Behavioral Ecology Vol. 15 No. 4: 555–563
DOI: 10.1093/beheco/arh051
Repertoire size, sexual selection, and offspring
viability in the great reed warbler: changing
patterns in space and time
Wolfgang Forstmeier and Bernd Leisler
Max Planck Research Centre for Ornithology, Vogelwarte Radolfzell, Schloss Moeggingen,
Schlossallee 2, D-78315 Radolfzell, Germany
Only a few studies have focussed on the consistency of sexual selection patterns in space and time. One such case is the great reed
warbler (Acrocephalus arundinaceus), for which studies in Germany in 1981–1982 and Sweden in 1987–1991 strongly suggested that
the size of a male’s song repertoire was the target of mate choice and sexual selection. Studying the same German population
once again in 1994–2000, we investigated the consistency of these patterns over time as well as between populations. Our
reanalysis of the data from 1981–1982 shows that male repertoire size was positively correlated with male pairing success (harem
size) and with clutch size (adjusted for seasonal effects), whereas no such correlations were found during 1994–2000 in the same
population. We suggest that the earlier correlations were probably caused indirectly by covariation with territory quality, and that
a decline in population size has changed the role of territory quality. In the Swedish population, an earlier study found a striking
correlation between the size of a male’s repertoire and the viability of its offspring, suggesting good-gene effects. In contrast, we
found no such correlation in the German population, neither in 1981–1982 nor in 1994–2000. We conclude that repertoire size
does not seem to be a very reliable indicator of variation in male quality. Interestingly, the analysis of data from 1994–2000
showed that male pairing success was strongly correlated with measures of strophe length and immediate versatility, traits that
have been found to reflect male longevity. Future studies will have to show whether these performance-related traits are more
powerful indicators of male quality than is repertoire size. Key words: female choice, good genes, male-male competition, song,
territory quality. [Behav Ecol 15:555–563 (2004)]
he case of the great reed warbler, Acrocephalus arundinaceus, is probably the most frequently cited example
showing that song is subject to directional female mating
preferences. In the laboratory, females preferentially solicited
copulations from males with large syllable repertoires (i.e., the
number of different syllable types in their song; Catchpole et
al., 1986). Moreover, two field studies found that males with
large repertoires attracted more females and tended to mate
polygynously, whereas males with small repertoires often remained unmated (Catchpole, 1986; Catchpole et al., 1985;
Hasselquist, 1994, 1998). Finally, Hasselquist et al. (1996)
showed that females paired to males with small repertoires
sought extrapair copulations from neighboring males that
had larger repertoires. Females may have profited from this
behavior because offspring viability was positively correlated
with the repertoire size of the genetic father (Hasselquist et
al., 1996). Thus, several studies suggested that repertoire size
may be an indicator of male quality and that females preferentially mate with good singers.
Only a few studies have investigated the consistency of
patterns of mate choice and sexual selection in space and time
(see Dale et al., 1999). The aim of the present study is to
present new data on sexual selection in the great reed warbler,
which allows us to make such comparisons between different
periods of time as well as between different study populations.
Two great reed warbler populations have so far been studied
extensively. First, there is a German breeding population at
the Fränkische Weihergebiet in northern Bavaria, where de-
T
Address correspondence to W. Forstmeier, who is now at Department of Animal and Plant Sciences, University of Sheffield, Western
Bank, Sheffield S10 2TN, UK, E-mail: [email protected].
Received 25 November 2003; revised 28 August 2003; accepted 28
August 2003.
tailed information on the mating system has been collected
from 1977 onward (Beier 1981; Catchpole et al., 1985; Leisler
et al., 1995, 2000). This population has been studied in
terms of how song relates to sexual selection during 1981
(Catchpole, 1986) and 1982 (partly published in Leisler
et al., 1995). Second, there is a breeding population at lake
Kvismaren in south-central Sweden, where detailed investigations started in 1983 (Bensch and Hasselquist, 1991; Bensch et
al., 1998). Studies focussing on song characteristics of male
great reed warblers have been conducted between 1987 and
1991 (Hasselquist, 1994, 1998; Hasselquist et al., 1996).
Here we present new data collected in the above-mentioned
German population during a 7-year period from 1994–2000,
and we examine these data in relation to the earlier findings.
To facilitate the comparison with the new data, we combine
the data from 1981 (Catchpole, 1986) with those from 1982
(Leisler et al., 1995), and we reanalyze some of the former
results with an increased power. We also investigate the
consistency of patterns between the German and the Swedish
population. We focus on three aspects of male reproductive
success: (1) harem size, (2) clutch size, and (3) offspring
viability.
Studying the German great reed warbler population in
1981, Catchpole (1986) found a strong positive correlation
between the size of a male’s repertoire and its success in
attracting females (measured as harem size). A similar, but
less strong, pattern was also observed in 1982 (Leisler et al.,
1995). However, in both years, territory quality was a better
predictor of harem size than was male repertoire size, and the
analyses published so far do not support any conclusion as to
whether there was a female preference for good singers
beyond that which was explained by territory quality. Thus,
females may have aimed for direct benefits resulting from
high territory quality and may not have chosen particular
males for their high genetic quality. A similar problem applies
Behavioral Ecology vol. 15 no. 4 International Society for Behavioral Ecology 2004; all rights reserved.
