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Individual Variability in Migration Timing Can Explain Long-Term, Population-Level Advances in a Songbird


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Migratory animals may be particularly at-risk due to global climate change, as they must match their timing with asynchronous changes in suitable conditions across broad, spatiotemporal scales. It is unclear whether individual long-distance migratory songbirds can flexibly adjust their timing to varying inter-annual conditions. Longitudinal data for individuals sampled across migration are ideal for investigating phenotypic plasticity in migratory timing programs, but remain exceptionally rare. Using the largest, repeat-tracking data set available to date for a songbird (n = 33, purple martin Progne subis), we investigated individual variability in migration timing across 7,000–14,000 km migrations between North American breeding sites and South American overwintering sites. In contrast to previous studies of songbirds, we found broad, within-individual variability between years in the timing of spring departure (0–20 days), spring crossing of the Gulf of Mexico (0–20 days), and breeding site arrival (0–18 days). Spring departure and arrival dates were fairly repeatable across years (depart r = 0.39; arrive r = 0.32). Fall migration timing was more variable at the individual level (depart range = 0–19 days; gulf crossing range = 1–15 days; arrive range = 0–24 days) and less repeatable, with fall crossing of the Tropic of Cancer being the least repeatable (r = 0.0001). In this first, repeat-tracking study of a diurnal migratory songbird, the high within-individual variability in timing that we report may reflect the greater influence of environmental and social cues on migratory timing, as compared to the migration of more solitary, nocturnally migrating songbirds. Further, large, within-individual variability in migration dates (0–24 days) suggest that advances in spring arrival dates with climate change that have been reported for multiple songbird species (including purple martins) could potentially be explained by intra-individual flexibility in migration timing. However, whether phenotypic plasticity will be sufficient to keep up with the pace of climate change remains to be determined.
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published: 06 September 2019
doi: 10.3389/fevo.2019.00324
Frontiers in Ecology and Evolution | 1September 2019 | Volume 7 | Article 324
Edited by:
Brett K. Sandercock,
Norwegian Institute for Nature
Research (NINA), Norway
Reviewed by:
Jason Courter,
Malone University, United States
Kristen Covino,
Loyola Marymount
University, United States
Kevin C. Fraser
Specialty section:
This article was submitted to
Behavioral and Evolutionary Ecology,
a section of the journal
Frontiers in Ecology and Evolution
Received: 27 March 2019
Accepted: 13 August 2019
Published: 06 September 2019
Fraser KC, Shave A, de Greef E,
Siegrist J and Garroway CJ (2019)
Individual Variability in Migration
Timing Can Explain Long-Term,
Population-Level Advances in a
Songbird. Front. Ecol. Evol. 7:324.
doi: 10.3389/fevo.2019.00324
Individual Variability in Migration
Timing Can Explain Long-Term,
Population-Level Advances in a
Kevin C. Fraser 1
*, Amanda Shave 1, Evelien de Greef 1, Joseph Siegrist 2and
Colin J. Garroway 1
1Department of Biological Sciences, University of Manitoba, Winnipeg, MB, Canada, 2Purple Martin Conservation
Association, Erie, PA, United States
Migratory animals may be particularly at-risk due to global climate change, as they
must match their timing with asynchronous changes in suitable conditions across
broad, spatiotemporal scales. It is unclear whether individual long-distance migratory
songbirds can flexibly adjust their timing to varying inter-annual conditions. Longitudinal
data for individuals sampled across migration are ideal for investigating phenotypic
plasticity in migratory timing programs, but remain exceptionally rare. Using the largest,
repeat-tracking data set available to date for a songbird (n=33, purple martin Progne
subis), we investigated individual variability in migration timing across 7,000–14,000 km
migrations between North American breeding sites and South American overwintering
sites. In contrast to previous studies of songbirds, we found broad, within-individual
variability between years in the timing of spring departure (0–20 days), spring crossing of
the Gulf of Mexico (0–20 days), and breeding site arrival (0–18 days). Spring departure
and arrival dates were fairly repeatable across years (depart r=0.39; arrive r=0.32).
