Download Influence of phenotypic and social traits on dispersal in a family

Behavioral Ecology
doi:10.1093/beheco/ars108
Advance Access publication 14 August 2012
Original Article
Influence of phenotypic and social traits on
dispersal in a family living, tropical bird
Corey E. Tarwater
Program in Ecology, Evolution, and Conservation Biology, University of Illinois at Urbana-Champaign,
Urbana, IL, USA, Present address: Ecosystem Sciences Division, Department of Environmental Science,
Policy & Management, University of California at Berkeley, Berkeley, USA
Individual variation in natal dispersal behaviors has extensive ecological and evolutionary consequences. Traits such as offspring
sex, age, and body condition may influence dispersal, resulting in a potentially complex suite of associations in traits that can
affect fitness. Conceivably, individuals with particular phenotypes may breed in different habitats, thus potentiating the development of geographic variation. Moreover, studies typically underestimate dispersal distance owing to sampling issues and rarely
consider the direction of movement, limiting understanding of this important life history stage. I examined the influence of
phenotypic and social traits on dispersal distance and direction in a family living bird, the western slaty antshrike (Thamnophilus
atrinucha). When accounting for detection probability, juveniles dispersed 1–14 territories and 46–1268 m. The age at dispersal
and body mass upon leaving the nest influenced dispersal distance and direction. Older and heavier individuals dispersed shorter
distances. Younger individuals dispersed towards comparatively younger forest with a higher density of antshrike territories. Older
and heavier offspring may be more competitive and/or have increased experience with the local habitat, increasing their probability of acquiring nearby territories. Contrary to other studies, sex-biased dispersal distance was not observed. Instead, the sexes
dispersed in different directions, potentially to reduce the risk of inbreeding. This study revealed the importance of age at dispersal and body mass on variation in dispersal behaviors and highlighted the need to investigate sex biases in dispersal direction. Key words: baker method, family living, inbreeding avoidance, intraspecific competition, social species, tropical. [Behav Ecol]
Introduction
I
ndividual variation in natal dispersal may arise because the
costs and benefits of dispersal behaviors are conditioned by
phenotypic or social traits, such as offspring sex, body condition, and age (Clobert et al. 2009). Costs and benefits are
influenced by the distance and direction of movement, and
both metrics determine the location of the first breeding site.
Nevertheless, studies typically focus on distance alone (but
see Sharp et al. 2008; Delgado et al. 2010; Penteriani and
Delgado 2011). Associations between individual traits and
dispersal could lead to a geography of different phenotypes
breeding in different habitats, with implications for genetic
structure, selection on phenotypic traits, and population
growth (Bowler and Benton 2005; Benard and McCauley
2008). Despite recognition of the importance of individual
variation in dispersal, little is known about what traits underlie
this variation (Bowler and Benton 2005; Clobert et al. 2009).
The trait most commonly examined in studies of natal dispersal is offspring sex, and 2 main hypotheses have been proposed
to explain sex-biased dispersal. In birds, intraspecific competition often leads to female-biased dispersal because males benefit
to a greater extent by shorter dispersal distances owing to their
need to acquire a resource (e.g., territory) to attract females
and their increased probability of outcompeting nonlocal males
for territories (resource defense hypothesis: Greenwood 1980).
Second, sex-biased dispersal distance or direction may be a
Address correspondence to C. E. Tarwater. E-mail: tarwater@
berkeley.edu
Co-author C.E.T is now at Department of Environmental Science,
Policy & Management, 130 Mulford Hall, Berkeley, CA 94720-3114, USA.
Received 16 August 2011; revised 17 May 2012; accepted 25 May 2012.
© The Author 2012. Published by Oxford University Press on behalf of
the International Society for Behavioral Ecology. All rights reserved.
For permissions, please e-mail: [email protected]
mechanism to reduce the probability of inbreeding (inbreeding
hypothesis: Greenwood 1980; Pusey 1987; Sharp et al. 2008).
Other phenotypic traits, such as birth date and body mass,
may also influence variation in dispersal (Verhulst et al. 1997;
Forero et al. 2002). These traits may influence competitive
ability and number of potential competitors (Garnett 1981;
Arcese 1987; Zack and Rabenold 1989). More competitive
individuals, typically of higher body mass, or ones experiencing less competition, often early-born individuals, are predicted to have a higher probability of obtaining preferred
breeding territories (Gauthreaux 1978; Stamps 2006).
