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Behavioral Ecology
doi:10.1093/beheco/arq200
Advance Access publication 15 December 2010
Original Article
When climate change affects where birds sing
Anders Pape Møller
Laboratoire d’Ecologie, Systématique et Evolution, Centre National de la Recherche Scientifique UMR
8079, Université Paris-Sud, Bâtiment 362, F-91405 Orsay Cedex, France
Recent changes in temperature and precipitation have implications for transmission and excess attenuation of sounds, with
important consequences for the choice of vocal display sites by animals. Birds typically sing from within or at the top of the
vegetation, and the relative height of such song posts varies consistently among species. I estimated relative height of positions in
the vegetation used by singing birds in 1986–1989 and again in 2010 after spring and summer temperatures had increased by
20% and precipitation by 30%, predicting that these changes would increase the height of song post positions. Average song post
height increased by 18% or 1.2 m during the study. Because the increase in song post height should depend on relative costs and
benefits of such change, I predicted that sexually dichromatic species and species with increasing populations and hence intense
intraspecific competition for mates should cause increases in song post height, whereas high predation risk by the sparrowhawk
Accipiter nisus should prevent increases in song post height because sparrowhawks preferentially prey on birds high in the
vegetation. That was indeed the case. These results suggest that display sites for singing birds can change rapidly, with potential
consequences for optimal design of songs, variance in mating success, and predator–prey interactions. Key words: density dependence, predation risk, sexual selection, signaling, trade-offs. [Behav Ecol 22:212–217 (2011)]
lthough current climate change has altered phenology, demography, abundance, and distribution of many organisms, there are relatively few studies of changes in behavior
although behavior together with physiology constitutes the
mechanisms underlying responses to climate change. The few
studies published relate to mating behavior and its consequences (e. g., Møller 2004; Spottiswoode et al. 2006; Twiss et al.
2006). Surprisingly, there are so far no studies of climate change
and vocalizations.
Recent climate change has resulted in warmer but also more
humid conditions in many places due to increasing precipitation (IPCC 2007). Such changes in climatic conditions affect
sound transmission and hence vocal communication. Such
change could come about in a number of potential ways: First,
sound absorption increases with temperature for lower frequency sounds, such as those produced by human voices,
whereas the reverse applies to higher frequency sounds, such
as bird calls. However, the exact relationship between sound
absorption and climate is not straightforward because it interacts with atmospheric pressure, relative humidity, temperature, and frequency (Harris 1966; Griffin 1971; Wiley and
Richards 1978; Bass et al. 1984; Larom et al. 1997). Given
the as yet relatively small changes in temperature (in the order of a couple of degrees), changes in sound absorption are
expected to be small. Second, air temperature can have significant effects on turbulence that can affect high frequencies
of birdcalls but also cause scatter (Wiley and Richards 1978).
Third, not only temperature but also precipitation has
increased considerably in recent years in some parts of the
world, and this can affect sound propagation and therefore
timing of singing (Lengagne and Slater 2002) but not sound
absorption, which is mainly affected by humidity. Fourth, climate change may affect vocal communication through effects
of climate on vegetation. For example, plant phenology has
A
Address correspondence to A.P. Møller. E-mail: anders.moller@
u-psud.fr.
Received 25 August 2010; revised 2 November 2010; accepted 8
November 2010.
The Author 2010. Published by Oxford University Press on behalf of
the International Society for Behavioral Ecology. All rights reserved.
For permissions, please e-mail: [email protected]
advanced significant in many parts of the temperate zone of
Europe (e. g., Menzel et al. 2006) with important consequences for bird song. Such change in phenology will be particularly
important for bird species that use tall vegetation like trees as
song posts, whereas species singing in open habitat would be
less affected. Indeed, timing of leafing will affect reverberation
and sound transmission with important consequences for
choice of song post. Finally, the phenology of birds has
changed considerably (e. g., Rubolini et al. 2007, 2010;
Lehikoinen and Sparks 2010), with residents and shortdistance migrants advancing their phenology much more than
long-distance migrants (Rubolini et al. 2010). Such change in
spring phenology of birds could cause mistiming between the
optimal transmission of vocal signals and the physical environment with long-distance migratory birds experiencing
impaired environmental conditions.
