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Behavioral Ecology Vol. 11 No. 1: 84-92
© 2000 International Society for Behavioral Ecology
Behavioral asymmetries in a moving hybrid zone
College of Forest Resources and Burke Museum, University of Washington, Seattle, WA 98195, USA
S. F. Pearson is currently at the Department of Zoology, PO Box 118525, University of Florida, Gainesville, FL 32611, USA. E-mail: spearson{at}zoo.ufl.edu .
Received 9 December 1998; revised 5 June 1999; accepted 7 August 1999.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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Three narrow hybrid zones exist between hermit and Townsend's warblers in the Pacific Northwest. Character transition curves suggest that these zones are moving and that Townsend's warblers have a selective advantage over hermit warblers. This study compares year-to-year return rates, male persistence on territories, pairing success, pairing patterns, and territory quality of hermit and Townsend's warblers and their hybrids in the Washington Cascades hybrid zone. There was no difference in the year-to-year return rate between the parental species. Townsend's males were more successful in maintaining territories and attracting mates than hermit males. Among mated pairs there were few hermit males compared to females, independently supporting the inferiority of hermit males in competition for territories or female mate choice for Townsend's-like males. I found no difference in the quality of hermit and Townsend's territories; however, in poor habitats Townsend's males were more successful at attracting mates, suggesting female preference for Townsend's-like males. In high-quality habitats, there was no difference in pairing success between the parental species. The fitness of hybrids relative to parentals affects the width and movement of the zone. Hybrids were intermediate in their ability to maintain territories and to attract mates, which should increase the width of the zone and accelerate its movement. Fewer hybrid males returned to from one year to the next, and among mated pairs there were few hybrid females relative to males, suggesting hybrid inferiority, which should narrow the hybrid zone and slow its movement.
Key words: Dendroica occidentalis, Dendroica townsendi, habitat quality, hybridization, hybrid zones, interspecific competition, mate choice, pairing patterns, return rates.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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Hybrid zones provide unique opportunities to observe behavioral and ecological interactions between differentiated populations. Although rare (Hewitt, 1988
;
Shapiro, 1998), zones that are
moving because of fitness asymmetries between parental species are
particularly interesting because they allow researchers to observe the
behavioral and ecological forces that contribute to the displacement of one
population by another. Many studies of competition and community structure
have been forced to invoke past selective pressures to explain current
patterns (Brown, 1995;
Connell, 1983). Narrow, moving
hybrid zones provide an opportunity to investigate dynamic systems where
selective pressures are both strong and consistent in their direction. Hybrid
zones are uniquely well suited to discovering fitness asymmetries because
parentals and hybrids differ in appearance.
Hermit (Dendroica occidentalis) and Townsend's (Dendroica
townsendi) warblers are sister taxa and apparently diverged during the
middle Pleistocene in Rocky Mountain (Townsend's) and coastal (hermit) refugia
(Bermingham et al., 1992).
Today, these warblers meet and hybridize in three geographically separate
hybrid zones in Oregon and Washington, USA: one in the Olympic Mountains of
Washington, another in the southern Cascade Mountains of Washington, and a
third in the Cascade Mountains of Oregon south of Mt. Hood
(Rohwer and Wood, 1998). The
two hybrid zones in Washington are narrow relative to estimated
root-mean-square dispersal distances
(Rohwer and Wood, 1998).
Previously, I tested whether the narrowness of the Washington Cascades
hybrid zone could be explained by either habitat transition or habitat
modification and found no differences in habitat use among phenotypes
(Pearson and Manuwal, 2000).
It is also unlikely that recency of contact
(Barrow-clough, 1980;
Endler, 1977) between these
warblers can explain the narrowness of these hybrid zones. If the
root-mean-square dispersal distance is 30 km, as estimated by Rohwer and Wood
(1998), the approximately 125
km width of the hybrid zones suggests that contact occurred less than 10 years
ago. Neither an analysis of historical specimens, nor 9 years of research on
these zones support such recent contact
(Rohwer and Wood, 1998).