The age at dispersal may also influence dispersal behaviors.
In family living species, a subset of juveniles remain with
parents past the age of independence, whereas the other
juveniles disperse as a result of sibling rivalry, parent–offspring
conflict, or territory quality (Ekman and Griesser 2002;
Ekman et al. 2002; Dickinson and McGowan 2005;). These
young dispersers are often less competitive or less familiar
with the surrounding habitat, and are therefore predicted
to have a lower probability of obtaining preferred breeding
sites (Pärt 1995; Ekman et al. 2002; Gienapp and Merila
2011). Although studies focus on cooperative breeders, family
living without helping is a more widespread behavior, and
thus studies are needed on how age at dispersal influences
dispersal movement in these species (Green and Cockburn
2001; Covas and Griesser 2007).
Sibling rivalry may influence when offspring disperse, and
number of siblings may influence the distance and direction
of dispersal. Juveniles with fewer siblings may have a higher
probability of obtaining preferred breeding habitat because
they have access to more resources on the natal site, potentially increasing their competitive ability (Forero et al. 2002;
Tinbergen 2005; Smiseth et al. 2007).
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Tarwater • Variation in dispersal in a tropical bird
I examined natal dispersal distance and direction and
the influence of phenotypic and social traits in variation in
dispersal in a family living bird, the western slaty antshrike
(Thamnophilus atrinucha), in a lowland tropical forest. Unique
to many dispersal studies, antshrikes were studied in a saturated environment, where breeding vacancies are rare, juvenile survival is high, and delayed reproduction is common
(Robinson et al. 2000; Tarwater et al. 2011). Furthermore,
rather than direction being governed by large-scale habitat
variation (Selonen and Hanski 2004; Gosselink et al. 2010),
I examined whether small-scale differences in habitat within
a contiguous forest would influence direction. First year survival estimates are higher for juveniles that are born earlier in
the breeding season, have a higher body mass up on leaving
the nest, and for older dispersers (Tarwater et al. 2011). Given
that these traits affect offspring fitness, I assessed their influence on dispersal behaviors as well. Antshrikes exhibit variation in age at dispersal (38–346 days after fledging or leaving
the nest), mass at fledging (12–18 g), and date of fledging
(February–September) (Tarwater and Brawn 2010a; Tarwater
et al. 2011). Large variation in these traits provides a unique
opportunity to inform on the link between phenotype and
dispersal. I predicted that males and females would disperse
similar distances if intraspecific competition influenced dispersal because, contrary to many other species, the sexes have
similar roles in territoriality (Tarwater, unpublished data).
Underestimates of dispersal distance are common because
studies typically occur in finite areas and the probability of
detecting dispersal decreases with distance (Baker et al. 1995;
Cooper et al. 2008). Although corrections for this sampling
bias have been suggested and are shown to improve estimates
(Baker et al. 1995; Cooper et al. 2008), typically, these corrections are not made (but see Sharp et al. 2008). Here I report
both observed and corrected dispersal distances.
MATERIALS AND METHODS
Study species and population
Slaty antshrikes were observed from 2003–2009 on a 104-ha
study site within Parque Nacional Soberanía; a 22 000 ha
contiguous, lowland tropical moist forest in the Republíc of
Panama (Karr 1971; Robinson et al. 2000). Antshrikes have a
clutch size of 2, are pair breeding, and weigh on average 23 g
in mass (Oniki 1975; Roper 1996; Tarwater and Kelley 2010).
They are year-round territorial residents with 0.8- to 0.9-ha
breeding territories, and approximately 120 breeding pairs
are on the study plot (Oniki 1975; Roper 1996; Tarwater,
unpublished data). The breeding season of antshrikes is from
approximately January to September, and up to 2 successful
broods may be produced during this period (Tarwater and
Brawn 2008; Tarwater and Brawn 2010b).