Although the physical environment in which songs are
transmitted may change in response to climate change, realized changes in positions used for vocal display may furthermore be affected by costs and benefits of such changes in
circumstances of display. If competition for mates is more intense in certain species, this should result in greater changes
in song post position than in species with little mate competition. For example, sexually dichromatic bird species have
higher levels of extrapair paternity than monochromatic species (Møller and Birkhead 1994; Owens and Hartley 1998),
implying that males of dichromatic species should compete
more intensely and hence change their display sites more
than monochromatic species. Likewise, individuals belonging
to species with increasing population trends should experience more intense competition than species with stable or declining populations. Just as increasing benefits may spur
greater changes in display site choice associated with changing climatic conditions, high costs may prevent such change.
Sexual display occurs at the cost of predation and parasitism
(Zuk and Kolluru 1998). Therefore, species displaying at exposed sites should suffer higher risks of predation than those
displaying at concealed sites. Indeed, male chaffinches Fringilla
coelebs sing at lower braches in the vegetation after having
been exposed to a raptor model (Krams 2001), and bird
Møller • Climate change and song posts
species using high song posts and hence singing from exposed sites in the vegetation are more susceptible to sparrowhawk predation than birds singing from lower in the
vegetation (Møller et al. 2008).
The objectives of this study were, first, to test for temporal
change in song post position in a community of birds during
a period when climate change was rapid and hence altered
the transmission and attenuation properties of the environment. Second, I tested to which extent differences in costs
and benefits of vocal display high in the vegetation may
account for interspecific differences in change in song post
position. Third, I attempted to evaluate 4 different hypotheses
that could explain temporal change in song post position: 1)
air temperature and turbulence, 2) number of rainy days, 3)
changes in vegetation, and 4) changes in migration phenology. This was done by analyzing extensive data on song post
position for 34 species of birds studied in the same area during 2 periods separated by more than 20 years.
MATERIALS AND METHODS
Study site
I studied the position of singing birds around the village of
Kraghede (lat 5712#N, long 1000#E), Denmark during April
to July 1986–1989 (Møller et al. 2008) and 2010. This farmland area mainly consists of open fields with scattered hedgerows, woodlots, shrub, and trees around farms and houses.
The exact same areas were studied during the 2 periods.
Song posts
I systematically recorded all singing birds while walking through
the 25 km2 study area, only ever recording a single individual
in each site to avoid pseudoreplication. Because birds are
highly sedentary within their breeding territories during the
breeding season, this procedure should prevent recording of
the same individual multiple times. The total number of recordings was 1614 in 1986–1989 and 1085 in 2010.
I recorded the position of singing birds in the vegetation
and the maximum height of the vegetation using my own
height as a yardstick. Height of singing birds and maximum
height of the herb, shrub, or tree were recorded in multiples
of my own height. For 300 observations in 2010, I subsequently used a Nikon Forestry 550/Hypsometer based on laser technique that allows precise recording of the height of
the vegetation. Estimates of song post position based on visual
judgment and the hypsometer were strongly positively correlated (F ¼ 26 613.74, degrees of freedom [df] ¼ 1, 298, r2 ¼
0.99, P , 0.0001). Relative song post height was defined as
song post height divided by the maximum height of the vegetation (Scherrer 1972; Møller et al. 2008). Møller et al.
(2008) have previously shown that song post estimates from
Denmark are strongly positively correlated with estimates
from France by Scherrer (1972). Hence, height recordings
were reliable.
Explanatory variables
Habitat
Song post height may depend on the height of the habitat
(Scherrer 1972), and higher song posts may arise as a consequence of higher vegetation. Therefore, I scored the
breeding habitat of all species on a 3-point scale, where
0 equals grassland, 1—bush and scrub, and 2—forest based
on Cramp and Perrins (1977–1994), with species occurring
in more than one category being assigned to the highest
category.