Character transition curves for the Washington hybrid zones are consistent
with Townsend's warblers having a selective advantage over hermit warblers;
Townsend's warbler plumage characters are introgressing into hermit warbler
populations (Rohwer and Wood,
1998). These data suggest that Townsend's warblers are replacing
hermit warblers, supporting Hewitt's
(1988) superior parental (or
dominant homozygote) model as the explanation for the narrowness of these
warbler zones. Differences in clutch size
(Pearson and Rohwer, 1998) and
in the aggressiveness of males across the zone
(Pearson and Rohwer, 2000)
further support this fitness asymmetry.
Possible explanations for the differential introgression of Townsend's
warbler genes into hermit warbler populations include female preference for
Townsend's-like males and superior competitive abilities of Townsend's males
in territorial disputes, or both. Although hybrid zones are often significant
barriers for the exchange of neutral alleles, alleles with even a small
selective advantage can penetrate relatively quickly
(Barton and Hewitt, 1985).
Parsons et al. (1993) found
that female choice in a lekking species had moved a plumage trait beyond a
hybrid zone. Thus, female mate choice could be causing the introgression of
Townsend's traits into hermit populations.
Pure Townsend's males are aggressively superior to pure hermit males
(Pearson and Rohwer, 2000).
This should make them better at obtaining female-worthy territories when they
disperse into and across the hybrid zone, thus causing the zone to move.
Hermit, Townsend's, and hybrid warblers overlap broadly in habitat
characteristics associated with territories and are interspecifically
territorial (Pearson and Manuwal,
2000), indicating that competition occurs. Because tertiary sex
ratios are male biased in these warblers (see below), the competitive
asymmetries between parentals may be reinforced by pairing patterns.
The present study compared return rates, male persistence on territories,
pairing success, and the quality of territories among parentals and hybrids.
The pattern of introgression found by Rohwer and Wood
(1998) predicts that
Townsend's males will be superior to hermits in these behavioral measures. The
competitive ranking of hybrids to parentals could not be inferred from
asymmetries in the character transition curves
(Rohwer and Wood, 1998). If
the hybrids are inferior to both parentals in these behavioral measures, then
the movement of the zone will be slowed; however, if hybrids are intermediate
between the parentals, then the movement of the zone will be accelerated.
Female mate choice provides additional information about behavioral differences between these warblers. To assess how female choice might be contributing to fitness differences between the phenotypes, I examined the relative contributions of territory quality and phenotype to pairing success. Pairing patterns also help dissect fitness differences. If more female than male hermit warblers pair, then Townsend's or hybrid males must be attracting these hermit females. This could be due either to female preference for Townsend's-like phenotypes or to a shortage of hermit males holding territories in the hybrid zone. If Townsend's males are attracting hermit females as mates while unmated hermit males are available, then a preference by female hermits, either for Townsend's-like phenotypes or for the territories they hold, is implicated.
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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Natural history
Hermit and Townsend's warblers and their hybrids live in the canopy of western coniferous forests in summer and in coniferous and mixed coniferous forests of Mexico and Central America during the winter. In southwestern Washington, adult males arrive on the breeding grounds during the first week of May; yearling males and females arrive 10 and 18 days later on average (Pearson and Rohwer, 1998
).
Male territories contain nesting and foraging sites and are maintained until
late June or early July. These species are interspecifically territorial in
the Washington Cascades hybrid zone and occupy similar habitats
(Pearson and Manuwal, 2000).
In regions of hybridization, hybrids predominate and vary continuously in
plumage characters between the parental species
(Rohwer and Wood, 1998).
I conducted research near the phenotypic center of the Washington Cascades
hybrid zone along tributaries to the Cowlitz River between the towns of Randle
and Packwood in the Gifford Pinchot National Forest (46°30' N,
121°45' W). Territories varied from 313 m to 1313 m in elevation.