Adults were color banded for individual identification and
nests of 25–60 breeding pairs were monitored depending upon
the year. Nestlings (n = 225) were color banded prior to fledging, and offspring were monitored 1–2 times per week until natal
dispersal (for details see Tarwater and Brawn 2010a, 2010b). An
offspring was considered dispersed when the natal territory was
searched 3 times and the individual was not observed (Tarwater
and Brawn 2010a). Offspring that were still dependent upon
their parents when they were not observed were classified as dead
rather than as dispersed (for details see Tarwater et al. 2011).
Dispersal distance and direction
I used 2 methods to estimate dispersal distance and direction: resighting and radiotelemetry. Dispersed juveniles
were resighted either through opportunistic resightings
(2003–2007) or systematic searches of the entire study plot
to identify every breeding individual (2008–2009). I attached
radiotransmitters to a subset of birds prior to dispersal (n = 24,
of which 13 were radiotracked when floating). Floating is
when an individual wanders within other breeding pairs’ territories in search of a vacancy (Smith 1978). Radio signals lasted
for 1–9.5 months depending upon survival of the bird and/or
the transmitter (for details see Tarwater et al. 2011).
Dispersal distance was defined as the distance between the
natal nest and either the first breeding territory or the area
an individual floated in. Direction was defined as the bearing
(from north) associated with the movement from a juvenile’s
natal nest to their breeding or floating area. All antshrike
juveniles float up on dispersal and after 1 long-distance
dispersal movement; they float within a given area until
territory acquisition (Tarwater, unpublished data). For the
birds resighted when floating, the location at which the bird
was resighted was used as the endpoint of dispersal. For the
radio-tagged birds, the weighted mean of the area an individual
floated in was used. GPS (Global Positioning System) location
data taken every 3–4 days was used to calculate the weighted
mean. Twelve juveniles were observed both while floating and
after territory acquisition. The median difference in dispersal
distance and direction between “true” and “floating” dispersal
was 84 m (~1 territory different) and 9.0°, respectively. Thus,
floating dispersal is a good proxy for dispersal distance and
direction onto a breeding site. A total of 29 juveniles were
found on a breeding territory, 24 juveniles were resighted as
floaters, and 13 juveniles were radiotracked when floating.
Phenotypic and social traits
I analyzed the influence of mass at fledging, number of siblings within a brood, date of fledging, offspring sex, and age at
dispersal on variation in dispersal distance and direction. Too
few sibling pairs were resighted to assess the influence of sibling sex on dispersal. Offspring were weighed (to the nearest
0.1 g) 1–3 days prior to fledging, during which time mass does
not change (Pearson correlation coefficient = 0.13, P = 0.42,
n = 44). Individuals were scored as having one sibling or none
if the sibling died within 2 weeks of fledging (Tarwater and
Brawn 2010b). Fledge date was determined by the monitoring
of nests. Offspring sex can be distinguished based on plumage by 3 weeks after fledging (prior to dispersal, Tarwater and
Brawn 2010a). I determined age at dispersal by observations
of banded and radio-tagged fledglings on their natal territories (for details see Tarwater and Brawn 2010a, 2010b). None
of these traits were strongly correlated with each other (all
P values from correlation coefficients were >0.09).
Statistical analyses
I used the linear distance from the natal nest to the breeding
territory or center of the floating area to estimate dispersal
distance and to calculate the number of territories that a bird
traversed (straight line distance divided by the average length
of a territory, 89 m) (Greenwood and Harvey 1982; Eikenaar
et al. 2008). Based on the area searched (127 ha for resighted
birds and 238 ha for radio-tagged birds), maximum detectable dispersal distances for resighted birds was 1750 m (19.7
territories) and for radio-tagged birds, 2100 m (23.6 territories). I used the Baker method to control for the finite size of
the study area (for details see Baker et al. 1995; Cooper et al.
2008). Surrounding habitat was considered suitable because
the plot is located within a larger forest. I calculated a probability of detection for a function of dispersal distances that
ranged from the minimum to the maximum dispersal distance observed. The probability of detection for every possible
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dispersal distance was calculated by starting from 1 point on
the study plot, drawing a circle from that point with a radius
that was equal to dispersal distance, and then dividing the area
of the circle that is inside the study plot by the total area of the
circle (including area that is outside the study plot). A probability of detection was calculated for each dispersal distance
every 20 m on the plot. Spacing of 20 m results in a large
distribution of detection probabilities from across the plot,
increasing the accuracy of the median probability of detection
for a given dispersal distance. Short dispersal distances have
a detection probability close to 1.0 because they are typically
within the plot and are thus more often detected. Longer distances have a detection probability of <1.0, with the longer the
distance, the lower the probability of detection. The corrected
dispersal distances were calculated by multiplying the median
probability of detection for each dispersal distance by the density function of the observed dispersal distances.