213
Population trends
Trends were estimated as the long-term trend for the period
1976–2005 based on the Danish breeding bird monitoring
program that uses point counts, with a value of 0 reflecting
a stable population, whereas negative values reflect declining
populations and positive values increasing populations
(Heldbjerg 2005).
Susceptibility to sparrowhawk predation and abundance of raptors
I estimated susceptibility to predation by sparrowhawks expressed as log-transformed observed number of prey minus
log-transformed expected number of prey according to standardized point counts during the breeding season in
Denmark (Grell 1998; Møller et al. 2010). Thus, a susceptibility index of 0 implies that prey are consumed according to
expectation from their abundance, an index of 11 implies
that a given prey species is consumed 10 times more often
than expected, whereas an index of 21 implies that a given
prey species is consumed 10 times less often than expected
from its abundance. I have previously described these estimates and their reliability in detail elsewhere (e.g., Møller
et al. 2008, 2010).
I searched the few possible breeding sites for raptors in
1986–1989 and 2010 for active nests.
Sexual dichromatism
I scored all species as sexually dichromatic if males and females
differed in coloration and monochromatic otherwise using
Mullarney et al. (2000) as a source. Previous studies using this
approach have shown biologically meaningful results that are
compatible with results from models based on avian vision
(Møller and Birkhead 1994; Seddon et al. 2010).
Migration distance
As an estimate of migration distance, I used the global northernmost and southernmost latitude of the breeding and the
wintering distributions, respectively, to the nearest 10th of a
degree based on information from maps in Cramp and
Perrins (1977–1994). Migration distance was calculated as
the mean of the 2 latitudes during breeding minus the mean
of the 2 latitudes during winter.
Body mass
I recorded body mass from the breeding season using Cramp
and Perrins (1977–1994) and preferably the estimate with the
largest sample size if more than a single estimate was available.
Summary statistics for all variables are reported in Supplementary Material 1.
Climate
Danish Meteorological Institute provided mean temperature
and rainfall data for April to August 1971–2010 from Aalborg
that is located 25 km south of the study area.
Statistical analyses
Migration distance and body mass were log10 transformed
before analyses (the former by adding a constant of one to
avoid problems of inclusion of resident species). I tested for
change in song post height among species using a paired
t-test. Subsequently, I tested how several different explanatory
factors accounted for variation in change in song post height.
Finally, I developed a statistical model that included all significant predictors from the bivariate analyses. To assess possible
problems of collinearity, I calculated variance inflation factors
that in all cases were less than 3, which is much less than the
commonly accepted levels for significant collinearity of 5–10
(McClave and Sincich 2003).
214
Closely related species have similar song posts due to such
species sharing song features, predators, and habitat preferences. I controlled for similarity in phenotype due to common
ancestry by calculating standardized independent linear contrasts (Felsenstein 1985) using comparative analysis with independent contrasts (Purvis and Rambaut 1995). I tested the
statistical and evolutionary assumptions of the continuous comparative procedure (Garland et al. 1992) by regressing absolute
standardized contrasts against their standard deviations (SDs).
In order to reduce the consequent problem of heterogeneity of
variance, 1) outliers (contrasts with studentized residuals .3)
were excluded from subsequent analyses (Jones and Purvis
1997), and 2) analyses were repeated with the independent
variable expressed in ranks. In neither case did these new analyses change any of the conclusions.
The composite phylogeny used in the analyses was based on
Davis (2008) (Supplementary Material 2). Because information for the composite phylogeny came from different sources
using different methods, consistent estimates of branch
lengths were unavailable. Therefore, branch lengths were
transformed assuming a gradual model of evolution with
branch lengths being proportional to the number of species
contained within a clade. Results based on these branch
lengths were compared with those obtained using constant
branch lengths (a punctuated model of evolution). Finally, I
used a standard bird taxonomy (Howard and Moore 1991) to
test for consistency in findings independent of phylogenetic
hypothesis. Nowhere were results qualitatively different (results not shown).
Regressions based on contrasts were forced through the origin because the comparative analyses assume that there has
been no evolutionary change in a character when the predictor
variable has not changed (Purvis and Rambaut 1995).
