; rs = -.72, n = 30, p <.0001) and may also be related to avian perception of hue.
From the video recordings we noted the number of feeding visits by each parent as well as the time spent brooding by females. From these values, we calculated the absolute feeding rates for males and females, as well as the relative proportion of feedings provided by males. When calculating female feeding rate, we excluded time spent brooding because females are unable to feed the nestlings while brooding and, at this stage, females only brood their young during or immediately after rainfall. Since there was some variation in the time of day we recorded feeding rates, we tested for a potential correlation between the time of day and parental feeding rates. However, we found no differences in feeding rates between early, intermediate and late recordings (male feeding rate, H = 3.63, df = 2, p =.16; female feeding rate, H = 1.56, df = 2, p =.46). We calculated male body condition as residuals from a linear regression between body mass and tarsus length. In analyses involving brood mass at day seven after hatching (as a measure of nestling growth during days zero-seven after hatching), we only included cases where nestlings had not been added or removed (n = 13). One male was excluded from analyses on the blue throat patch because the reflectance curve had an aberrant form probably due to measurement error (a very low reflectance peak in the blue part of the spectrum). For analyses on feeding rates, one male was excluded because he gave alarm calls for more than 2 h during the recording. We used parametric statistics whenever the assumptions of such tests were met. When reporting descriptive data, we present means ± SE. We used StatView 5.0 for the Macintosh for the statistical analyses.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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The reflectance curve for the blue throat patch showed a decrease in reflected radiance from 380 to 700 nm (Figure 1a), identical to those reported in earlier studies (Andersson and Amundsen, 1997
; Johnsen et
al., 1998). For the chestnut spot, the reflectance curve showed an
increase in reflected radiance from 380 to 700 nm
(Figure 1b). In contrast to
other studies on the same species
(Andersson and Amundsen, 1997;
Andersson S, unpublished data; but see
Johnsen et al., 1998), we
found no statistically significant differences in the plumage reflectance of
first-year and older males (Table
1). Average tarsus length, used as an estimate of body size, was
30.53 ±.12 mm. Average size standardized body mass, used as an estimate
of body condition calculated as residuals from a regression of tarsus length
on body mass, was.00 ±.17. There were no significant correlations
between any color measure and male body size or body condition
(Table 2).
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Median hatching date was 26 June (range, 21 June-2 July). The mean brood mass on day seven after hatching was 76.3 ± 1.8 g, corresponding to a mean nestling mass of 12.7 ±.3 g. Mean feeding rates on day seven after hatching were 12.1 ±.7 (range, 5.2-19.8) and 13.9 ±.8 (range, 7.7-23.4) visits/h for males and females, respectively. On average, males provided 47.0 ± 2.3 (range, 20.0-68.0) percent of the feeding visits to the nest. There were no significant correlations between any color measure and the absolute feeding rate of males, although there was a close-to-significant negative correlation between the brightness of the chestnut spot and the absolute feeding rate of males (Table 3). Neither did any color measure correlate with the absolute feeding rate of females, or the relative proportion of feedings provided by males (Table 3). Finally, there were no significant correlations between any color measure and brood mass at day seven after hatching or hatching date (Table 4).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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We found no relationship between male plumage coloration and male or female parental care. Thus, there was no support for the hypothesis that male coloration signals male parental quality (Heywood, 1989
;
Hoelzer, 1989) or for the
hypothesis of differential investment by females mated to attractive males
(Burley, 1986). Moreover, we
found no support for the suggestion that male coloration reveals phenotypic
quality, since male coloration was not related to age, body size or body
condition. Finally, male coloration was not significantly related to hatching
date, a parameter that may indicate female mating preferences (e.g.,
Alatalo et al., 1984).
The main aim of our study was to test for a relationship between male
plumage coloration and paternal care. We found a trend for a negative
correlation between brightness of the chestnut spot and absolute male feeding
rate. However, this trend may have been accidental because, if performing
sequential table-wide Bonferroni correction
(Rice, 1989), the critical
is reduced to.0028 and the result is then far from significance. For
the other measures of male plumage reflectance, there were no trends for a
correlation with measures of nestling provisioning. Nestling provisioning is
an important aspect of avian parental care, and the measures of nestling
provisioning used in our study (absolute feeding rates of males and females,
and relative proportion of feedings provided by males) were similar to those
used in a number of previous studies (e.g.,
Linville et al., 1998;
Studd and Robertson, 1985b;
Sundberg and Larsson, 1994;
Wiehn, 1997). We also recorded
nestling growth (brood mass at day seven after hatching) as an indicator of
the total amount of food brought to the nestlings, since load size or food
quality may vary among parents and territories (e.g.,
Sætre et al., 1995).
