Behavioral Ecology Vol. 10 No. 2: 128-135
© 1999 International Society for Behavioral Ecology
Aerodynamic costs of long tails in male barn swallows Hirundo rustica and the evolution of sexual size dimorphism
Laboratoire d'Ecologie, Université Pierre et Marie Curie, CNRS-URA 258, Bât A, 7e étage, 7, quai Saint Bernard, Case 237, F-75252 Paris, Cedex 05, France
Address correspondence to A. Barbosa, who is now at the Departmento de Ecología Evolutiva, Museo Nacional de Ciencias Naturales, CSIC, C/José Gutierrez Abascal, 2, 28006 Madrid, Spain. E-mail: Abarbosa{at}snv.jussieu.fr
Received 23 October 1997; revised 29 June 1998; accepted 5 July 1998.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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Exaggerated tail feathers of birds constitute a standard example of evolution of extravagant characters due to sexual selection. Such secondary sexual traits are assumed to be costly to produce and maintain, and they usually are accompanied by morphological adaptations that tend to reduce their costs. The aerodynamic costs for male barn swallows Hirundo rustica of having long tails were quantified using aerodynamics theory applied to morphological data from seven European populations. Latitudinal differences in tail length were positively correlated with differences in flight costs predicted by aerodynamics theory. A positive relationship between aerodynamic costs of long tails and the degree of sexual size dimorphism was found among populations. Latitudinal differences in foraging costs may result in tail length being relatively similar in males and females in southern populations, whereas the low foraging costs for males in northern populations may allow them to cope with higher aerodynamic costs, giving rise to large sexual size dimorphism. Enlargement of wingspan in males can alleviate but not eliminate the costs of tail exaggeration, and therefore differences in aerodynamic costs of male ornaments were maintained among populations. Sexual size dimorphism in the barn swallow arises as a consequence of latitudinal differences in the advantages of sexual selection for males and the costs of long tails for males and females.
Key words: clinal variation, flight costs, sexual selection, tail shape.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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Secondary sexual characters are more exaggerated than homologous characters in closely related species due to the effects of sexual selection and its two main components, male-male competition and female choice (Darwin, 1871
). Sexual
selection arises because individuals of the chosen sex experience advantages
over others of the same species and sex in relation to reproduction. Two
different categories of sexual selection models may account for exaggeration
of male traits. The Fisherian model of mate choice suggests that female
preferences coevolve with the male trait, resulting in the production of
exaggerated, attractive, and costly secondary sex traits
(Fisher, 1930). At
evolutionary equilibrium this cost of the secondary sexual character is
balanced by its advantages in terms of increased mating success
(Fisher, 1930;
Kirkpatrick, 1982;
Lande, 1981;
Pomiankowski et al., 1991;
Sutherland and de Jong, 1991).
Alternatively, handicap models assume that the level of signaling adopted by a
male is an outcome of individual optimization. Low-quality individuals will be
unable to produce a larger sex trait because the cost of such a trait is
relatively greater for low- than for high-quality individuals
(Andersson, 1986;
Grafen, 1990;
Heywood, 1989;
Iwasa et al., 1991;
Kodric-Brown and Brown, 1984;
Price et al., 1993;
Zahavi, 1975). Thus all these
models of sexual selection assume that higher levels of display are prevented
because of the costs of signaling.
Feather ornaments in birds, and particularly extravagant tails, are
standard examples of exaggerated traits that are maintained by sexual
selection. Several experimental studies of birds demonstrate that males with
long tails experience a mating advantage (e.g.,
Andersson, 1982,
1992;
Evans and Hatchwell, 1992;
Møller, 1988) at a
viability cost (Evans and Thomas,
1992; Møller,
1989; Møller and de
Lope, 1994; Møller et
al., 1995b; Saino and
Møller, 1996; Saino et
al., 1997) and support the hypothesis that sexual selection is the
mechanism responsible for the evolution of long tails
(Møller et al., 1998).