Fall migration timing was more variable at the individual level (depart range =0–19 days;
gulf crossing range =1–15 days; arrive range =0–24 days) and less repeatable, with
fall crossing of the Tropic of Cancer being the least repeatable (r=0.0001). In this first,
repeat-tracking study of a diurnal migratory songbird, the high within-individual variability
in timing that we report may reflect the greater influence of environmental and social
cues on migratory timing, as compared to the migration of more solitary, nocturnally
migrating songbirds. Further, large, within-individual variability in migration dates (0–24
days) suggest that advances in spring arrival dates with climate change that have been
reported for multiple songbird species (including purple martins) could potentially be
explained by intra-individual flexibility in migration timing. However, whether phenotypic
plasticity will be sufficient to keep up with the pace of climate change remains to
be determined.
Keywords: phenotypic plasticity, spring phenology, repeatability, climate change, avian, long-distance migration,
Fraser et al. Individual Variability in Songbird Migration
Phenotypic plasticity in animal migration timing could provide
the means for rapid acclimation to environmental change,
as compared to adaptive responses through genetic change
(Charmantier and Gienapp, 2014). To what extent phenotypic
plasticity and/or micro-evolution are the mechanisms
responsible for population-level advances in the spring
migration timing of some landbirds has been hotly debated
(Knudsen et al., 2011; Charmantier and Gienapp, 2014). Steep
population declines among migratory species (Both et al., 2010),
lends urgency to determining whether constraints on adaptive
timing are a contributing factor.
Longitudinal data are ideal for research on these
themes because they provide the opportunity to investigate
phenotypic variation within individuals in response to varying
environmental conditions across years (Charmantier and
Gienapp, 2014). For songbird migration, most previous studies
have focused on the use of observational data to determine
the individual repeatability (r) of spring migration departure
and arrival dates. These studies report broad variation (r=
0.04–0.51), both within and among migratory species (Potti,
1998; Brown and Brown, 2000; Moller, 2001; Ninni et al., 2004;
Cooper et al., 2009; Studds and Marra, 2011). Direct-tracking
technologies (Stutchbury et al., 2009) provide the means to
examine complete annual migration tracks, but studies where
multiple migrations by the same individual are monitored
are still rare (Both et al., 2016). In a Neotropical songbird
(wood thrush, Hylocichla mustelina), within-individual spring
migration timing was remarkably repeatable, with a mean
difference of just 3 days in spring arrival date between years,
suggesting limited plasticity (n=10; Stanley et al., 2012). In
a Palearctic example (red-backed shrikes, Lanius collurio),
within-individual variability was similarly low, where mean
within-individual differences were 3–12 days, and breeding
arrival date (n=2) varied by only 1–4 days (Pedersen et al.,
2018). Thus, repeatability of migration timing of songbirds using
direct tracking is generally reported to be high, particularly in
spring. However, studies to date have relied on low sample size
(<20 individuals), are thinly spread across species and migratory
systems, and have focused on nocturnally migrating species
(Both et al., 2016).
In many migratory species, population-level advances in
spring migration timing have been observed over decadal scales
and linked to temperature increase with climate change (Butler,
2003; Mayor et al., 2017; Lehikoinen et al., 2019). Across
European and North American migratory landbird systems, the
mean advance over several decades in spring migration timing
was 1 week (Lehikoinen et al., 2019); with advances within
some species reported to be >2 weeks (Butler, 2003). There is
much debate as to whether these timing advances are the result
of phenotypic plasticity, micro-evolutionary change, or both
(Knudsen et al., 2011). For species with high, within-individual
repeatability in migration timing, these rapid population-level
advances in timing are difficult to reconcile. In Icelandic
black-tailed godwits (Limosa limosa islandica), population-level
arrival dates advanced by approximately 17 days over 20 years
(Gunnarsson and Tomasson, 2011) but over this same time
span individual arrival dates were highly consistent (r=0.51;
Gill et al., 2014). These results for black-tailed godwits suggest
that advances were not driven by individual plasticity of adult
migrants, but rather ontogenetic effects during development
could underlie these rapid advances in timing (Gill et al.,
2014). Such comparisons of individual variability vs. population-
level advances in timing remain rare, and need to be further
explored in other species and systems. There is currently a
deficit of knowledge about the mechanisms of adaptive change
(microevolutionary and/or phenotypic plasticity) in response
to environmental conditions during migration (Pulido and
Berthold, 2004) and the impact of longer-scale climatic effects
on the flexibility of migration patterns (Knudsen et al., 2011),
especially for long-distance migratory songbirds.