I evaluated the magnitude of associations between song post
height and predictor variables based on effect sizes according
to Cohen’s (1988) criteria for small (Pearson r ¼ 0.10, explaining 1% of the variance), intermediate (9% of the variance), and large effects (25% of the variance).
RESULTS
Mean temperature during April to August 1971–2009 increased, with an accelerating trend since 1980 (Supplementary Material 3). The predicted increase amounted to more
than 2 C or almost 20% between 1986–1989 and 2010. Likewise, precipitation increased significantly during 1971–2009
(Supplementary Material 3) from approximately 250 mm in
1986–1989 to 325 mm in 2010 or by approximately 30%. The
number of rainy days in 1986–1989 was 61–78, whereas it was
75 in 2010.
Song post height was highly repeatable among species with
species accounting for 44% of the variance (F ¼ 21.32, degrees of freedom [df] ¼ 32, 2666, r2 ¼ 0.43, P , 0.0001). Song
post height was 0.677 (standard error [SE] ¼ 0.042), N ¼ 34
species during 1986–1989, but increased to 0.802 (SE ¼
0.032) in 2010 or an increase by 18%. Change in song post
height was on average 0.125 (SE ¼ 0.025), range ¼ 20.200 to
0.406, N ¼ 34, differing significantly from zero (Figure 1;
paired t-test, t ¼ 4.97, df ¼ 33, P , 0.0001). Effect size accounted for 43% of the variance, thus equaling a strong effect
(sensu Cohen 1988). Because mean height of the vegetation
used for singing was 9.7 m (SD ¼ 5.3), an increase by 0.125
implies an average increase by 1.2 m in song post height
between 1986–1989 and 2010.
The relationships between relative song post height in 2010
and the different variables are shown in Figure 2. Species with
increasing population trend increased their song post height
(Figure 3; species-specific data: F ¼ 3.94, df ¼ 1, 32, r2 ¼ 0.11,
Behavioral Ecology
Figure 1
Song post position of different bird species in 2010 in relation to
position in 1986–1989. The line is y ¼ x.
P ¼ 0.056, slope [SE] ¼ 2.911 [1.466] and contrasts: F ¼
10.88, df ¼ 1, 32, r2 ¼ 0.25, P ¼ 0.0024, slope [SE] ¼ 2.737
[0.830]).
Species singing in taller vegetation increased their song post
height more than species singing in low vegetation (speciesspecific data: F ¼ 13.75, df ¼ 1, 32, r2 ¼ 0.30, P ¼ 0.0008, slope
[SE] ¼ 0.098 [0.027] and contrasts F ¼ 11.22, df ¼ 1, 32, r2 ¼
0.26, P ¼ 0.0021, slope [SE] ¼ 0.109 [0.033]).
There was no significant relationship between change in
song post height and time of day in an analysis of trends within species (quadratic effect of time of day: F ¼ 0.32, df ¼ 1,
2696, r2 ¼ 0.00, P ¼ 0.98).
Change in song post position was not significantly related to
migration distance (species-specific data: F ¼ 2.19, df ¼ 1, 32,
r2 ¼ 0.06, P ¼ 0.15 and contrasts: F ¼ 2.86, df ¼ 1, 32, r2 ¼
0.08, P ¼ 0.101).
Sexually dichromatic species increased their song post
height more than monochromatic species in the analysis of
contrasts (species-specific data: F ¼ 0.11, df ¼ 1, 32, r2 ¼ 0.00,
P ¼ 0.74 and contrasts: F ¼ 11.31, df ¼ 1, 32, r2 ¼ 0.26, P ¼
0.0020, slope [SE] ¼ 0.161 [0.048]).
Species that were particularly susceptible to sparrowhawk
predation increased their song post height less than unsusceptible species (species-specific data: F ¼ 11.24, df ¼ 1,31, r2 ¼
0.27, P ¼ 0.0021, slope [SE] ¼ 20.230 [0.069] and contrasts:
F ¼ 5.28, df ¼ 1, 29, r2 ¼ 0.15, P ¼ 0.029, slope [SE] ¼ 20.166
[0.072]). There were 3 pairs of sparrowhawks and no other
breeding raptors during the study years, so differences in the
abundance of predators could not account for the change in
song post height.