Tails of birds are functional units that are shaped by both natural and sexual
selection. Aerodynamics theory suggests that only the proximal part of the
tail until the point of maximum continuous width is aerodynamically functional
and that any area beyond this point does not increase lift, but increases drag
(Thomas, 1993). Drag is
proportional to the area beyond this point of maximum continuous width, and
such tail drag contributes significantly to the parasite drag of the bird
(Evans and Thomas, 1992;
Thomas and Balmford, 1995),
which can cause an increase in the power required for flight
(Norberg, 1995). In a forked
tail such as that of the barn swallow Hirundo rustica, the maximum
lift for any given drag is produced by a spread, triangular tail. The optimum
tail shape is one with the outermost tail feathers just slightly more than
twice the length of the central feathers when the spread tail just exceeds
120° (Thomas and Balmford,
1995). An increase in tail length exceeding the optimum ratio of
two would increase tail drag and the power of flight and therefore increase
the cost of flight.
Recently, Norberg (1994)
suggested a mechanism that would improve maneuverability by increasing lift
and reducing drag. Basically, tail streamers through aeroelastic properties of
distal parts of the feather cause a rotation in their sockets, deflecting the
leading edge and acting like certain high lift devices in aircraft
(Norberg, 1994). Evans and
Thomas (1997) predicted from a
theoretical study a decrease in turning radius when Norberg's mechanism was
operating. Hence they suggested that the outermost feathers would not be
costly but would actually confer a natural selection advantage to males with
long streamers. This claim cannot be easily tested because certain parts of
Norberg's hypothesized mechanism are difficult to evaluate and relate to
improved flight. For example, there is no quantification of the relationship
between the degree of the deflecting leading edge mechanism and the length of
the tail streamer. Obviously, a certain streamer length causes torsion of the
feather, but this torsion could also be produced by a short streamer, as in
females or short-tailed males. In fact, Norberg
(1994) assumed such a
possibility, and although he proposed possible differences in feather
characteristics such as flexural stiffness, feather shaft curvature, and
torsional rigidity in relation to the length of the streamer, the consequences
of such differences remain to be demonstrated. Therefore, the mechanism
described by Norberg (1994)
does not make any explicit predictions about the cost of a long tail. However,
there are several pieces of evidence suggesting that the Norberg effect can be
considered a constant acting on the whole range of tail lengths in barn
swallows (Barbosa and Møller, in
press; Møller et al.,
1998). A discussion of the effects of the Norberg mechanism in
barn swallows can be found in Møller et al.
(1998) and
Barbosa and Møller (in
press) and in the Discussion of the present paper.
Costs of secondary sexual characters can be reduced by the presence of
cost-reducing traits (Møller,
1996). For birds with elongated tails, elongation and enlargement
of wings and narrowing of the outermost tail feathers have been demonstrated
to act as cost-reducing traits (Andersson
and Andersson, 1994; Balmford
et al., 1994, Møller et
al., 1995a). Although geographical variation in sexual size
dimorphism in tail length and foraging costs have been reported for the barn
swallow (Møller, 1995;
Møller and de Lope,
1994; Møller et al.,
1995b), aerodynamical costs sensu stricto of long tails
remain to be quantified.
In this study, we calculated the aerodynamic costs of long tails in male barn swallows from seven European populations. We used a theoretical framework to study the relationship between such costs and the degree of sexual dimorphism in tail length and the size of cost-reducing characters along a latitudinal gradient in size dimorphism. If long streamers in male barn swallows have evolved by sexual selection, we predicted a significant positive relationship between costs of long tails and sexual size dimorphism.
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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Barn swallows are small (approximately 20 g), monogamous, semicolonial passerine birds that feed on insects caught on the wing. Sexual size dimorphism is slight with the exception of the outermost feathers of the forked tail. Male tail length is only weakly correlated with structural body size, and individual males are highly consistent in tail length among years (Møller, 1991
). Male
barn swallows attempt to attract a mate by performing displays of their tail
ornaments in flight or while perched
(Møller, 1988). Female
barn swallows visit several males before making their mate choice, and males
with long and symmetrical tails are preferred over ones with short and
asymmetrical tails (Møller,
1988, 1990,
1992a,
1994). Long-tailed males also
experience a number of other sexual selection advantages
(Møller, 1990,
1992b,
1994). Nestlings are fed by
both parents for an average of 3 weeks before fledging.