556
to the Swedish study population. Male repertoire size was
positively correlated with harem size, but this pattern was
confounded by variation in male age (Hasselquist, 1994,
1998). In this population, repertoire size and pairing success
both increased with age, and the correlation between repertoire and harem size was nonsignificant when male age was
statistically controlled for (Hasselquist, 1994, 1998).
Catchpole (1986) found a particularly strong positive correlation between male repertoire size and reproductive success. This was primarily owing to variation in harem size, but
also to a positive trend between repertoire size and clutch size
(r ¼ .27; p ¼ .092; data from 1981) (Catchpole, 1986). This
latter correlation may have arisen from (1) differential
allocation (i.e., females paired with attractive males increased
their investment by producing larger clutches; see Burley,
1986; Sheldon, 2000), (2) differential attraction (i.e., good
singers attracted higher-quality females; see Burley, 1986),
or (3) variation in territory quality (i.e., good singers held
higher-quality territories which facilitated laying larger
clutches). Although the available data do not allow us to
distinguish between these alternatives, we want to test whether
the observed trend between clutch size and repertoire size
would still hold for an increased data set.
Studying the Swedish great reed warbler population,
Hasselquist et al. (1996) found a positive correlation between
male repertoire size and the viability of the offspring, suggesting that repertoire size reflects the genetic quality of
males. We use the data collected during the first (1981–1982)
and second (1994–2000) study period to test whether this
finding also holds for the German population.
In addition to repertoire size, we focus on a song parameter
termed ‘‘syllable switching,’’ which is the number of syllabletype switches within a strophe. This measure reflects strophe
length as well as immediate versatility. As described elsewhere,
we found that syllable switching (but not repertoire size) was
positively correlated with male longevity, suggesting that this
song measure might reveal aspects of male quality (Forstmeier
W, Hasselquist D, Bensch S, and Leisler B, unpublished data).
Unfortunately, measures of syllable switching are not available
for the earlier studies.
METHODS
Population data
The study site is located in the Franken area of southern
Germany (49 409 N, 10 519 E, 290 m above sea level). For
a detailed description of the area, see Beier (1981) and Leisler
et al. (1995). The great reed warbler population breeding at
this site has been monitored intensively since 1973. Data have
been collected at least weekly each year from the end of April
onward during the whole breeding season, and every second
day during the peak breeding-season in June (averaging
approximately 38 visits per year). We provided almost all adult
birds with color rings and nestlings with metal rings. We
mapped all singing males and searched all reed fringes for
nests from the water side. For each nest we tried to determine
the date of deposition of the first egg (assuming that females
lay one egg per day) as well as clutch size and the number of
young fledged. We defined the size of a male’s harem as the
number of females that were nesting together with him
(males defend the nests of their females).
For the present study, we focus on the breeding seasons
from 1994–2000, as song recordings of males were collected
during this period (see below). On average, 23.1 males
(range ¼ 18–32) were holding a territory in the area each
year, resulting in a total of 162 male breeding seasons. This
sample comprised 87 individual males, and 86 of them were
Behavioral Ecology Vol. 15 No. 4
color-ringed. We tried to collect one song recording from
each male in each year, but because of time limitations and
restrictions of the permit, we managed to sample only a subset
of the males present. No male was recorded twice within the
same season. On average, we recorded 15.6 males (67% of
those present) per breeding season (range ¼ 12–22), resulting
in a total of 109 recordings stemming from 62 individual
males (71% of all males). We carefully examined whether
taking this subset introduced any systematic bias into our
sample. However, males that we managed to record did not
differ significantly from those that we failed to record in any
trait measured: wing length (see below; t 73 ¼ 1.2; p ¼ .25),
body mass (see below; t 73 ¼ 0.3; p ¼ .76), harem size (KruskalWallis test; v2 ¼ 2.0; df ¼ 2; p ¼ .39), male age (first-year versus
older; Fisher’s Exact test; v2 ¼ 2.9; p ¼ .12), or whether
male age was known from ringing as a nestling or not (see
below; Fisher’s Exact test; v2 ¼ 0.6; p ¼ .51).
We measured male pairing success in terms of the number
of females attracted (harem size). Out of the 109 male breeding seasons, males remained unmated in 28 cases (26%),
monogamy occurred in 59 cases (54%), bigamy was recorded
21 times (19%) and the status of one male could not be
determined with certainty (leaving us with n ¼ 108). The
population-wide mean of harem size varied considerably
between the years (from 1.22–0.61). We used two different
methods of accounting for this interannual variation.
First, when estimating the effect of song characteristics on
harem size, we used generalized linear models with a Poisson
error distribution (using SAS, procedure: ‘‘genmod’’; options:
‘‘error ¼ poisson’’ and ‘‘dscale’’) for which year could be
entered as a separate factor (i.e., a dummy variable accounting for sex-ratio variation). We used male breeding seasons
(n ¼ 108) as the independent statistical unit. This widely used
approach maximizes the power for detecting patterns but
assumes that there is no major effect of the individual male.
We selected this approach (rather than deleting repeated
measures of the same male), because we were concerned with
making a type II error (not finding what earlier studies have
found) rather than falsely rejecting the null hypothesis.