Dispersal direction was analyzed using circular statistics
(Fisher and Lee 1992). Standard errors (SE) were calculated
using maximum-likelihood estimates from a von Mises distribution (Fisher and Lee 1992). Juveniles were able to disperse
in any direction, and half of the juveniles dispersed from territories on the east side of the plot and the other half dispersed from the west side of the plot.
To assess how phenotypic and social traits influenced
dispersal, I constructed models in R 2.14 (R Development
Core Team 2011). For dispersal distance, linear models
with a Gaussian distribution were constructed, distance was
cube-root transformed, age at dispersal was log transformed,
and distance was the response variable. For direction, circular
regression based on the von Mises distribution was used, and
radian was the circular response variable (Fisher and Lee
1992). The von Mises distribution is the circular analog of the
Gaussian distribution (Lee 2010). I examined correlations
between fixed effects and found that the variance inflation
factor was less than 1.4 for all correlations (values <3 have
low collinearity; Zuur et al. 2007). Owing to the small sample
size, interactions were not posited, and I first tested whether
method of estimating dispersal (resighted when floating,
radio-tagged when floating, found on breeding territory)
influenced dispersal (all t-tests had P values >0.1 for distance
and direction). Therefore, in subsequent analyses, method
was not included as a fixed effect.
For dispersal distance, I used Akaike’s Information corrected for small sample sizes (AICc) for model selection.
Model selection was used to test whether including natal
territory (full and half siblings), sibling pair (full siblings
from the same nest), or method of estimating dispersal
as random effects improved the model. Sibling status was
determined based on individual identification of the female
and male parents caring for offspring. The model without
any random effects had the lowest AICc. The overall fit of
the saturated model was assessed. I constructed 32 models
with all combinations of variables and an intercept-only
model. Models with a difference (ΔAICc) of ≤ 2 are as
parsimonious as the best-fit model (lowest AICc), and
ΔAICc > 7 is considered strong evidence that the likelihood
of the candidate models differ from one another (Lebreton
et al. 1992). Relative importance of the traits was assessed
and model-averaged estimates were derived to account for
model uncertainty (Burnham and Anderson 2002).
For dispersal direction, there is no accepted measure of
overall fit of a saturated model for circular data, and model
averaging techniques are not well developed (Scapini et al.
2002). Therefore, similar to other studies, I used backwards
stepwise elimination to compare nested models. I began with
the saturated model, removed the variable with the highest P
value, and compared the new model with the saturated model
Behavioral Ecology
using log-likelihood ratio tests. This procedure was performed
until only variables with significant P values were retained.
RESULTS
Natal dispersal distance and direction
The observed estimated median dispersal distance for all juveniles was 286.5 m (interquartile range = 338 m) with a range
of 46–1012 m (n = 54 juveniles, Figure 1). This is equivalent to
a median distance of approximately 3.3 territories and a range
of 1–12 territories. By using the Baker method, the distribution of dispersal distances shifted to the right (Figure 1). The
corrected median dispersal distance was 414.1 m (interquartile
range = 431.9 m), equivalent to 4.7 territories, and a range of 1–14
territories (maximum of 1268 m, Figure 1). Considering only
juveniles that acquired breeding territories, the observed median
dispersal distance was 290.0 m (interquartile range = 346.5, range
of 67–1012 m, n = 29). No juveniles bred on their natal territory.
Juveniles, on average, dispersed to the south with a
median dispersal direction of 187.5° and an average
Figure 1 The percent of juvenile slaty antshrikes that dispersed different
distances away from the natal territory (n = 54 juveniles). The
observed (white bars) and corrected (black bars) distances are
shown. Corrected distances are based on the Baker method and
a density distribution of the observed distances. Each column
represents dispersing one territory farther.