The forest is dominated by Douglas fir (Pseudotsuga menziesii); other
major tree species include western hemlock (Tsuga heterophylla),
western red cedar (Thuja plicata), grand fir (Abies
grandis), and Pacific silver fir (Abies amabilis). Pearson and
Manuwal (2000) provide a
detailed habitat description.
Bird surveys
I used U.S. Forest Service roads to survey appropriate habitat for new
arrivals every 2-3 days from mid-April through early June from 1994 to 1996.
Different drainages were used each year to avoid using birds and territories
sampled previously. Males were captured for color banding using mist nets and
song playback. These males were aged as yearlings (first breeding season) or
adults (second or later breeding season) following Jackson et al.
(1992). I scored phenotypes of
captured males following Rohwer and Wood
(1998), using photographs of
their voucher specimens. For each male, a single phenotypic score that ranges
from 0 (Hermit extreme) to 1 (Townsend's extreme) was derived by summing the
scores from seven standardized plumage characteristics and dividing by seven
(Rohwer and Wood, 1998). The
seven plumage characters used in the index are extent of yellow on the crown,
intensity of yellow on breast, extent of yellow on the breast, back color, bib
corner, mid-flank streaking, and lower-flank streaking. By definition, hermit
male scores ranged from 0 to 0.25, pure Townsend's from 0.75 to 1, and hybrids
between 0.25 and 0.75. Thus, hybrids include F1 individuals and
back-crossed individuals. Black on face was also scored but not included in
the composite index because it is controlled by a single dominant allele
(Rohwer and Wood, 1998).
Because most females could not be captured, I used binoculars to score females
using the hybrid index of C. M. Smith (personal communication). I omitted one
character, extent of yellow on the breast, because it was difficult to assess
through binoculars. Like the male index, the female index produces a single
phenotypic score ranging from 0 (hermit extreme) to 1 (Townsend's extreme). By
definition, hermit female scores range from 0 to 0.2, pure Townsend's from 0.8
to 1, and hybrids between 0.2 and 0.8. Females could not be aged using
binoculars.
To avoid including migrants, only males present on the days after capture were included in this study. Territorial persistence was evaluated by reading the resident's color bands every 3-7 days throughout the breeding season. During each revisit, a locally recorded song was played to lure males out of the canopy; if no male responded, I quit these playbacks after 10 min. When original owners were absent and new males were singing in the same area, I considered the original owner replaced. New territory owners were captured, banded, aged, and scored as described above. New owners that could not be captured were scored with binoculars for as many characters as possible. For every replacement, I checked all neighboring territories to make sure the original owner had not moved to a neighboring territory or given up a portion of his original territory. Territories were followed until the final owner left his territory. When original owners disappeared without being replaced, I revisited the territory two additional times, 3-5 days apart, before considering the territory abandoned. To be considered maintained, owners had to hold their territories more than 35 days, the minimum time needed to establish a territory, attract a mate, and fledge young. The territories of all adult hermit, adult Townsend's, and all yearling males captured before female arrival were monitored through the breeding season. Because more adult hybrid males were captured than could be monitored, a random subset (10 or 11 per year) of the adult hybrid territories were monitored throughout the breeding season.
Males and territories that attracted a female were considered successful.
Changes from first- to second-category song were considered evidence of
pairing; in these and other warblers, first-category song type dominates early
in the season before pairing and second-category song dominates after pairing
(Kroodsma, 1981;
Kroodsma et al., 1989;
Morse, 1967;
Pearson and Rohwer, 1998;
Spector, 1992). Evidence of
pairing also included the presence of a female on the territory on at least
two occasions, nest building, food carrying, and the presence of
fledlings.
To assess year-to-year spring return rates, I searched for returning birds both on their previous territory and on adjacent territories.