Second, to avoid the possible effect of pseudoreplication
inherent in the first approach, we used lifetime averages of
males (n ¼ 62) in the second approach. We first standardized
all harem sizes in relation to the population mean of harem
size of the respective breeding season (referred to as residual
harem size; ranging from 1.22 to 1.24 females). This
procedure is to scale male pairing success in relation to the
number of females available. Second, we calculated the mean
of these residuals for each male (averaged among years).
Multiple measures of a male’s song characteristics were
also averaged among years (without prior standardization).
Neither repertoire size nor syllable switching of individual
males increased with age (Forstmeier W, Hasselquist D,
Bensch S, and Leisler B, unpublished data), thus taking
lifetime averages did not introduce any bias caused by
variation in male longevity.
We adopted the same procedures when reanalyzing the data
from 1981 (mean harem size ¼ 1.00; song recordings available
for n ¼ 37 males; Catchpole, 1986) and 1982 (mean harem
size ¼ 1.03; song recordings available for n ¼ 27 males, 15 of
which were new to the study area; Leisler et al., 1995). This
time, however, we did not enter a year effect into the generalized linear models, owing to the very small interannual
variation in mean harem size.
To examine variation in clutch size in relation to song
characteristics, we adopted the same two methodological approaches as above: (1) we used generalized linear models with
Poisson error structure, considering each clutch as statistically
independent (1994–2000: n ¼ 105; 1981–1982: n ¼ 66) and
Forstmeier and Leisler
•
Song and sexual selection
entering date of clutch initiation as a covariate to account for
seasonal changes in clutch size, and (2) we first controlled for
seasonal effects by taking the residuals from a regression of
clutch size over the date of deposition of the first egg measured in days after the first of May (y ¼ 5.665 0.032x; r ¼
.50; p , .0001; n ¼ 966; data from 1973–2000). Second, we
calculated mean residual-clutch size for each male (averaged
among all nests in a male’s lifetime; 1994–2000: n ¼ 47 males;
1981–1982: n ¼ 39 males).
When capturing males in mist-nets, we measured wing
length and body weight. Nineteen out of the 62 males were
trapped in more than one season, so multiple measures were
sometimes available. Male age was known exactly from ringing
as a nestling for 25 of the 62 males (40%). For the others,
which we observed for the first time as adults, we can only
provide an estimate of minimum age (at least 1 year when first
recorded as adult), but it is likely that many of these birds
were actually 1 year old (data not shown).
We measured the survival of the offspring produced by the
62 males included in this study (data from 1994–2000). These
males altogether fledged 418 young (in the years from 1989–
2000), and 44 of them (9.5%) were seen again as recruits in
the following years (up until 2001). As 98 of 165 adult breeders
(59%) in the study area (all males and females from 1994–
2001) had not been ringed as nestlings in the area, we
estimate that only about two of five recruits will return to their
natal area. This sets some limitations on the estimation of
offspring survival rates in relation to song characteristics of
their father. A second problem is that we cannot be sure
whether all of these young were actually sired by their social
fathers. A paternity study conducted in this population (in
1992–1997) showed that 19 out of 194 nestlings (9.8%) were
sired by an extrapair male (Leisler et al., 2000). This paternity
analysis included 140 offspring, which are part of the 441
fledglings (33%) considered here. Fourteen of these (10%)
were sired by an unknown extrapair male. By excluding these
14 young from our analysis, we can expect to reduce the
confounding effect of extrapair paternity by approximately
one third.
We also looked at offspring survival of those males that had
been recorded from 1981–1982 (n ¼ 48 color-ringed males).
These males altogether fledged 291 young (in the years from
1977–1986), and 18 of them (6.2%) were seen again as
recruits in later years.
Song characteristics
Male great reed warblers have two different kinds of song that
differ in length. Unmated males sing long strophes (long
song) consisting of a wide variety of high-amplitude elements,
whereas mated males that are guarding their female produce
short strophes (short song) that consist of relatively few,
stereotyped, low-amplitude syllables (Catchpole, 1983). Apparently in an effort to attract a second female, some mated
males start producing long song again once their first female
is incubating (Hasselquist and Bensch, 1991). Playback
experiments in the laboratory as well as in the field have
shown that long song is attractive to females, whereas short
song functions as a territory defense (Catchpole, 1983;
Catchpole et al., 1986).
We recorded males that were singing a minimum of 30
consecutive strophes of long song. Thirty-three males were
recorded in only 1 year, 17 males in 2 years, seven males in 3
years, four males in 4 years, and one male in 5 years (109
recordings in total). Recordings were made by using a Sony
TC-D5 PROII recorder and a Sennheiser K6M67 microphone
and were analyzed with Avisoft SASLab Pro 3.4 using the
following settings: sampling frequency, 22,050 Hz; 16 bits;
557
time resolution, 5.8 ms; and bandwidth, 111 Hz. From each of
the 109 recordings, we extracted the following two song
parameters.
Repertoire size
We categorized syllable types (all done by the same person to
ensure consistency of classifications) by visual inspection of
printed spectrograms. We defined the number of different
syllable types found in 30 consecutive strophes as repertoire
size. As shown in Catchpole (1986), new syllable types only
rarely turned up after the first 20 strophes had been analyzed.