Because nestling growth is due to the combined efforts of the male and the
female, this measure provides no information on the relative provisioning by
males or the females. Therefore, data on nestling growth can only supplement
information on sex-specific provisioning in testing the good parent and the
differential-allocation hypotheses
(Smiseth and Amundsen,
2000).
In our study, we focused on both the structurally derived blue patch and
the melanin-based central chestnut spot of male bluethroats. Gray
(1996) suggested that both
structural and melanin-based colors were poor candidates for being honest
signals of phenotypic quality compared with carotenoid-based colors. In
support of this suggestion, Hill and Brawner
(1998) found that, in the
house finch (Carpodacus mexicanus), coccidial infection had a
negative effect on carotenoid-based but not on melanin-based coloration.
According to Gray (1996),
honest signaling by structural and melanin-based colors requires additional
honesty-maintaining mechanisms, such as costs induced from social
interactions. In house sparrows (Passer domesticus), for instance,
social costs apparently maintain the honesty of the males' melanin-based
social badge (Møller,
1987a,b).
In bluethroats, males display the throat patch to other males
(Peiponen, 1960), suggesting
that it is involved in male-male competition. In contrast to what might be
expected if male plumage reflectance functions as a social badge in
competition with other males, field manipulations of the plumage reflectance
had no effect on the ability of males to retain territories
(Johnsen et al., 1998).
However, since the manipulations of male coloration were performed after
territory establishment, ownership asymmetries may have had a stronger effect
on the males' ability to retain their territories than plumage coloration.
Thus, it is unclear whether or not the social costs suggested by Gray
(1996) to be necessary for
maintaining the honesty of structural or melanin-based colors are present in
bluethroats.
Gray's (1996) suggestion
that structural colors are poor at revealing phenotypic quality has recently
been challenged by Fitzpatrick
(1998). She suggested that
structural colors, which are based on either wavelength-differential
reflections or interference patterns, might be honest signals of phenotypic
quality since minute variations during feather growth would produce changes in
plumage reflectance. Thus, according to Fitzpatrick
(1998), structural colors
might reveal developmental stability during feather growth. Based on the
morphology of structural UV and violet feathers, Andersson
(1999) speculated that both
amount and regularity of the color-producing nanostructure (present also in
bluethroats; Andersson S, personal communication) might carry production
costs. Recently, Keyser and Hill
(1999) found some support for
this suggestion in a study on blue grosbeaks (Guiraca caerulea). In
their study, a combined measure of patch size and color (measured
spectrometrically) was positively correlated with tail feather growth rate,
which they used as an index of nutritional condition. Keyser and Hill
(1999), therefore, concluded
that structural colors could be a condition-dependent signal. However, it
should be noted that, at present, the costs and constraints in production of
structural colors, and thus their potential for being honest signals of
phenotypic quality, are unknown.
Our results on bluethroats contrasts with a number of previous studies on
monogamous birds which have found either a positive
(Grant and Grant, 1987;
Hill, 1991;
Keyser and Hill, 2000;
Linville et al., 1998; Norris,
1990,
1993;
Palokangas et al., 1994;
Reyer et al., 1998;
Sætre et al., 1995;
Wiehn, 1997) or a negative
relationship (Burley, 1986,
1988;
Studd and Robertson, 1985b;
Sundberg and Larsson, 1994)
between male plumage coloration and parental care. Very few previous studies
have reported zero effects or non-significant relationships (but see
Lozano and Lemon, 1996). In
many cases where a positive or a negative relationship between male coloration
and male care have been found, the plumage colors seem to be either
carotenoid-based (Hill, 1990;
Linville and Breitwisch, 1997;
Sundberg and Larsson, 1994) or
social badges that function in competition with males
(Järvi and
Bakken, 1984;
Järvi et
al., 1987; Møller
1987a,b;
Studd and Robertson, 1985a).
This pattern might support Gray's
(1996) suggestion that
structural or melanin-based colors that lack additional honesty-maintaining
mechanisms are poor candidates for being honest signals of phenotypic quality.