Change in song post height was not significantly related to
body mass (species-specific data: F ¼ 0.70, df ¼ 1, 32, r2 ¼ 0.02,
P ¼ 0.41 and contrasts: F ¼ 0.44, df ¼ 1, 32, r2 ¼ 0.01, P ¼
0.51).
A model that included the 4 significant predictors listed
above accounted for 58% of the variance in the speciesspecific analysis and 32% of the variance in the analysis of
contrasts (Table 1). Monochromatic species barely increased
their song post height (least mean squares [SE] ¼ 0.067
[0.030]), whereas dichromatic species increased a lot (0.183
[0.029]). Species that were susceptible to sparrowhawk predation only marginally increased song post height compared
with unsusceptible species (Table 1). Furthermore, species
that have increased in abundance increased their song post
height, whereas those that have declined in abundance did
not change their song post position (Table 1). Finally, species
Møller • Climate change and song posts
215
Figure 2
Relationship between song post height (SE) in 2010 and (A) Danish breeding population trend, (B) habitat (0—grassland, 1—shrub, and
2—trees), (C) sexual dichromatism (0—monochromatic and 1—dichromatic), (D) susceptibility to sparrowhawk predation, (E) migration
distance, and (F) body mass (g).
breeding in forests increased their song post height more
than those using shrub or grassland (Table 1). These effects
were confirmed in a comparative analysis (Table 1).
DISCUSSION
The main findings of this study were that height of the position
in the vegetation used by singing birds increased considerably
between 1986–1989 and 2010. Song post height changed
more in species using trees than using shrub and grassland
for singing. Higher song posts in the vegetation were modified by costs and benefits. Species with more intense compe-
tition for mates such as sexually dichromatic birds and birds
with increasing population trends increased their song post
height more than sexually monochromatic species and species with stable or decreasing populations. Fitness costs also
contributed to interspecific differences in response because
bird species that were susceptible to sparrowhawk predation
increased much less in song post height than species that only
infrequently fell prey to sparrowhawks.
Common birds have considerably increased the height from
where they sing in the vegetation during a period of 20 years
when temperatures and precipitation have increased. These
changes with a large effect amounted to an average increase
Behavioral Ecology
216
Figure 3
Increase in song post position between 1986–1989 and 2010 in
relation to population trend of different species of birds. The line is
the linear regression line.
in height by 1.2 m or 12%. I hypothesized that 4 different
mechanisms potentially could account for such change. I discarded the first hypothesis based on changes in sound absorption with changes in temperature and precipitation because
the effects of such changes would be small at best given the
observed changes in temperature and rainfall. Second, air
temperature can have significant effects on air turbulence
that can affect high frequencies of birdcalls (Wiley and
Richards 1982). Although I have no direct measurements of
turbulence, this hypothesis predicts stronger effects of turbulence around noon than during calm periods around dawn
and dusk. However, relative song post height did not change
with time of day during the study, thus not providing support
consistent with the hypothesis. Third, increasing precipitation
could affect sound propagation and therefore timing of singing (Lengagne and Slater 2002). However, there has been no
significant increase in the number of rainy days, whereas the
increase in precipitation is caused by heavier rainfall when it
rains. Fourth, climate change may affect vocal communication
through effects of climate on plant phenology that has
advanced significantly in parts of the temperate zone of
Europe (e.g., Menzel et al. 2006). Such change in phenology
will be particularly important for bird species that use tall
vegetation like trees as song posts, whereas species singing
in open habitat would be less affected (Martens 1980). Timing
of leafing will affect reverberation and sound transmission
with important consequences for choice of song post. Song
post position has particularly increased in species that use
forest but less so in species in shrub and not at all in grassland
species. This effect was not simply a consequence of grassland
species only being able to sing from a limited range of song
post positions because single trees and hedges were present
throughout the study area. Finally, the breeding phenology of
birds has advanced considerably for residents and migrants
(e.g., Rubolini et al. 2007, 2010; Lehikoinen and Sparks
2010). Such change in spring phenology could cause mistiming between the optimal transmission of vocal signals and the
phenological stage of the vegetation. In particular, long distance migratory birds that have advanced their phenology
only little and hence are increasingly asynchronous with the
phenology of the vegetation should experience impaired environmental conditions for vocal communication. However,
change in song post position was not significantly related to
migration distance. Although this assessment is qualitative
rather than quantitative, the conclusion is that particularly
species that sing in forests have changed their song post
position, and the evidence suggests that this may be due to
advancing phenology of the vegetation.