Our measure of a male’s repertoire size showed significant
repeatability among years (R ¼ .34; p ¼ .005; for details, see
Forstmeier W, Hasselquist D, Bensch S, and Leisler B,
unpublished data). Differences in mean repertoire size that
seem to exist between various studies (see Catchpole, 1986;
Fischer et al., 1996; Hasselquist, 1994; this study) simply
reflect methodological differences, that is, splitting versus
lumping of syllable types by different observers (Forstmeier W,
Hasselquist D, Bensch S, and Leisler B, unpublished data).
Note that the estimates of repertoire size of birds from the
first period of study (1981–1982) were made by a different
person (Catchpole, 1986) and are therefore not directly
comparable to our estimates of males from 1994–2000. In
cases in which such direct comparison was needed, we ztransformed repertoire estimates within studies.
Syllable switching
For each strophe, we counted the number of syllable-type
switches within the strophe. This parameter is highly
correlated with strophe length measured in seconds (r ¼
.78), as long strophes contain more syllable switches than do
short strophes. Besides that, syllable switching can be
interpreted as a measure of immediate variety, as it typically
equals repertoire size of the strophe minus one. Nevertheless,
there was only a very weak positive correlation between syllable
switching (measured per strophe and averaged among 30
strophes) and total repertoire size (measured per 30 strophes;
r ¼ .22, p ¼ .023, n ¼ 109 recordings; and r ¼ .14, p ¼ .27, n ¼
62 male lifetime means). Our measure of a male’s syllable
switching showed significant repeatability among years (R ¼
.36; p ¼ .003) (see Forstmeier W, Hasselquist D, Bensch S, and
Leisler B, unpublished data). Strophe length as measured in
seconds was slightly less repeatable (R ¼ .28; p ¼ .017) and
therefore did not seem worth a separate analysis. However, as
strophe length is a widely used measure in the birdsong
literature, we present some of the main results for strophe
length as well.
RESULTS
Throughout this section, generalized linear models (GLMs)
based on male breeding seasons as the statistical unit are
presented in the main text, whereas the corresponding
regression analyses based on lifetime averages of males are
used for graphical illustration and are presented in the figure
legends.
Repertoire size and male pairing success
During 1981–1982, male repertoire size had a strong positive
effect on harem size (GLM, v21;62 ¼ 9.6; p ¼ .002) (Figure 1a).
This correlation was not confounded by variation in male age
(GLM, age: v21;61 ¼ 0.9; p ¼ .34; repertoire size: v21;61 ¼ 0.9; p ¼
.004). However, as outlined in Catchpole (1986), variation in
territory quality seemed to be a major confounding factor.
Territory quality was estimated in terms of the amount of reedwater interface (‘‘edge length’’ measured in meters), which
558
Figure 1
Relation between a male’s repertoire size and his success in
attracting females in 1981–1982 (r ¼ .42; n ¼ 52; p ¼ .002) (a) and
in 1994–2000 (r ¼ .04; n ¼ 62; p ¼ .78) (b). The difference between
the two effect sizes is significant (p ¼ .042). Residual harem size is
a measure of male pairing success relative to that of other males
present in the same year. Average values are shown for individuals
present in more than one season. The fact that repertoire size
estimates in panel b are considerably larger than in panel a reflects
methodological differences (splitting versus lumping of syllable
types by different observers).
appears to be important in both feeding and nesting ecology
(Bensch et al., 2001; Catchpole et al., 1985; Leisler et al.,
1995). Harem size was strongly positively correlated with edge
length (GLM, v21;60 ¼ 16.9; p , .0001; data for two males
missing), and repertoire size was also strongly correlated with
edge length (r ¼ .44; n ¼ 62; p , .001). When both parameters
were entered together, edge length, but not repertoire size,
had a significant effect on harem size (GLM, edge length:
v21;59 ¼ 7.9; p ¼ .005; repertoire size: v21;59 ¼ 2.4; p ¼ .125). The
same result was obtained with the second method of analyzing
lifetime averages. By using partial regression (as done by
Catchpole, 1986), we found no significant effect of repertoire
size on residual harem size when edge length was statistically
controlled for (redge¼const ¼ .20; df ¼ 47; p ¼ .18). Thus, we
could not detect a female preference for males with large
repertoires beyond that which was already explained by
variation in territory quality.
In contrast to these patterns, we found no positive effect of
repertoire size on harem size during the second period of
Behavioral Ecology Vol. 15 No. 4
study, 1994–2000 (GLM, year: v26;100 ¼ 13.2; p ¼ .041;
repertoire size: v21;100 ¼ 0.8; p ¼ .376) (Figure 1b). When
the full data set from both study periods within the
same model was used, and separate slopes for the repertoire-size effect for the two periods were estimated, the
difference in slopes was not large enough to reach statistical
significance (p ¼ .092). However, with the second approach of
analyzing lifetime averages, the effect sizes of repertoire size
differed significantly between the two study periods (p ¼ .042)
(Figure 1).