Alternatively, it could simply reflect that most reported studies have been
conducted on species with carotenoid-based colors or colors that function as
badges in competition with other males. It is noteworthy that the first two
studies addressing relationships between parental care and structural colors
produce different results: our study on bluethroats found no correlation
between these two variables, while Keyser and Hill's
(2000) study on blue grosbeaks
found a weak, but statistically significant, positive correlation. Because
structural colors are widespread among birds (e.g.,
Gray, 1996), we encourage a
stronger emphasis on such colors, and not only on carotenoid-based colors, in
future research.
We found no differences in the plumage reflectance of first-year and older
males (see also Johnson et al., 1998). In contrast, other studies of the same
population found that older males had a higher reflectance in the UV part of
the spectrum, but not in the human-visible part, than first-year males
(Andersson and Amundsen, 1997;
Andersson S, unpublished data). This measure of chroma, which is based
primarily on wavelengths to which the UV cone is the most sensitive, is
somewhat different from ours, which is based on wavelengths to which both the
UV and the short-wavelength cones are the most sensitive. Thus, the difference
in the findings between these studies might be due to differences in
methodology. However, this is not likely to explain the absence of a
correlation between plumage reflectance and paternal care in our study since a
post hoc analysis of our data revealed no difference in the amount of
care provided by first-year and older males (mean feeding rate, 12.3
±.7 and 12.0 ± 1.0 visits per h for first-year and older males,
respectively; t =.19, df = 28, p =.85). Hence, females seem
not to obtain direct benefits by mating with older males. Since age may
reflect phenotypic quality, the positive results are consistent with indicator
models of sexual selection (see Andersson,
1994). However, regarding indirect (genetic) benefits, it is
currently controversial whether old males are of higher quality than the
younger ones (Hansen and Price,
1995; Kokko and
Lindström, 1996;
Manning, 1985;
Trivers, 1972).
Hatching date may be used as an indicator of male mating success and female
preferences since females of early-mated males may start egg laying (and hence
hatch their eggs) earlier (e.g., Alatalo et
al., 1984). In a previous study on blue-throats, females mated to
males with experimentally reduced UV reflectance had a later laying date than
control males, suggesting that female mating preferences are affected by male
coloration (Johnsen et al.,
1998). We found a no evidence of a relationship between male
coloration and hatching date. Hatching date may be influenced by a number of
factors other than male mating success (e.g., female quality, territory
quality and weather conditions between mating and egg laying). Therefore, it
is unclear to what extent hatching date can be used as an indicator of female
preferences. It is noteworthy that Johnsen et al.'s
(1998) UV reduction treatment
extended well beyond the natural variation in plumage reflectance, while our
study was restricted to the natural variation in plumage reflectance. This
suggests that strong treatment effects are required to overcome confounding
factors. Within the natural range of plumage reflectance, a female preference
has yet to be demonstrated.
In conclusion, we found no support for the good parent
(Heywood, 1989;
Hoelzer, 1989) or the
differential-allocation (Burley,
1986) hypotheses. Since the throat patch of blue-throats consists
of structural and melanin-based colors, rather than carotenoid-based ones,
this finding is consistent with Gray's
(1996) suggestion that, in
absence of additional honesty-maintaining mechanisms, structural and
melanin-based colors are poor candidates for being honest signals of
phenotypic quality. However, more studies are needed on potential
honesty-maintaining mechanisms for melanin-based and structural colors before
any firm conclusion can be drawn on this issue. More research is also needed
to clarify which color characteristics (brightness, chroma, or hue) are likely
to be associated with phenotypic quality.
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ACKNOWLEDGEMENTS |
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We thank Rolf J. Bu, Aase Kristin Eikenæs, Per-Anders Elvertrø, Asle Moen, Sonja Mork, and Roar Sandodden for field assistance, and Arild Johnsen and Jan T. Lifjeld for much appreciated cooperation during fieldwork. We thank R. J. Bu and A. K. Eikenæs for help with the video analyses and Innes C. Cuthill, Amber Keyser, Cristophe Pélabon, and two anonymous referees for comments on the manuscript. The study was financially supported by the Norwegian Research Council and the Nansen Endowment. The work was analyzed and written up while P.T.S. and T.A. visited the Department of Zoology, Göteborg University, in spring 1998. The visits were supported by the Nordic Academy for Advanced Study (P.T.S.) and the Norwegian University of Science and Technology (T.A.). P.T.S. and T.A. thank Malte Andersson for kind hospitality during the stay. We adhered to the legal requirements of performing experiments on animals in Norway, and the study protocol was approved by the Norwegian Committee for Research on Animals.
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