We investigated the phenotypic plasticity of migration timing
by using a diurnal, long distance, Neotropical migratory songbird
(purple martin, Progne subis) that travels 10, 000–20, 000 km
annually between North American breeding sites and South
American overwintering sites (Fraser et al., 2012, 2013a,b).
Purple martins are aerial insectivores, and like other swallows,
are thought to use a fly-and-forage strategy during their diurnal
migration (Brown and Tarof, 2013). At the population-level, this
species has advanced spring migration timing by 8–20 days over
the last 100 years (Arab and Courter, 2015). However, arrival
dates did not advance in response to a record-setting, early spring
in 2012 (Fraser et al., 2013a). Data on the variation in individual
timing across multiple migrations are therefore required to
further investigate the potential for phenotypic plasticity in
purple martins in response to environmental conditions and
to determine whether this can explain long-term advances in
timing. We used the largest repeat-track data set available for
a songbird, comprised of 33 individuals tracked across 2 years
by using light-level geolocators. Our objectives were to (1)
determine within-individual variation and repeatability (r) in
timing across both spring and fall migration, and (2) assess
whether the degree of within-individual variability provides a
potential mechanism to explain population-level advances in
spring timing reported for martins and whether this species can
serve as a model for similar investigations in other songbirds.
Geolocator Analysis Methods
Light-level geolocators were deployed on adult purple martins
at 8 North American breeding sites (latitudinal range 38.36N
to 53.02N; Supplementary Table 1) using a leg-loop backpack-
style harness made of Teflon ribbon (Rappole and Tipton, 1991;
Stutchbury et al., 2009) and retrieved in the following year (or
subsequent year, n=2) at the same locations (2009–2016). This
study was conducted in accordance with the recommendations
of the Ornithological Council’s Guidelines to the Use of Wild
Birds in Research’ and was approved by the University of
Manitoba and York University Animal Care Committees (2009-
2 W, F14-009/1–3).
We defined sunrises and sunsets (twilights) from the raw
geolocator light data using the preprocessLight function in the
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Fraser et al. Individual Variability in Songbird Migration
R-package BAStag version 0.1.3 (Wotherspoon et al., 2016). We
used a light intensity threshold of 32 to define the separation of
day and night. Events that influenced the geolocator’s light sensor
outside of sunrise and sunset times (e.g., shading during day, light
during night) indicated false twilights. We used the initiation
of heavy shading (false twilights during daylight periods) in
spring, that clearly indicated entrance and exits of nest cavities,
to identify breeding arrival date. After defining arrival date, all
false twilights were removed.
The twilight dataset was used to define daily locations
and movement periods by using the R-package GeoLight
version 2.0 (Lisovski and Hahn, 2012). We used the coord
function to determine spatial coordinates throughout entire
migratory tracks. We calculated an appropriate sun elevation
angle using twilight data at each bird’s known breeding
location, before fall migration. Latitudes impacted by spring
and fall equinox periods were omitted by using a tolerance
level of 0.13 (Lisovski and Hahn, 2012). The resulting data
were used to determine daily coordinates and movement
periods and to identify spring and fall arrival and departure
dates, as well as the date individuals crossed the Tropic
of Cancer (23.4N). We used the changeLight function to
determine residency and movement periods, and shifts in
latitudinal and longitudinal coordinates, to identify departure
and fall arrival dates. We defined overwintering locations as
a tenure of >7 days within the known non-breeding range.
Most stopovers in this region were <7 days (Van Loon
et al., 2017), thus this provided a conservative estimate of
when birds had completed migration and arrived at their
overwintering destination.