Territory quality
I marked the first location from which I heard a male singing on each of my
first three visits to his territory. These three locations marked the centers
of 0.04-ha circles used to sample the following variables: aspect (orientation
of slope), tree height diversity, canopy cover, canopy height, average tree
diameter, basal area of canopy trees (in the following groupings: Douglas
fir/true fir, western hemlock/western red cedar, and deciduous trees), and
basal area of small coniferous and deciduous trees (see
Pearson and Manuwal, 2000, for
details on these measurements). The values for these variables were averaged
for the three circles. Additional singing locations were marked if any of the
0.04-ha circles overlapped.
Statistical analysis
Asymmetries in character transition curves suggest that Townsend's warblers
are competitively superior to hermit warblers; thus the primary focus of this
paper is the competitive ranking of the two parental species. Of secondary
interest was the competitive ranking of hybrids (which affects the width of
the zone and speed of movement). Consequently, I compared the competitive
ranking of the three phenotypic groups, but when samples were small, I also
compared the competitive ranking of the parental species without hybrids. I
used a chi-square test for comparisons of return rates, pairing success, and
territory maintenance among all three phenotypes. For the two group
comparisons between the parental species I used a chisquare test when samples
were large and a Fisher's Exact test when samples were small (50% of the cells
with an expected frequency <5).
I evaluated differences in territory quality by discriminating between
territories that did and did not attract females. Discriminant scores from
this analysis were used to quantify territory quality in all subsequent
analyses of territory quality and are hereafter referred to as
"territory quality" or "territory quality score(s)." I
used an interactive stepwise procedure to discriminate the two groups of
territories. For this interactive analysis, I used F-to-remove
statistics to determine the relative importance of variables and tolerance
levels to examine correlations between variables
(Wilkinson, 1990). The
selected model minimized the difference between the classification matrix and
jackknifed classification matrix. Variables violating the assumptions of
normality (Kolmogorov-Smirnov one-sample test) or homogeneity of group
variances (Bartlett's F test, residual scatter plots) were
transformed using either log or arcsine transformations
(Zar, 1984).
Differences in territory quality among male phenotypes were evaluated with
an analysis of variance and t test. I used linear regression to
examine the effect of territory quality on pairing date. I used a forward
stepwise logistic regression to evaluate the relative influence of territory
quality and male phenotype on male pairing success. The criterion for entry of
a variable into the model was 
= 0.05 for the score statistic and, for
removal from the model, = 0.10 for the log-likelihood ratio statistic
(Norusis, 1994).
I used linear regression to examine the relationship between male and
female phenotypes of mated pairs. All analyses were performed using SYSTAT
(Wilkinson, 1990) or SPSS
(Norusis, 1994).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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Return rates
I visited 12 adult male territories in 1995 and 31 adult male territories in 1997 where I had banded males the previous year. I could see birds well enough to identify leg bands on 41 of these 43 territories; 9 of the 41 birds returned. Return rates between hermit, Townsend's, and hybrids were not different (
2 = 4.82, df = 2, p =.09), nor were return
rates different for parentals (37% of 11 Townsend's and 50% of 4 hermits
returned; Fisher's Exact p =.54). Return rates of hybrids, however,
were lower (12% of 26) than those of parentals (40% of 24), suggesting either
that they survive less well or are less able to reclaim territories (Fisher's
Exact p =.044).
Pairing success
The pairing success of yearlings and adults had to be analyzed separately
within the zone because 65% of the 75 original territory-holding adults
paired, but only 13% of the 24 original territory-holding yearling males
paired (Table 1;
2 = 20.35, df = 1, p <.0001). For adult males,
pairing success for original owners and replacements was the same for each
phenotype within the hybrid zone; thus, I combined them in subsequent analyses
(Table 1). The three-group
comparison (including hybrids) was not significant
(Figure 1; 2 =
4.79, df = 2, p =.091). However, in the two-group comparison, more
Townsend's males than hermit males attracted mates
(Figure 1; 2 =
4.66, df = 1, p =.03), supporting the parental fitness asymmetry
model of narrow hybrid zones. Although sample sizes for yearling males were
too small to analyze statistically, the trends for yearlings were similar to
that of adults, with more yearling Townsend's males pairing than yearling
hermit males (Table 1). Hybrids
were intermediate between the parental species in pairing success
(Figure 1).