To understand this discrepancy between the two study
periods, it would be important to know how territory quality
was related to repertoire size and harem size during this
second study period. Unfortunately, no territory characteristics were measured after 1982. Thus, we have no direct way
of estimating these correlations, but we can test whether
specific territories were associated with these parameters in
some way. During the 7 years of the second study period, 20
territories had been occupied consecutively by two or more
different males (53 territory owners). An ANOVA shows that
there was not a significant association between those territories
and the repertoire size of the males occupying them (F19,33 ¼
1.1; p ¼ .37), nor was there an association between territories
and residual harem size (F19,33 ¼ 1.3; p ¼ .23). In contrast, we
found a highly significant association between territories and
the body weight of the territory owners (F17,22 ¼ 3.4; p ¼ .004),
which means that owners of the same territory were more
similar in body weight to each other than expected by chance.
This indicates that the above analysis had the power to detect
such associations. However, it might be argued that such an
analysis comprises only the territories of highest quality (those
that were occupied repeatedly), and that the main variation in
territory quality might be found between multiply occupied
territories and those that had been occupied only once. Yet
again, the owners of these two types of territories did not
differ in repertoire size (t107 ¼ 0.7; p ¼ .49; using male
breeding seasons as independent samples), and these
territories also did not differ in residual harem size (t106 ¼
0.7; p ¼ .48).
Clutch size in relation to repertoire size
For the first period of study, we found a significant positive
effect of a male’s repertoire size on the clutch size produced by
its females (GLM, accounting for seasonal effects: date: v21;63 ¼
17.6; p , .0001; repertoire size: v21;63 ¼ 4.6; p ¼ .032) (Figure
2a). Variation in territory quality did not seem to be
a confounding factor, as the correlation between edge length
and clutch size was very weak (GLM, date: v21;63 ¼ 18.9; p ,
.0001; edge length: v21;63 ¼ 0.4; p ¼ .536). When edge length as
a covariate was entered, there was still a positive effect of
repertoire size on clutch size (GLM, date: v21;62 ¼ 17.4; p ,
.0001; edge length: v21;62 ¼ 0.1; p ¼ .745; repertoire size: v21;62 ¼
4.2; p ¼ .040).
In contrast, repertoire size was not positively correlated with
clutch size during the later years (GLM, date: v21;102 ¼ 69.2;
p , .0001; repertoire size: v21;102 ¼ 0.4; p ¼ .529) (Figure 2b).
When the full data set from both study periods within the
same model was used, and separate slopes for the repertoiresize effect for the two periods was estimated, these slopes
differed significantly (p ¼ .034).
Correlation between repertoire size and offspring viability
Thirty-five of the 48 males from the first period of study
produced at least one fledgling in any of the years from 1977–
1986 (mean ¼ 8.3, range ¼ 2–23 fledglings). Eighteen of
these 291 fledglings (6.2%) were resighted as recruits.
Forstmeier and Leisler
•
Song and sexual selection
Figure 2
Relation between a male’s repertoire size and the size of the
clutches produced by his females in 1981–1982 (r ¼ .30; n ¼ 39;
p ¼ .065) (a) and in 1994–2000 (r ¼ .21; n ¼ 47; p ¼ .16) (b).
The difference between the two effect sizes is significant (p ¼ .024).
Residual clutch size is adjusted for variation in laying date.
Following the methods used by Hasselquist et al. (1996: Figure
2a), we plotted the number of a male’s lifetime recruits over
the number of lifetime fledglings, excluding the males that
produced no fledglings. The equation of this regression was
y ¼ 0.072x 0.084 (r ¼ .428; n ¼ 35; p ¼ .010). As males without fledglings cannot produce any recruits, we forced this
regression through the origin (y ¼ 0.065 [60.014 SE]x; t34 ¼
4.6; p , .0001) and took the residuals from this regression.
In contrast to the findings of Hasselquist et al. (1996), the
residual number of lifetime recruits was not significantly
correlated with repertoire size (r ¼ .10; n ¼ 35; p ¼ .55)
(Figure 3a). However, this correlation coefficient did not
differ significantly from that obtained by Hasselquist et al.
(1996) (r ¼ .32; n ¼ 48 versus r ¼ .10; n ¼ 35; p ¼ .32), and the
power of our test, assuming an effect size as found by
Hasselquist et al. (1996), was only 62%.
During the second period of study, 44 of the 62 males
produced at least one fledgling in any of the years from 1989–
2000 (mean ¼ 9.5, range ¼ 3–26 fledglings). Up until 2001, 44
of these 418 fledglings (9.5%) were resighted in the study area
559
Figure 3
Relation between a male’s repertoire size and the relative
postfledging survival of his offspring in 1981–1982 (r ¼ .10; n ¼ 35;
p ¼ .55) (a) and in 1994 –2000 (r ¼ .11; n ¼ 44; p ¼ .50) (b). The
difference between the two effect sizes is not significant (p ¼ .374).
The relative survival of a male’s offspring is quantified by taking the
residuals from a regression of the number of lifetime recruits over
the number of lifetime fledglings that were produced by a male.
as recruits. We took residuals from a regression of the number
of lifetime recruits over the number of lifetime fledglings (y ¼
0.107 [60.017 SE]x; t43 ¼ 6.4; p , .0001). Also in this sample,
the residual number of lifetime recruits was not positively
correlated with repertoire size (r ¼ .11; n ¼ 44; p ¼ .50)
(Figure 3b). This correlation coefficient differed significantly
from that obtained by Hasselquist et al. (1996) (p ¼ .046), and
the power of our test was 71%. When the data from 1981–1982
were pooled with those from 1994 –2000 (adjusting for the
differences in repertoire size estimates), the overall correlation was close to zero (r ¼ .03; n ¼ 79; p ¼ .77) and the
power of this test was 91%.