The studies took place at seven different sites. Kraghede, Denmark (57°
N, 10° E) is an open farmland site with scattered plantations, ponds, and
hedgerows. The barn swallows usually breed on farms either solitarily or in
colonies of up to 50 pairs. A detailed description of the population is given
in Møller (1994). Two
study sites in the Ukraine were situated on large cooperative farms in open
farmland habitat near Chernobyl (52° N, 29° E) and Kanev (50° N,
31° E), separated by a distance of 300 km. The main crops were grass and
wheat. The barn swallows breed inside stables and cowsheds. The number of
breeding pairs per colony ranged from 20 to 120 pairs.
Pärnu, Estonia (58° N, 24° E), is a
mixed farmland and forest habitat with solitary pairs or colonies of up to 50
pairs of swallows breeding inside stables. The study site at Tiszatelek,
Hungary (48° N, 21° E), is an open farmland habitat with breeding
sites at farms having single pairs up to more than 25 pairs. The study site at
Milano, Italy (45° N, 9° E), is an open farmland area. The main crops
are maize, grass, wheat, and soybeans. Fields are bordered by hedges and
trees. Barn swallows breed mainly in cowsheds, milking rooms, garages, and
sometimes outdoors. The number of nests in the colonies ranged from 18 to 59.
The Badajoz, Spain (39° N, 7° W), study site consists of open farmland
with scattered groups of trees around farms and rivers. The main crops are
grass, maize, and wheat. Barn swallows breed solitarily or colonially (up to
50 pairs) in farmhouses and other buildings.
The distance between study sites and the nearest neighboring site was on
average 908 km (SE = 160, range 300-1500 km, n = 7). The average
natal dispersal of barn swallows is known to be 0.7 km for males and 2.5 km
for females based on ringing recoveries
(Cramp, 1988), and other
estimates are of a similar magnitude
(Glutz von Blotzheim and Bauer,
1985). Breeding dispersal is also restricted and rarely exceeds 1
km (Cramp, 1988;
Glutz von Blotzheim and Bauer,
1985). This means that the average distance between study sites is
several hundred times the average natal dispersal distance. Gene flow between
study sites should therefore be minimal
(Endler, 1977;
Slatkin, 1985). The
statistical dependence of data from different populations is likely to be
related to geographical proximity, and this may affect the reliability of
statistical analyses in a similar way to phylogenetic analyses
(Felsenstein, 1985). To avoid
this problem, we adopted the independent contrast method using geographical
distance as an estimate of genetic divergence between populations based on an
assumption of isolation by distance. As geographic variation in most
morphological traits of the barn swallow shows a latitudinal cline
(Møller, 1995), we
built a cladogram based on latitudinal coordinates of each study site
(Figure 1), using the
unweighted pair-group average method (UPGM;
James and McCulloch, 1990;
Sneath and Sokal, 1973).
Statistical analyses of relations between characters were determined using
contrasts derived from the geographical distance cladogram. These contrasts
were calculated as the value of a trait (e.g., tail dimorphism) in one
population (or node) of the cladogram subtracted from the value of the closest
population (or node). Thus each population or node was compared to its closest
population. Euclidean distances were used as branch lengths in contrast
analyses because Felsenstein's method requires knowledge of branch lengths in
the cladogram (see Felsenstein,
1985, for more details). To be able to identify covariation
between variables, regression through the origin was used to test the
relationship (Garland et al.,
1992). To make unique representations of bivariate scatterplots,
one must set all contrasts for one trait (e.g., the independent variable)
positive, while switching signs for the other trait's corresponding contrasts.
Regression through the origin yields the same results whether contrasts are
thus positivized (Garland et al.,
1992).
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Most adult barn swallows were captured in mist nets at the breeding sites.