Table 1 Model selection results for phenotypic and social traits that influence
variation in dispersal distance of juvenile slaty antshrikes
Modela
AICcb
△AICcb
AICcb wt
1. Age + mass + date
2. Age + mass
3. Age + mass + date + sex
4. Age + mass + date + sibling
5. Age + mass + sex
6. Age + mass + sibling
7. Age + mass + date + sibling + sex
8. Age + mass + sibling + sex
127.10
127.95
128.88
129.00
129.75
129.77
130.68
131.45
0.00
0.85
1.78
1.90
2.64
2.67
3.57
4.34
0.31
0.20
0.13
0.12
0.08
0.08
0.05
0.03
a Models
1-4 are the top models (≤ 2 ΔAICc). Models >5 ΔAICc are not
shown.
AICc = difference between the AICc value for the best model and
this AICc value.
bΔ
1245
Tarwater • Variation in dispersal in a tropical bird
direction of 180.1 ± 22.8° (range 1–357°, n = 54 juveniles).
Nevertheless, juveniles tended to disperse uniformly in all
directions with a variance of 0.76 (0, disperse in one direction; 1, uniform dispersal) and a P value of 0.05 for the
Rayleigh test of uniformity (significant P values indicate
nonuniform dispersal).
Influence of phenotypic and social traits on
dispersal distance
The saturated model (mass at fledging, date of fledging, number of siblings, offspring sex, and age at dispersal) was significant (R2 = 0.34, P = 0.03). There were 4 top-ranked models
(≤2 ΔAICc), and these included all traits examined (Table 1).
Figure 2 The influence of age at dispersal (a), mass at fledging (b), date of fledging (c), offspring sex (d), number of siblings (e), and method of
estimating distance (f) on natal dispersal distance in juvenile slaty antshrikes (n = 54 juveniles).
1246
Behavioral Ecology
Table 2 Model-averaged results for phenotypic and social traits that influence
variation in dispersal distance of juvenile slaty antshrikes
Traits
β estimate
SE
Lower CI
Upper CI
Intercept
Age at dispersal
Mass at fledging
Date of birth
Offspring sex
Number of siblings
15.400
−1.030
−0.506
0.006
0.056
−0.044
2.710
0.895
0.171
0.008
0.174
0.159
9.930
−2.830
−0.850
−0.009
−0.290
−0.361
20.800
0.767
−0.162
0.022
0.403
0.273
Model-averaged results indicate that the relative importance
weights were the same for age at dispersal and mass at fledging (1.00), followed by date of fledging (0.54), sex of offspring (0.24), and number of siblings (0.23). Age at dispersal
and mass at fledging were in all the top models and were the
only traits with strong support. Individuals that dispersed
at an older age and were heavier upon leaving the nest dispersed shorter distances than individuals that dispersed earlier and were lighter (Figure 2a,b). Based on model-averaged
results, the upper and lower confidence intervals (CI) for
mass at fledging do not include zero (Table 2). Owing to the
large standard error for age at dispersal (high variation in
ages), the CI overlap zero; however, the upper and lower CI
are not evenly spread around zero (Table 2). Furthermore,
in a comparison of juveniles that disperse during their natal
breeding season or late wet season (postbreeding season)
with those that disperse during the dry season the next calendar year (older dispersers), the median dispersal distance
was 370.0 m (range 46–1012, n = 33) compared with 185.5
m (range 117–392, n = 12, Mann–Whitney, U test 396.0,
P = 0.00), respectively. Territory turnover is uncommon during the wet season (breeding season and postbreeding season) compared with turnover during the dry season months
(January–March). For the other traits, low relative important weights were observed, and the upper and lower CI fall
evenly above and below zero, indicating no support for these
traits (Table 2, Figure 2c-e).
Influence of phenotypic and social traits on dispersal
direction
The saturated model showed a significant effect of 3 out of
the 5 traits examined: mass at fledging, offspring sex, and
age at dispersal (all P values <0.03). Through backwards
elimination, the final model included mass at fledging, sex
of offspring, and age at dispersal (Table 3). The final model
(AICc = −10.9) is a better fit model than the intercept-only
model (AICc = 4.36, ΔAICc = 15.3) and the saturated model
(AICc = −4.42, ΔAICc = 6.48). Males tended to disperse to
the southwest (mean = 188.7 ± 18.5°, median = 197.9°),
whereas females tended to disperse to the southeast
Table 3 Final model results for phenotypic and social traits that influence
variation in dispersal direction of juvenile slaty antshrikes
Traits
β estimate
SE
t value
P value
Age at dispersal
Mass at fledging
Offspring sex
0.028
−0.125
−0.629
0.009
0.054
0.359
3.163
2.323
1.750
0.001
0.010
0.040
In final model µ = −1.65 (SE = 0.22), κ = 1.24 (SE = 0.30).