A problem with this result is that the analysis does not
control for the occurrence of extrapair paternity, which
should concern approximately 10% of the offspring (Leisler
et al., 2000). Paternity data are available for 140 out of 418
fledglings (33%) from the second period of study, and these
include 14 extrapair young (10%). By deleting these 14 offspring with unknown genetic fathers from the above analysis
(three out of 44 males were concerned), we can hope to
reduce the confounding effect of extrapair paternity by
approximately one third. However, the correlation between
Behavioral Ecology Vol. 15 No. 4
560
residual lifetime recruits and repertoire size did not change
toward a positive direction (r ¼ .12; n ¼ 44; p ¼ .45). Finally,
we repeated the analysis, basing it on the survival of the 126
fledglings for which we could confirm that the social father
had also been the genetic father. Note that this leaves us with
a greatly reduced sample of only 21 males. However, again, we
could not find a positive correlation between offspring
survival and repertoire size of the father (r ¼ .10; n ¼ 21;
p ¼ .60).
Syllable switching and sexual selection
Syllable switching was measured only during the second
period of study. This song trait showed a significant positive
effect on harem size (GLM, year: v26;100 ¼ 12.9; p ¼ .045;
syllable switching: v21;100 ¼ 6.6; p ¼ .010) (Figure 4a). The
effect changed only marginally when the outlier (Figure 4a)
was removed (GLM, year: v26;99 ¼ 11.6; p ¼ .071; syllable
switching: v21;99 ¼ 6.6; p ¼ .010). When the more widely used
parameter strophe length as a substitute of syllable switching
was used, a significant effect was found as well (GLM, year:
v26;98 ¼ 15.4; p ¼ .017; strophe length: v21;98 ¼ 5.9; p ¼ .015;
data for two males lacking). Unlike in the first study period,
male age was a potentially confounding factor in this analysis,
as it significantly affected harem size (GLM, year: v26;100 ¼ 15.9;
p ¼ 0.015; age: v21;100 ¼ 6.0; p ¼ .014). However, when age was
entered as a covariate, the effect of syllable switching was still
significant (GLM: year: v26;99 ¼ 16.3; p ¼ .012; age: v21;99 ¼ 4.9;
p ¼ .027; syllable switching: v21;99 ¼ 5.5; p ¼ .019). This was also
true when strophe length was used as a substitute of syllable
switching (data not shown). Territory quality did not seem to
be a confounding factor, as there was no association between
multiply occupied territories and the syllable switching of the
males occupying them (F19,33 ¼ 1.1; p ¼ .40). Moreover,
multiply occupied territories did not differ from singly
occupied territories in terms of syllable switching of their
owners (t107 ¼ 1.2; p ¼ 0.24; using male breeding seasons as
independent samples).
A male’s syllable switching had no positive effect on clutch
size (GLM, date: v21;102 ¼ 66.4; p , .0001; syllable switching:
v21;102 ¼ 1.2; p ¼ .276) (Figure 4b) or offspring viability (r ¼
.09; n ¼ 44; p ¼ .57) (Figure 4c). Excluding the 14 extrapair
young from the latter analysis did not affect the outcome (r ¼
.07; n ¼ 44; p ¼ .65) ,and restriction to the cases in which
paternity was known did not yield a significant result either
(r ¼ .04; n ¼ 21; p ¼ .87).
DISCUSSION
Repertoire size and male pairing success
Figure 4
The number of syllable-type switches per strophe in the songs of
males in relation to a male’s pairing success (r ¼ .36; n ¼ 62;
p ¼ .004) (a), clutch size produced by a male’s females (r ¼ .14;
n ¼ 47; p ¼ .36) (b), and postfledging survival of a male’s offspring
(r ¼ .09; n ¼ 44; p ¼ .57) (c). With the outlier removed these
correlations were as follows: (a) r ¼ .36, n ¼ 61, p ¼ .005; (b)
r ¼ .19, n ¼ 46, p ¼ .21; and (c) r ¼ .06, n ¼ 43, p ¼ .69. For
further details see legends of Figures 1 through 3.
Two previous studies have found a positive correlation between the repertoire size of male great reed warblers and
harem size.
First, studying the German great reed warbler population in
1981, Catchpole (1986) found a very strong correlation
between repertoire size and harem size (r ¼ .49), and this
correlation remained to some extent when territory quality
was statistically controlled for by using partial regression
(redge¼const ¼ .28; n ¼ 37; p ¼ .093; Catchpole, 1986). After
combining these data with those collected in the same
population in the following year, our analysis shows that this
partial correlation is very weak at best (redge¼const ¼ .20; n ¼
50; p ¼ .18). We conclude that the strong correlation between
repertoire size and harem size (r ¼ .42; based on the full data
set) is probably caused indirectly. Males with large repertoires
occupied the territories of highest quality, and females
preferentially settled in the best territories.