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Does pairing success of parentals differ inside and outside the zone? I can
address this question only for hermit warblers. In, 1995, 9 of 10 yearling
(n = 1) and adult (n = 9) males just south of the Washington
Cascades hybrid zone attracted a mate, whereas only 11 of 22 yearling
(n = 1) and adult (n = 21) males at the center of the hybrid
zone attracted mates (2 = 4.48, df = 1, p =.034). Age
ratios were similar between these areas, and there were no differences in the
habitat variables associated with territories between these areas
(Pearson and Manuwal, 2000).
This result suggests that competition with Townsend's and hybrid males or
female mate choice is responsible for the relatively low hermit warbler
pairing success within the zone.
Male persistence on territories
Of 75 original adult territory owners, 65% held their territories long
enough to breed, 21% were replaced by other males, and 13% abandoned their
territories (Table 1). All
males that paired also maintained their territories long enough to reproduce.
For original owners and replacements combined, there were no differences in
frequency of replacements (some replacements were replaced) among the three
phenotypic classes (Figure 1;
2 = 4.3, df = 2, p =.43). There was also no
difference in frequency of replacements between the parental species
(Figure 1; 2 =
0.59, df = 1, p =.44). The quality of territories where replacements
occurred was intermediate and not different from either maintained territories
(Figure 2, t = -0.77,
df = 63, p =.44) or abandoned territories
(Figure 2; t = 1.23 df
= 23, p =.27). The mean date of replacement was nearly identical to
the mean date of female arrival (Figure
3; t = 0.05, df = 37, p =.96).
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For original owners and replacements combined, there were no differences in
frequency of territory abandonments among the three phenotypic classes
(Figure 1; 2 =
4.04, df = 2, p =.133). However, in a two-group comparison, fewer
Townsend's males abandoned territories than hermit males, suggesting female
preference for Townsend's males or a shortage of hermit females
(Figure 1; 2 =
3.81, df = 1, p =.051). Two lines of evidence suggest that unmated
males abandon their territories: (1) The mean date of territory abandonment
was 5 days after the mean date of female arrival, although this difference was
not significant (Figure 3; t = 1.48, df = 38, p =.146), and (2) abandoned territories
were of poorer quality than successful territories
(Figure 2; t = 2.12,
df = 56, p =.038).
Territory quality and pairing success
Pairing success may be influenced by territory quality if females are
selecting mates based in part on territory quality. Discriminant analysis
significantly separated successful from unsuccessful territories, predicting
group membership correctly in 72% of the cases
(Figure 4; Wilks' 
=
0.81, F = 3.84, df = 4, 89, p <.001). Thus, females may
be selecting territories based at least in part on territory quality.
Figure 4 plots the distribution
of successful and unsuccessful territories along the discriminant axis for the
four habitat variables (see Table
2). Successful territories had a larger basal area of fir and
higher canopy closure. Unsuccessful territories tended to be on north-facing
slopes and had a higher basal area of small conifers (indicative of wet and
cool sites). I used yearlings and adults for this analysis, but repeating it
just for adults (which differed significantly from yearlings in territory
quality; t = -2.6, df = 32, p =.013) produced similar
results (74% predictability; Wilks' = 0.85, F = 3.07, df =
4,69, p =.022).
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As an independent test of the significance of territory quality on female choice, I regressed pairing date on territory quality. Females settled earliest on high-quality territories (Figure 5; F = 7.7, df = 1,77, r = -.48, p =.01).
Given the asymmetries in character transition curves, I expected Townsend's males to dominate high-quality habitats. However, I found no difference in territory quality among adult male phenotypes (Figure 6; F = 0.02, df = 2,71, p =.98). There was also no difference in territory quality between adults of the parental species (Figure 6; t = -0.11, df = 46, p =.91). Dominance of female Townsend's warblers in highquality habitats could give them a reproductive advantage. I found no difference in territory quality among paired (see below) female phenotypes (F = 0.21, df = 2,20, p =.81).