All birds were provided with a numbered aluminium band to allow
identification. Individuals were sexed from the presence (females) or absence
(males) of a brood patch and by the shape of the cloacal protuberance
(Svensson, 1984).
Morphological characters (Table 1) were measured by A.P.M. (Kraghede, Pärnu, Chernobyl, Kanev) or using exactly the same methods by other field workers given instructions by A.P.M. in order to reduce interobserver variability. However, we included the person who took the measurements as a dummy variable (0 for other persons, 1 for A.P.M.) to test whether interobserver variability had affected the results. The morphological characters were used as dependent variables in multiple linear regression analyses with latitude, the person dummy variable, and the latitude-person variable interaction as independent variables. None of the regression coefficients for the dummy variable or the interaction were statistically significant (p >.05), which suggests that variation in morphology along the latitudinal gradient was unbiased by the person who had taken the measurements.
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The length of the outermost tail feathers, hereafter called tail length, and the central tail feathers were measured with a ruler to the nearest millimeter. We measured wing span to the nearest millimeter as the distance between wing tips when the wings were stretched maximally. Body mass was recorded to the nearest 0.1 g using a Pesola spring balance. We determined sexual size dimorphism in tail length as the residuals from a regression of mean values for males regressed on mean values for females in each population. The regression was statistically highly significant and positive [male tail length = 51 + 0.36 (female tail length), F = 18.57, df = 1,5, r =.88, p =.007].
To calculate tail drag and power curves of power consumption in each
population, we used the mean tail length and the hypothetical optimum tail
length for each population in the aerodynamic models (tail length divided by
central feather length equaling two;
Thomas and Balmford, 1995).
The tail was assumed to have a triangular shape with streamers in the former
case and a triangular tail without streamers projecting beyond the maximum
continuous span in the latter case. The Norberg effect has been considered
constant because there is no empirical evidence of differential effects among
different individuals (Barbosa and
Møller, in press;
Møller et al., 1998;
see also Norberg, 1994).
Tail drag calculations were performed using the formula from Prandtl and
Tietjens (1934) for a flat
plate with Reynolds number less than 106
(Evans and Thomas, 1992)
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= 1.23 (kg/m3) is the density of air, s is
tail span, l is tail length, v = 1.45 x
10-5 (m2s-1) is the kinematic viscosity of
air, and u is velocity.
Power curves of power consumption were calculated using a modified version
of the computer programs in Pennycuick
(1989) adding tail drag. We
used Pennycuick's model because it provides the most realistic estimate of
flight costs in comparison with other aerodynamic models
(Welham, 1994; see also
Evans and Thomas, 1992). This
model allows calculations of aerodynamic performance in relation to flight
speed on the basis of body mass and wingspan of birds
(Pennycuick, 1989).
Increases in power consumption and drag due to tail length in each population were calculated as the percentage increase for the mean tail length with respect to the hypothetical optimum tail length (see above). The increase in wingspan needed to compensate for the flight costs due to tail elongation were calculated. Wingspan values were manipulated in the computer models until flight costs at the mean speed considered for a bird with an elongated tail were the same as for a bird with the optimal tail length.
Latitudinal variation in tail length has been related to latitudinal
variation in foraging costs (Møller
et al., 1995a). We have explored the possible variation in such a
foraging costs comparing the size of prey available for barn swallows with the
size of prey captured by barn swallows at different latitudes. We have
compared the percentage increase of mean size between available prey and
selected prey in the three populations for which this kind of data were
available (Kraghede, 57° N, Møller, AP, unpublished data; Stirling,
56°, Turner, 1980;
Lippstadt, 51°, Loske,
1993).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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Table 2 shows differences in drag at different flight speeds between optimal and elongated tails in the seven populations of barn swallows. Assuming a mean flight speed of barn swallows to be about 10 m/s (Harrison, 1931
; Meinertzhagen,
1955), the percentage increase of drag in males with a long tail
in relation to the hypothetical optimum tail length ranged from 7.0% in the
Badajoz population to 15.3% in the Pärnu
population (Figure 1). The
effect of increase in drag due to long tails relative to the optimal tail
length on power of flight is shown in Table
3. At the mean speed considered, the percentage of power increase
with respect to the optimum tail length ranged from 0.9% in the Badajoz
population to 1.5% in the Kanev population
(Figure 1). Mean percentage of
drag and flight power increase between optimal and long-tailed phenotypes was
10.2% and 1.3%, respectively.