Forstmeier and Leisler
•
Song and sexual selection
Second, studies on the Swedish breeding population also
found a significant positive correlation between repertoire
size and harem size (r ¼ .36; Hasselquist, 1994, 1998). In this
case, it appeared that the correlation was indirectly caused by
covariation with male age. After the age-related increase in
repertoire size was controlled for (which exists in the Swedish
but not the German population; Forstmeier W, Hasselquist D,
Bensch S, and Leisler B, unpublished data), there was not
even a positive trend for males with larger age-adjusted
repertoire size to attract more females than others (Hasselquist, 1994, 1998).
In conclusion, both studies could not detect a significant
female preference for pairing with males having large
repertoires in addition to what was attributable to variation
in age or territory quality. The new results presented in this
article (from 1994–2000) are in agreement with this interpretation, as no female preference was apparent. The strong
correlation observed between repertoire size and territory
quality during the first period of study suggests that repertoire
size might be related to male-male competition (as found in
several other species: Hiebert et al., 1989; Krebs et al., 1978;
Yasukawa, 1981) rather than to female choice.
For the remaining question, namely, why the relationship
between repertoire size and territory quality has changed over
the past two decades, we suggest the following explanation.
When the first study was conducted, there were on average 38
males holding territories in our study area. In the later years
the numbers of territory owners ranged from 18 to 32 males
only (mean ¼ 23). If one assumes that this population decline
was not caused by deterioration of the breeding habitat
(which is suggested by an increasing annual reproductive
success; data not shown), but by other factors (data not
shown), then the intensity of male-male competition may have
been relaxed during the later period. It might well be that all
males had the opportunity to settle in territories of high
quality, and with the lack of variation in territory quality, the
correlation with repertoire size may have disappeared. This
idea is supported by the fact that we could not detect any
female preferences for certain territories (ANOVA using
repeatedly occupied territories, and comparison between
singly and repeatedly occupied territories).
Clutch size in relation to repertoire size
During the first period of study, we found that females paired
to males with large repertoires laid larger clutches, which,
assuming that repertoire size reflects male quality, would be
consistent with idea of differential allocation (Burley, 1986;
Sheldon, 2000). However, this finding should be interpreted
with caution. First, we cannot distinguish between differential
allocation and differential attraction, that is, the possibility
that males with large repertoires attracted higher-quality females. Second, we cannot exclude the possibility that females
adjusted their clutch size in response to some aspect of
territory quality other than edge length, and that repertoire
size was also correlated with this unknown aspect of territory
quality. In other words, controlling statistically for variation in
edge length does not mean that all aspects of territory quality
are held constant (see also Sheldon, 2000). Thus, given that
we could not detect a female preference for males with large
repertoires (partial regression), it appears somewhat unlikely
that females differentially allocated resources in response to
male repertoire size. Rather it might be that repertoire size
was strongly correlated with territory quality (which is only
partly described by edge length) and that females adjusted
their clutch size in relation to territory quality. Interestingly,
the correlation between clutch size and repertoire size has
changed significantly between the two periods of study
561
(Figure 2), just as the correlation between harem size and
repertoire size has disappeared (Figure 1). We consider it
likely that these changes over time were caused by the same
factor, namely, by a loss of the covariation between repertoire
size and territory quality.
Repertoire size and offspring viability
Despite high statistical power, we found no positive correlation between a male’s repertoire size and the probability of
survival of his offspring. Although we were not able to check
the paternity of most of the offspring included in the analysis,
there were no indications that partially controlling for this
confounding factor would shift the correlation in a positive
direction. Moreover, if it is true that males with low repertoire
size are typically cuckolded by extrapair males with larger
repertoires (Hasselquist et al., 1996), we should expect that
males with large repertoires do not lose paternity in their own
nests. This means that our estimates of offspring survival (the
y-axis in Figure 3b) would be most reliable for males with large
repertoires. However, for the top 25% of males with largest
repertoires offspring survival was still 0.35 SD below the
overall mean. This indicates that the lack of a positive
correlation between repertoire size and offspring survival
cannot solely be attributable to the failure to fully determine
genetic paternity in our study.
A recent meta-analysis of good-genes effects in sexual
selection (Møller and Alatalo, 1999) suggests that the effect of
male genetic quality on offspring survival may be quite small
(r ¼ .122). Thus, unfeasibly large sample sizes would be
required to nullify the existence of such a small effect in
a particular case study (a one-tailed power of 95% would be
reached with a sample size of 718 males).
The distinction between preference and choice
In the bird-song literature, there seems to be overwhelming
support for the idea that females prefer to mate with males
having large repertoires (Catchpole and Slater, 1995). There
are two lines of evidence: demonstrations of female mating
preferences in the laboratory (see Catchpole, et al., 1986),
and correlative findings of field studies in which males with
large repertoires were more successful in attracting females
(see Catchpole, 1986). Although these two approaches seem
to perfectly complement each other, they often do not
measure the same thing. It is important to make the distinction between female mating preferences and female choice.
In the laboratory, females may readily display their mating
preferences because all other factors are being controlled for.