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Female mate choice
Why do Townsend's warblers have the highest pairing success when the
territories they occupy are no higher in quality than those of hybrids and
hermits? The answer seems to be that Townsend's males were nearly twice as
successful in attracting mates in poor-quality habitats, suggesting female
preference for Townsend's males (Figure
7).
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I further analyzed the relative influence of male phenotype and territory
quality on female choice using a logistic regression. For this analysis, I
used only the territories occupied during female arrival and the phenotypic
score of the males occupying those territories. When only territory quality
was included in the model, 86% of the territories were classified correctly
(2 = 15.27, df = 1, p =.0001). When the scores for
each of the eight phenotypic characters (including face patch) were included
in the model along with territory quality, both face patch (Wald = 5.85,
p =.016) and lower-flank streaking (Wald = 8.46, p =.0036)
improved the overall model significantly. The resulting model classified 93%
of the territories correctly (2 = 27.77, df = 3, p
<.0001). This logistic model indicates that territory quality best explains
the difference between successful and unsuccessful territories; however, some
of the poor-quality territories were incorrectly classified. Because
Townsend's warblers have relatively high pairing success even in relatively
low-quality habitats (Figure
7), adding Townsend's characters (heavy lower-flank streaking and
black face patch) to the model improves its predictability. Individual
character scores were added to the model rather than the composite phenotypic
score for two reasons. First, the composite index does not include face patch
(see Methods), which may be important to female choice. Second, within
individual warblers in these hybrid zones, there is a strong correlation among
characters associated with black (face patch, flank streaking, bib corner, and
crown) and among characters associated with yellow (extent and intensity of
yellow on the breast and back color), but there are weaker correlations
between black and yellow characters
(Rohwer and Wood, 1998).
Because female warblers may select mates based on individual characters or
several characters associated with either black or yellow, it is necessary to
analyze phenotypic characters individually.
Pairing patterns
Figure 8 presents the
phenotypic relationship between males and females of mated pairs. The overall
pattern of pairing is assortative (Figure
8; F = 6.84, df = 1,26, r =.46, p
=.01). It is the assortative pairing of the parentals and not the hybrids that
is causing this relationship. Despite the overall assortative pairing, the
most striking pattern in this figure is that every hermit male was paired with
a pure hermit female, whereas an equal number of pure hermit females pair with
hybrid and Townsend's males.
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Among mated pairs, I observed fewer hermit males (n = 7) than
hermit females (n = 13), which may reflect reduced hermit male
survivorship or the inability of hermit males to maintain territories or
obtain mates (Figure 8). There
are half as many hybrid females as hybrid males
(Figure 8). Given normally
prevailing male-biased sex ratios (Lack,
1954; Promislow et al.,
1992), females are typically all paired and males are not. Thus,
the shortage of paired hybrid females relative to paired hybrid males likely
represents a true female shortage in the population (although I expect a small
male bias in the overall population).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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Behavioral asymmetries
Character transition curves from Rohwer and Wood (1998
) suggest Townsend's
warblers are competitively superior to hermit warblers. Field data from a
locality in the center of the Washington Cascade hybrid zone yielded the
following behavioral asymmetries supporting this predicted difference.
Townsend's males were more successful in establishing and maintaining
territories and in attracting mates than hermit males
(Figure 1). Among mated pairs
there were few hermit males compared to females
(Figure 8), independently
supporting the inferiority of hermit males in competition for territories or
female mate choice. Although I found no difference in the quality of
territories held by Townsend's and hermit males
(Figure 6), in poor habitats
Townsend's males were more successful at attracting mates than hermit males
(Figure 7), suggesting female
preference for Townsend's males. In high-quality habitats, there was no
difference in pairing success between parentals
(Figure 7).