Interspecific heterogeneity in increase in song post height
should depend on differences in benefits and costs of such
an increase. Sexually dichromatic birds compete intensely
for mates as reflected by high levels of extrapair paternity
(Møller and Birkhead 1994; Owens and Hartley 1998). Thus,
males of sexually dichromatic species should compete more
intensely for sexual display sites than those of monochromatic
species. Indeed, species with sexually dichromatic plumage
increased their song post height more than monochromatic
species, with a large effect size accounting for 36% of the
variance (sensu Cohen 1988). Likewise, males of bird species
with increasing population trends should compete more intensely for mating opportunities (Møller and Ninni 1998)
resulting in increasing song post heights. Again, effect size
was large at 28%. The fact that not all species responded
similarly to changing conditions for sound transmission and
attenuation implies that responses are under the influence of
the ecology of different species.
Sparrowhawks are the most common predators on passerine
birds in the western Palearctic (Newton 1986). Møller et al.
(2008) have shown that birds that sing from positions high in
the vegetation are particularly susceptible to sparrowhawk
predation. Here, I extend these findings by showing that
increases in song post height depend on susceptibility to sparrowhawk predation, with species less susceptible to predation
showing the largest increases in song post height. Thus,
Table 1
Change in song post position in relation to sexual dichromatism, susceptibility to sparrowhawk predation, and population trend
Variables
Species:
Sexual dichromatism
Susceptibility to sparrowhawk predation
Population trend
Habitat
Error
Contrasts
Sexual dichromatism
Susceptibility to sparrowhawk predation
Population trend
Habitat
Error
Sum of squares
df
0.091
0.040
0.049
0.090
0.300
1
1
1
1
28
0.049
0.005
0.017
0.018
0.065
1
1
1
1
27
P
Slope (SE)
8.52
3.72
4.62
8.40
0.0069
0.064
0.040
0.0072
0.057
20.069
1.549
0.074
(0.020)
(0.036)
(0.721)
(0.025)
21.19
2.35
7.19
7.93
,0.0001
0.136
0.012
0.008
0.163
20.070
1.736
0.076
(0.035)
(0.046)
(0.648)
(0.027)
F
The models had the statistics F ¼ 9.49, df ¼ 4, 28, r2 ¼ 0.58, P , 0.0001 and F ¼ 13.23, df ¼ 4, 28, r2 ¼ 0.29, P , 0.0001.
Møller • Climate change and song posts
climate change through elevated song post positions may affect the relative susceptibility of different species to predation
and thereby affect predator–prey interactions (Millon et al.
2009).
Finally, are the hypothesized effects of climate change listed
here restricted to vocal signals? Similar arguments could readily be made for visual signals. If tree leafing advances due to
increasing temperatures or other climate effects, we should expect a change in the signaling environment, especially for migratory species that have not changed their phenology. A
comparison of changes in color over time for residents versus
migrants would constitute a particularly powerful test.
In conclusion, birds now sing from positions much higher in
the vegetation than just a couple of decades ago during a period
when temperature and precipitation increased rapidly. Interspecific heterogeneity in increase in song post height was explained by differences in costs (susceptibility to sparrowhawk
predation) and benefits of high song posts (species with more
intense mating competition showing a stronger response),
demonstrating that changes in behavior vary in consistent ways
with the ecology of different species. Change in song post position was most pronounced in forest species and least in grassland, consistent with the hypothesis that changes in plant
phenology may be the cause of the change in song post height.
SUPPLEMENTARY MATERIAL
Supplementary material can be found at http://www.beheco.
oxfordjournals.org/.
T. A. Mousseau kindly lent me his hypsometer.
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