(mean = 124.8 ± 90.2°, median = 117.3°). Age at dispersal
rotates in a clockwise direction, with individuals that disperse
at a younger age dispersing in a southeast direction, most
offspring (average dispersers) dispersing to the west and
north, and older dispersers dispersing primarily to the east
(Figure 3a). Mass at fledging rotates in a counterclockwise
direction, with lighter individuals dispersing to the north,
most offspring (average mass) dispersing to the west, and the
heaviest offspring dispersing to the southwest (Figure 3b).
DISCUSSION
This is, to my knowledge, the first study to examine variation
in dispersal distance and direction in a tropical, family living
species. This study emphasizes the importance of examining
direction when assessing potential sex biases in dispersal and
highlights the long-term effects on offspring of natal conditions, influencing offspring mass, and parental behaviors,
influencing when offspring disperse (Tarwater and Brawn
2010b). Older dispersers and heavier individuals dispersed
shorter distances, dispersed in different directions, and had
higher first year survival compared with younger dispersers
and lighter individuals (Tarwater et al. 2011). The large variation in dispersal distance and direction observed reflects, in
part, the range in dispersal ages in antshrikes. Large variation in age at dispersal is likely prevalent in other tropical and
southern hemisphere species, where family living is commonly
observed (Russell et al. 2004; Griesser and Barnaby 2010).
Therefore, results presented here may be broadly applicable.
The influence of age at dispersal and body mass on
dispersal distance
Age at dispersal and body mass at fledging were both inversely
related to dispersal distance in antshrikes. Individuals that
disperse at a later age and are heavier upon leaving the nest
may be able to disperse shorter distances because they are
more competitive and have more experience with the local
habitat, increasing their probability of obtaining a nearby
breeding site (Stamps 1987; Raihani et al. 2010; Piper 2011).
On the other hand, lighter and younger juveniles may
increase their probability of obtaining a territory by dispersing
longer distances in search of good floating areas, where there
are fewer older juveniles (born the previous year) to compete
with. Competitive ability often increases with age because
the longer a juvenile remains with parents, the longer they
have access to food resources and the more time they have
to develop foraging and predator-detection skills prior to
dispersal (Ekman et al. 2001; Raihani et al. 2010; reviewed in
Griesser and Barnaby 2010). Similarly, competitive ability may
be greater in heavier offspring because these individuals are
often from higher quality natal sites (Garnett 1981; Galeotti
et al. 1997; Van de Pol et al. 2006). Juveniles that disperse
when older have more time to prospect nearby breeding sites,
to evaluate the probability of acquiring these sites, and to gain
experience with the area (Zack and Rabenold 1989; Bruinzeel
and Van de Pol 2004; Eikenaar et al. 2008). Likewise,
individuals of higher mass may have more experience with
the territory holders in an area if they do not need to spend
as much time acquiring food (Sullivan 1989).
The influence of age at dispersal and body mass on
dispersal direction
Even in contiguous habitat, phenotype influenced the direction
of antshrike dispersal. Dispersal direction may be influenced
by abiotic factors affecting initial movement, such as wind
1247
Tarwater • Variation in dispersal in a tropical bird
Figure 3 The influence of age at dispersal (a) and mass at fledging (b) on natal dispersal direction in female (thick solid line) and male (thick dashed
line) juvenile slaty antshrikes (n = 54 juveniles). Predictions based on circular regression model are shown. 95% CI are included as thinner
solid (females) and dashed (males) lines.
direction, or factors that influence habitat selection, such as
competitive ability and habitat quality (Stamps 2001; Delgado
et al. 2010). The heaviest and oldest individuals tended to disperse during the dry season, but dispersed in different directions (southwest and northeast, respectively), suggesting that
seasonal changes in abiotic factors may be less important in
dispersal direction. Instead, individuals may select their habitat depending upon their own phenotype (Clobert et al. 2009).