How fast the zones move depends on the competitive ranking of hybrids
relative to the parental species. If hybrids are inferior to both parental
species, then the movement of the zone should be slowed. If hybrids are
superior to hermits but inferior to Townsend's, then the movement of the zone
should be accelerated. Hybrid territory quality was nearly identical to that
of the parental species (Figure
6). Hybrid males were intermediate between parentals in their
ability to maintain territories and attract mates
(Figure 1). Fewer hybrid males
returned from one year to the next and, among mated pairs, there were few
hybrid females relative to males (Figure
8), suggesting hybrid inferiority. Fewer returning hybrid males
may reflect reduced overwinter survival, poor breeding success during the
previous summer, or inability to reclaim territories
(Holmes et al., 1996). Reduced
overwinter survival is a likely explanation because hybrids were more
successful than hermits in maintaining territories and attracting mates
(Figure 1). The shortage of
paired hybrid females relative to males suggests that hybrid females are less
viable or suffer greater infertility than hybrid males. Haldane's rule
(1922) predicts that when
reduced fertility or viability occurs in hybrids, it should occur in the
heterogametic sex; in birds, females are the heterogametic sex. Alternatively,
hybrid females may have difficulty finding mates, but this seems implausible
because 20% of 75 adult male territories and 63% of 24 yearling male
territories held throughout female arrival never attracted females, indicating
a surplus of unmated males.
Table 3 provides a summary of the ranking of parentals and hybrids based on nine criteria that I presume are related to fitness. No measure contradicts the competitive superiority of Townsend's over hybrid and hermit warblers. Two assessments, arrival dates and territory quality, reveal no difference among the three phenotypes. Of nine comparisons, eight can be used to infer fitness asymmetries between hybrids and hermits. Hybrids are superior to hermits in three comparisons, equal to hermits in three comparisons, and inferior to hermits in two comparisons.
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Why do Townsend's males pair more successfully than hermit
males?
The relatively high rate of Townsend's warbler pairing can be explained by
the superior competitive ability of Townsend's males in territorial disputes
or by female mate choice. Two lines of evidence suggest that a Townsend's
competitive advantage may contribute to this asymmetry. Townsend's males
respond more aggressively to simulated territorial intrusions than hermit
males (Pearson and Rohwer,
2000), suggesting a competitive advantage. Townsend's males are
more successful in maintaining territories and are rarely replaced
(Figure 1).
If Townsend's males are competitively superior to hermit males, then they
should predominate in high-quality habitats. Surprisingly, I found no
difference in territory quality between these species. Competitive sorting
(see Pearson and Rohwer, 2000)
could explain the lack of a relationship between phenotype and territory
quality. Competitive sorting results in males, regardless of their phenotype,
being similar in aggressiveness within any given locality within the hybrid
zone. Because Townsend's males are more aggressive, localities across the
hybrid zone should be rank ordered in aggression. If males within any given
locality are similar in aggressiveness, then they should occupy similar
quality territories. This work was performed at essentially one locality near
the mid-point of the zone, and thus there is no reason to expect a difference
in territory quality among phenotypes. However, hermit males could establish
territories in poor habitats near the center of the zone. Perhaps hermit males
move to a location on the hermit side of the zone where they can obtain a good
territory and a mate rather than establishing territories in poor habitat
where they are unlikely to attract a mate.
Female mate choice may also explain Townsend's warbler pairing success. Territory quality appears to have a strong influence on hermit and hybrid male pairing success and less of an influence on Townsend's warbler pairing success (Figure 7). The logistic regression examining the influence of territory quality and phenotype on pairing success indicates that territory quality is primarily responsible for pairing success but that Townsend's-like males are more successful in attracting mates. Interestingly, there is no correlation between phenotype and territory quality, indicating that females are selecting Townsend's-like males over hybrid and hermit males. However, there is the possibility that Townsend's males hold territories that are slightly superior in some aspect that I did not measure, and consequently the Townsend's phenotypic characters that appear to be preferred by females may be a surrogate for this unmeasured aspect of territory quality.
Why do more hermit females successfully pair than hermit males?