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The results did not change using the independent contrast method instead of the species regression which considers each population to contribute a statistically independent data point. However, we present the results for the independent contrast method to correct for any possible effects of independence due to geographical proximity.
The increase in tail drag and flight power were significantly positively related to sexual size dimorphism in the seven populations [ß (SE) = 0.84 (0.24), t = 3.47, df = 5, p =.01; ß (SE) = 0.80 (0.26), t = 3.01, df = 5, p =.02; Figure 2). The percentage increase in wingspan was positively, but not significantly, related to the percentage increase in tail elongation [ß = 0.72 (0.30), t = 2.34, df = 5, p =.06; Figure 3].
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Table 4 shows the percentage difference in mean prey size captured relative to the available prey in the three populations. Northern populations selected much larger prey with respect to the size available than southern populations, consistent with a decrease in foraging costs toward the north.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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Exaggeration of secondary sexual characters such as tail feathers is costly according to current models of sexual selection (Andersson, 1986
;
Fisher, 1930;
Grafen, 1990;
Heywood, 1989;
Iwasa et al., 1991;
Pomiankowski et al., 1991;
Zahavi, 1975). A number of
different costs of elongated tails have been reported. Møller
(1989) and Møller and
de Lope (1994) provided
experimental evidence for the presence of a considerable viability cost of
long tails in male barn swallows. Long-tailed males were better able to
survive tail elongation, whereas short-tailed males survived relatively better
when their tails were shortened. Foraging costs of tail exaggeration have also
been reported in barn swallows
(Møller, 1989;
Møller and de Lope,
1994; Møller et al.,
1995a): males with experimentally elongated tails captured more
and smaller prey than tail-shortened individuals, but males with naturally
long tails were less affected by experimental treatment. Optimal prey for the
barn swallow are large, actively flying insects according to optimal foraging
models (Bryant and Turner,
1982; Turner,
1982), but these prey are difficult to catch for long-tailed
males, both with naturally and experimentally elongated tails
(Møller, 1992b;
de Lope and Møller,
1993; Møller et al.,
1995a). Direct aerodynamic costs of long tails have been
demonstrated a few times. Evans and Thomas
(1992) showed that malachite
sunbirds (Nectarinia johnstoni) with experimentally elongated tails
spent less time flying than before manipulation. The theoretical aerodynamic
effects of tail elongation have been analyzed for bird species with different
tail morphologies (Balmford et al.,
1993; Norberg,
1995; Thomas,
1993; Thomas and Balmford,
1995). Here we calculated the aerodynamic costs (in terms of
increased drag and flight power) of tail elongation in male barn swallows from
seven European populations and investigated how such costs were related to
sexual size dimorphism, geographical variation in dimorphism, and the presence
of cost-reducing morphological traits.
The optimal tail shape for an aerially foraging bird with a forked tail
that gives rise to the maximum lift-to-drag ratio is one that forms a
triangular platform and a straight trailing edge when the tail is spread at an
angle of approximately 120° (Thomas,
1993). The outermost tail feathers will then be twice the length
of the central tail feathers. Male barn swallows in all seven populations had
outermost tail feathers that on average were much longer than twice the length
of the central tail feathers (see also
Møller, 1995;
Møller et al., 1995b).
Such elongation increases costs because any area beyond the point of maximum
continuous width of the tail does not increase lift but increases drag
(Thomas, 1993). Overcoming
drag is the major energetic cost of flight
(Gill and Wolf, 1975). Energy
expenditure on locomotion cannot be used for other demanding activities such
as an efficient immune system, and the energy cost of ornament exaggeration
thus imposes a reduction in the amount of resources available for immune
function
(König and
Schmid-Hempel, 1995; Saino and
Møller, 1996).