The forest in the northern section of the study plot is older
(mature secondary compared with secondary forest) compared with the south (Pyke et al. 2001), the east and west sides
of the plot differ in soil type (Turner and Engelbrecht 2011),
and breeding densities of antshrikes and one other understory
bird are greater on the west side compared with the east side
of the plot (Styrsky 2003; Tarwater, unpublished data). If a
higher density of territories result in more territory vacancies,
then younger dispersers may disperse to the west because, for
less-competitive individuals, this is the best strategy for obtaining a breeding territory. Older dispersers, on the other hand,
tended to disperse to the lower territory density and older forest region in the northeast section of the plot, suggesting that
this habitat is of higher quality. Further examination is needed
to assess how breeding habitat influences fitness; however,
results suggest that an interaction between an individual’s own
phenotype and habitat influenced dispersal direction.
Sex-biased dispersal
Results support both the resource-defense hypothesis and
the inbreeding hypothesis as mechanisms influencing
1248
dispersal in male and female slaty antshrikes. Both sexes
acquire and defend breeding territories, and males and
females do not differ in survival, age at dispersal, or age at
reproduction, suggesting that the sexes experience similar
levels of competition (Tarwater and Brawn 2010a; Tarwater
et al. 2011). Thus, males and females likely benefit equally
from dispersing short distances. These observations support the resource-defense hypothesis as the reason for no
sex-biased dispersal distance. Instead, the sexes differed in
dispersal direction, and this may reduce the probability of
inbreeding (Greenwood 1980; Sharp et al. 2008; Dale 2010).
Broader implications
Few studies have assessed the relationship between delayed
dispersal, short dispersal distances, and delayed reproduction (relative to non–family living species) within a single
species (but see Ekman et al. 1999; Green and Cockburn
2001; Gienapp and Merila 2011). However, these traits
are predicted to be intricately linked in family living species (with and without cooperative breeding) and to result
in higher lifetime reproductive success (Stearns 1992; Ens
et al. 1995; Covas and Griesser 2007). Results presented
here and in previous studies are in agreement with the link
between these traits (Tarwater and Brawn 2010a; Tarwater
et al. 2011). Within antshrikes, delayed dispersal, delayed
reproduction, and, on average, short dispersal distances relative to non–family living species are observed (Nilsson 1989;
Verhulst et al. 1997; Russell et al. 2004; Tarwater et al. 2011).
Tropical forests are being deforested at an alarming rate,
and these changes are the primary threat to global biodiversity (Achard et al. 2002). Tropical forest–dependent species, and in particular understory insectivores, are often the
first species to become extirpated from fragments (Stouffer
and Bierregaard 1995; Sekercioglu et al. 2002; Sodhi et al.
2004). This may arise in part due to short dispersal distances
of these species, yet empirical data are limited (Sekercioglu
et al. 2002). Here, I show that in a nonfragmented forest, antshrikes dispersed short distances, dispersing up to 1268 m and
on average, 414 m. This work suggests that even if these birds
are willing to cross light gaps (Moore et al. 2008), fragments
that are greater than 1300 m apart are unlikely connected by
antshrike movement. Furthermore, those that do disperse farther away are often the subdominant individuals, potentially
leading to reproductive isolation over small spatial scales.
FUNDING
National Science Foundation (Integrative Research
Challenges in Environmental Biology grant IBN-02125870);
University of Illinois at Urbana-Champaign; Smithsonian
Tropical Research Institute; Cooper Ornithological Society;
and Wilson Ornithological Society.
I thank S. Bassar, R. Bassar, D. Buehler, A. Castillo, and B. Lascelles
for help in the field. I am especially grateful to C. Batista, I. Gallo,
and I. Ochoa for multiple years of help resighting and radiotracking juveniles. Thanks to J.D. Brawn, E.A. Lacey, T.L. Morelli, J.A.
Woodruff, and 2 anonymous reviewers for helpful comments on the
manuscript. Thanks to J.P. Kelley for general support. The Autoridad
Nacional del Ambiente granted permission to work in the Republic
of Panama. Thanks to the Smithsonian Tropical Research Institute
for providing logistical support.
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