In contrast, these preferences may be of little importance
under natural circumstances (Searcy, 1992). The outcome of
female choice in the field may depend much more strongly
on territory characteristics than on the song characteristics of
the male (Searcy and Yasukawa, 1996). Because good-genes
effects are typically small (see above), it has often been argued
that direct benefits (e.g., resulting from high territory quality)
should be more important to females than are indirect
benefits. This view is supported by the present study (the data
from 1981–1982), in which female choice was strongly related
to measurable aspects of territory quality, but not to song
characteristics, when territory quality was controlled for,
which also seems to be the case in many other studies (Searcy
and Yasukawa, 1996). Under such circumstances, the outcome
of female choice will largely depend on the outcome of malemale competition for the best territories.
Assuming that male-male competition and territory quality
are the major components contributing to the patterns under
consideration (the relation between repertoire size and
Behavioral Ecology Vol. 15 No. 4
562
harem size), the low temporal and spatial stability of these
patterns is not so astonishing. The intensity of competition
may change as the size of the population changes, and it will
also depend on the magnitude of differences in habitat
quality. In other words, intrasexual selection is highly
dependent on ecological circumstances and will therefore
be as changeable as these circumstances are.
Are repertoires acoustic peacock tails?
If large repertoires were very costly to maintain (as costly as
a peacock’s tail), we would expect that the ornament would
reflect the overall quality of males (Zahavi, 1975). Individuals
well adapted to their environment (including resistance to
parasites and adaptation to autecological factors) would display larger repertoires than do less well adapted individuals or
individuals carrying deleterious mutations. Such differences
in the overall quality of individuals would very likely also affect
a male’s pairing success and the viability of his offspring.
Thus, we would at least have expected to find some positive
trend between repertoire size and these fitness-related parameters. As this was not the case, we have to conclude that
repertoire size is probably not a very reliable or not a very
powerful indicator of male quality.
The discrepancy between studies of how repertoire size
relates to offspring viability may again result from environmental variation. Offspring viability is a trait that depends on
the selective environment, and this selective pressure may vary
between populations (as well as over time). This means that
certain pleiotropic genes that have positive effects on both
repertoire size and offspring survival (i.e., good genes) may be
favored in one population, but need not be favored in
another population in which offspring survival may depend
on a different set of factors. Likewise, we might expect natural
selection to favor one type of female mating preferences in
one population at one time, but other preferences elsewhere
or at other times. This scenario of changing selective environments leading to different patterns of intra- and intersexual
selection in different populations seems most likely to apply
to situations in which sexually selected male characters are
not highly exaggerated. The literature seems full of such
examples, in which repeated studies of sexual selection patterns in the same species have yielded different or even contradictory results (see Collins and ten Cate, 1996; Dale et al.,
1999; Griffith et al., 1999; Jennions, 1998; Krokene et al.,
1998; Mateos and Carranza, 1996; Saetre et al., 1997). Thus, it
appears to be quite common that the forces of sexual selection
vary greatly in space and time.
Syllable switching as an indicator of male quality
In the present study, we found a positive correlation between
the number of syllable-type switches within a strophe and
a male’s pairing success. As shown elsewhere, syllable switching
of individual males did not increase with age but was a strong
predictor of male longevity (Forstmeier W, Hasselquist D,
Bensch S, and Leisler B, unpublished data). This latter
correlation causes an apparent increase in syllable switching
with age. As older males were more successful in attracting
females, male age should be controlled for when analyzing the
effect of syllable switching on male pairing success. When
doing this, the remaining effect was still significant, which
supports the idea that syllable switching reflects some aspects
of male quality. Note that we do not claim that there is a female
preference for such males, as we cannot disentangle the effects
of female choice from those of male-male competition.
Syllable switching might reflect variation in male quality for
two reasons. First, this song trait is tightly connected to
strophe length, and it may be energetically demanding or
exhausting to sing long strophes (Suthers and Goller, 1997).
Second, long series of syllable-type switches may also reflect
neural abilities, as great reed warblers sing with great immediate variety. Apart from the less variable start of a strophe,
males avoid introducing the same syllable type twice within a
strophe. Thus, singing long strophes requires a large repertoire of immediately accessible syllable types. In addition, such
measures of immediate variety seem easier to take by the
choosing female than are estimates of total repertoire size.
Note that the correlation between these two song traits is very
low (see Methods).
Future field studies will have to show whether syllable
switching is a more consistent correlate of male quality than
repertoire size seems to be. Laboratory experiments would
offer an alternative approach to discover which aspects of
song quality are likely to be perceived by females.
We thank Lucia Meyer, who analyzed the song recordings for us. We
are indebted to Josef Beier for monitoring the great reed warbler
population with great accuracy for nearly three decades. The
following persons helped carry out some of the field work: Meinrad
Kneer, Kirstin Kuczius, Karl-Heinz Siebenrock, and Edith Sonnenschein. Gösta Metzler helped with the analyses. We are grateful to
Thorsten Balsby, Clive Catchpole, Dennis Hasselquist, and Jonathan
Ekstrom for their valuable comments on earlier versions of the
manuscript, and to Hector Castillo, Andrew MacColl, and Markus
Neuhäuser for their help with the statistics. We thank the nature
conservation authorities (Erlangen-Höchstadt and Ansbach) for
permitting access to the various reserves. This project was supported
by the Deutsche Forschungsgemeinschaft (LE512/7–2).
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