Flight costs are affected by the different degree of tail elongation of
males in different populations of barn swallows
(Tables 2 and
3), increasing with latitude
(Figure 1). Tail length
manipulation affects flight costs and foraging efficiency, which may give rise
to the observed reduced survival of male barn swallows with experimentally
elongated tails and the increased survival of males with shortened tails
(Møller, 1989;
Møller and de Lope,
1994). The cost of barn swallow tail ornaments may vary
geographically if ambient temperature affects insect physiology and thereby
the energy cost of prey capture. Flight performance of insects, including
their ability to escape avian predators, depends on ambient temperature
(Beament and Treherne, 1968;
Taylor, 1963;
Wigglesworth, 1972). Barn
swallows are specialist predators of large, actively flying Diptera that
constitute the optimal diet (Bryant and
Turner, 1982). Large insects are more difficult to catch at the
higher temperatures that predominate at southern latitudes, as shown by
Møller et al. (1995a).
Geographic variation in the mean size of insect prey captured by barn swallows
relative to the mean size available is consistent with a latitudinal trend in
foraging costs (Table 4).
Although foraging costs decrease with increasing insect abundance due to a
reduction in the cost of searching for a new prey, the cost of prey capture
per item will be unaffected by prey abundance. Prey-searching flight does not
require the same maneuverability and agility as that of prey capture because
it consists of rapid flight with the tail furled and therefore has little
aerodynamic cost (Thomas,
1996). Our aerodynamic analyses are thus consistent with an
ecomorphological relationship between aerodynamic costs due to tail elongation
and the difficulty of capturing insects. The lowest flight costs are found in
southern populations in which foraging costs are the highest due to superior
insect flight performance. Aerodynamic costs increase with latitude and are
the highest in populations where insects can readily be caught. Therefore,
there appears to be a trade-off between foraging costs and aerodynamic costs
affecting the size of tail ornaments in male barn swallows.
Geographic variation in sexual size dimorphism in barn swallows has been
explained by geographic variation in the costs of long tails for the two sexes
(Møller, 1995). We
found a positive relationship between aerodynamic costs of long tails and the
degree of sexual size dimorphism among populations
(Figure 2), suggesting that
long tails in males have evolved by sexual selection. As shown above,
aerodynamic costs during foraging enforce the tail morphology of males at
southern latitudes to be close to the aerodynamic optimum and therefore close
to the morphology of female tails. At high latitudes, where foraging costs are
low, males are less constrained, and they can cope with higher aerodynamic
costs, and larger differences in sexual size dimorphism thus evolve (see also
Møller, 1995).
Our previous arguments have only considered males. However, the evolution
of sexual size dimorphism is a process governed by the differential effects of
natural and sexual selection on individuals of the two sexes. Although there
is a slight latitudinal increase in tail length in females, this is
considerably less than the increase in males
(Møller, 1995),
presumably because long tails are more costly for females than for males. Tail
elongation in females has been suggested to be a consequence of a correlated
response to sexual selection on males
(Cuervo et al., 1996) because
of a strong positive genetic correlation between the sexes
(Møller, 1993). Thus
geographic variation in sexual size dimorphism in the barn swallow seems to
arise as a consequence of foraging costs of long tails in both sexes, allowing
little divergence at southern latitudes, but more divergence in cold climates,
where large dipteran prey are relatively easily captured even by long-tailed
males.
Recently, Norberg (1994)
proposed a hypothetical mechanism for increasing lift by the barn swallow tail
due to the distal feather bending upward and backward with the torsion of the
feather deflecting the leading edge. This mechanism was suggested to provide
an explanation based on natural selection for the evolution of tail streamers
in the barn swallow (Evans and Thomas,
1997; Norberg,
1994; Thomas and Rowe,
1997). Norberg's paper only describes a possible mechanism that
relates feather bending and feather torsion to increased lift. Norberg does
not present data demonstrating that such relationships are dependent on tail
length. The functional relationship between streamer length and the degree of
deflection of the leading edge of the tail remains to be determined. Moreover,
differential lift related to different degrees of deflection also remains to
be demonstrated. The unknown relationship between the different parameters
involved in the Norberg mechanism makes the effect difficult to assess.