The asymmetry in pairing success between hermit males and hermit females
appears to be the result of interactions between phenotypes within the hybrid
zone because territorial hermit males south of the hybrid zone in similar
quality territories to those within the zone
(Pearson and Manuwal, 2000)
almost always attract mates, are rarely replaced by other males, and rarely
abandon their territories. Two possible explanations for this asymmetry
include female hermit warbler preference for Townsend's-like phenotypes and
the inability of hermit males to maintain territories when competing with
hybrid and Townsend's males. Hermit males abandon their territories twice as
frequently as Townsend's males (Figure
1), yet they occupy high-quality territories, suggesting that
hermit females prefer Townsend's-like males. Although not a statistically
significant difference, territorial hermit males are replaced by other males
nearly twice as frequently as Townsend's males and as a result may be less
successful in attracting mates. As mentioned previously, hermit males appear
to be less aggressive than Townsend's males
(Pearson and Rohwer, 2000),
indicating a competitive disadvantage in territorial disputes. The relative
contribution of female mate choice and male—male competition to the
shortage of paired hermit males is difficult to evaluate. However, the similar
rates of territory abandonment and replacement among hermit males
(Table 1) suggest that both
factors may be important.
Possible scenario explaining the asymmetric movement of Townsend's
warbler genes into hermit warbler populations
If aggressive differences among males are contributing to the apparent
movement of these hybrid zones, then Townsend's males must be better able to
disperse into and across the zone and to establish territories than hermit
warblers. Rohwer and Wood
(1998) provide evidence of
substantial dispersal. Mount presentation experiments
(Pearson and Rohwer, 2000)
suggest that Townsend's males may have a competitive advantage in establishing
territories in regions dominated by hybrids and hermits. Once established on
territories within the hybrid zone, Townsend's and Townsend's-like males near
the center of the zone are more likely to breed than are hermit males (Figures
1 and
8).
Dispersal of Townsend's females could also contribute to the asymmetric introgression of Townsend's warbler characters into hermit populations. However, long-distance dispersal of Townsend's females to the hermit side of the zone is unlikely because Townsend's females were never observed pairing with hermit males or hybrid males with a hybrid score below 0.4 (Figure 8).
If the movement of these zones is caused by the asymmetric dispersal of
Townsend's males, with all heterospecific pairing occurring between Townsend's
males and hermit females, and if hybrids are being absorbed by the advancing
Townsend's warbler population, then one would expect an asymmetric pattern of
hermit warbler haplotypes in the wake of a moving hybrid zone. Preliminary
genetic analyses indicates an asymmetric pattern of hermit warbler haplotypes
in Townsend's warbler populations well removed from the hybrid zone, and in
some Townsend's warbler populations hermit mtDNA is virtually fixed (Rohwer S,
personal communication). Similar patterns of directional hybridization, with
resultant one-way introgression of mtDNA, have been described for warblers
(Vermivora; Gill,
1997), canids (Canis;
Lehman et al., 1991), deer
(Odocoileus; Carr et al.,
1986), pocket gophers (Thomomys;
Patton and Smith, 1993), and
tree frogs (Hyla; Lamb and Avise,
1986). As in this warbler hybrid zone, the likely mechanisms
responsible for differential introgression include sex-specific dispersal
(Carr et al., 1986;
Gill, 1997;
Lehman et al., 1991) and
female mate choice (Patton and Smith,
1993).
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ACKNOWLEDGEMENTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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Keen insight from Sievert Rohwer, Dave Manuwal, and Matthias Leu led to many productive discussions of this work. I thank Peter Gibert for his field assistance in 1997 and the University of Washington Burke Museum staff for logistical support. For comments on the manuscript, I thank Sievert Rohwer, Matthias Leu, Dave Manuwal, Ron Ydenberg, and three anonymous reviewers. This research was supported by a Burke Museum Eddy Fellowship in Ornithology and by direct contributions from Garrett Eddy to the University of Washington Burke Museum.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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