Aerodynamic calculations can only include Norberg's mechanism when the
relationship between streamer length, the degree of distal feather bending
(upward and backward), the degree of feather torsion, and the amount of lift
achieved have been quantified. However, several pieces of evidence suggest
that the Norberg effect is independent of streamer length and that the
mechanism acts for short streamers such as those of female barn swallows or
those of species of hirundines with shallow, forked tails
(Norberg, 1994). Norberg
(1994) assumed that even
without streamers, it would be possible to achieve a similar effect to improve
flight (see also Møller et al.,
1998; Barbosa and
Møller, in press).
The following evidence suggests that the Norberg mechanism is unrelated to the length of tail feathers. First, differences in the length of streamers of male and female barn swallows are unrelated to differences in the flight costs of each sex (Barbosa et al., submitted). Second, the probability of feather damage is positively correlated with tail length in male barn swallows, increasing the flight costs for long-tailed individuals due to the effects of tail asymmetry on flight performance (Barbosa et al., submitted). Third, there are differences in the evolution of deep and shallow forked tails in hirundines (Barbosa and Møller, unpublished data). Fourth, sexual size dimorphism in juveniles during the first winter cannot be explained by Norberg effect because the outermost tail feathers do not extend beyond the central feathers when the tail is spread at 120°. The ratio of the length of the outermost tail feathers to that of the central feathers is on average 1.55 in juvenile males and 1.47 in juvenile females (Cadée et al., submitted). Therefore, it is unlikely that sexual size dimorphism in juveniles is explained by natural selection through the Norberg effect. Instead, sexual size dimorphism in juveniles could be explained in the light of sexual selection acting on adult dimorphism. Finally, Norberg assumed a relationship between feather traits such as the feather shaft, curvature, and flexion stiffness, but, as stated above, these hypothetical relations remain to be tested. Furthermore, if the Norberg mechanism was dependent on tail length, we should expect streamer length to be related to the functional part of the outermost tail feather. However, we have determined the relationship between streamer length and basal feather length and found no association in either sex in five different populations of barn swallows (Barbosa and Møller, unpublished data). These results also suggest that the deflecting leading edge mechanism is not determined by the relation between basal feather length and streamer length.
Several studies have shown that the presence of costly secondary sex traits
often results in the evolution of cost-reducing characters (see review in
Møller, 1996).
Aerodynamic costs due to tail elongation can be reduced by an enlarged
wingspan (Thomas, 1993). Birds
with exaggerated tail ornaments have longer wings than closely related species
without ornaments (Andersson and Andersson,
1994; Balmford et al.,
1994). Male barn swallows have reduced the aerodynamic costs of
their outermost tail feathers by increased wingspans and a reduced width of
the outermost tail feathers (Møller
et al., 1995b). Our analyses show that there is a positive, but
not significant, relationship between the increase in wing length needed to
reduce the flight costs and the degree of sexual size dimorphism among barn
swallow populations (Figure 3).
However, despite such cost reduction, a flight cost of long tails in males
still remains, as indicated by differences in flight costs among
populations.
In conclusion, long tail streamers of male barn swallows entail aerodynamic costs that are reduced by the presence of cost-reducing characters such as an increased wingspan. Geographical differences in male tail length and their inherent aerodynamic costs are constrained by foraging costs that are responsible for geographic differences in sexual size dimorphism.
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ACKNOWLEDGEMENTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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F. de Lope, N. Saino, M. Kose, and T. Szep kindly helped collecting data. A.P.M. was supported by grants from the Swedish and Danish Natural Science Research Councils. A.B. was supported by a Marie Curie postdoctoral grant of the European Communities (ERB4001GT951093). Anders Hedenström kindly reviewed a first version of the manuscript and made helpful suggestions. T. Garland kindly provided the programs for calculations of independent contrasts.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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