Repeatability of Spring and Fall Migration
To investigate if individuals are consistent in their migration
timing between years, repeatability was examined for birds
tracked for at least 2 years (individuals =33, geolocator tracks
=67). We examined the repeatability of migration departure
date (fall tracks =66, spring tracks =67), date passing the
Tropic of Cancer (23.4N; fall tracks =34, spring tracks =61),
and date of arrival at the breeding grounds or overwintering
grounds (fall tracks =67, spring tracks =67). We also
included 144 single-tracked (1 year only) individuals in the
analysis to better account for population level variability in
the analysis, resulting in a total of 5–57 tracks per breeding
location (Supplementary Table 1). Repeatability was calculated
as the fraction of variation in behavior between individuals, as
compared to the sum of phenotypic plasticity and measurement
error (Nakagawa and Schielzeth, 2010). Repeatability is a
proportion between 0 and 1, where low values indicate most
of the variation is due to plasticity and error. The adjusted
repeatability (value of repeatability calculated after controlling
for confounding effects) of aspects of migration timing were
calculated in linear, mixed-effects models using the package
MCMCglmm (Hadfield, 2010). In this case the confounding
effects of sex and age were set as fixed effects (males and older
birds may have earlier timing) with year, individual, and breeding
colony as random effects to control for repeated measures. We
did not include temperature or other weather factors in our
analysis owing to limitations in the number of repeat-track birds
per site. Confidence intervals for repeatability were estimated
by parametric bootstrapping with 1,000 replications. Results
were replicated with an uninformed prior which produced
quantitatively similar results (Table 1) with overlapping 95%
credibility intervals. All analyses were done in R version 3.5.3 (R
Core Team, 2018).
We found that spring migration timing (departure, crossing
23.4N, arrival) was more repeatable between years at the
individual level than timing during fall migration (spring range,
r=0.32–0.39; fall range, r=0.0001–0.001; Table 1,Figure 1).
The timing of spring departure was the most consistent across
years (r=39, CI =0.08–0.50), perhaps owing to strong
endogenous control of migration initiation. Spring crossing of
the Tropic of Cancer (23.4N) and breeding arrival date were
also fairly consistent across years (cross r=0.32; arrive r=
0.32). Fall arrival date was much less repeatable than breeding
arrival date (0.0009 vs. 0.32) (Figure 1). Variance explained
by the random factors of colony and year ranged from 32.47
to 90.76 and 3.09–30.94, respectively (Supplementary Table 2).
Age had a significant effect on timing across both spring and
fall migration, with ASY birds departing and arriving earlier
by 5.52–7.18 days as compared to SY birds. Sex impacted
the timing of spring departure and spring cross only, with
males migrating 6.46 and 6.08 days earlier than females
(Supplementary Table 3).
Within-individual variability between the first and second
year of tracking was broad (0–24 days), with individual timing
earlier (1–24 days), or later (1–23 days) in the second year of
tracking (Figure 2). In spring, departure date varied by 0–20
days, spring crossing of 23.4N by 0–20 days, and arrival at the
TABLE 1 | Adjusted repeatability estimates and 95% credibility intervals for spring
and fall migration (2008–2016) including departure dates (n=67; n=66), spring
and fall crossing the Tropic of Cancer (n=61, n=34), and spring and fall arrival
dates (n=67; n=67).
Timing Adjusted repeatability 95% CI
Spring departure from non-breeding site 0.39 0.08, 0.50
Spring crossing tropic of cancer 0.32 0.16, 0.54
Spring arrival at breeding site 0.32 0.12, 0.48
Fall departure from breeding site 0.001 <0.001, 0.38
Fall crossing tropic of cancer 0.0001 <0.001, 0.07
Fall arrival at non-breeding site 0.0009 <0.001, 0.47
Individuals were tracked for two spring migrations, except one individual tracked for three
spring migrations. Estimates and credibility intervals were calculated using MCMCglmm
(Hadfield, 2010)*. We included fixed effects of sex and age and controlled for non-
independence of year and individuals within breeding colonies by including them as
random effects. Uninformed prior distributions (V =1, nu =0.002) were used for
all variables.
Hadfield (2010)*.
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Fraser et al. Individual Variability in Songbird Migration
FIGURE 1 | Comparison of migration timing (day of year) at the individual level for the first vs. second year of tracking. Colors show breeding locations where
geolocators were retrieved and correspond to points indicated on the map. Line indicates 1:1, where points below the line indicate earlier migration in year two. The
map shows the year-round purple martin range in purple (Brown and Tarof, 2013).
breeding site by 0–18 days. Fall migration timing was generally
more variable at the individual level (depart range =0–19 days;
cross range =1–15 days; arrive range =0–24 days). Overall
within-individual variability over the year was therefore up to
24 days, which spans the 8–20 day population-level advance in
spring arrival date over 100 years, reported for purple martins
(Arab and Courter, 2015) (Figure 2).
Individual patterns of migration timing across years provide
invaluable clues regarding the phenotypic plasticity of migration
timing. We show that spring migration timing of a long-distance
migratory songbird was more repeatable, from start-to-finish,
than fall migration, with spring departure date being the most
consistent between years. We found broad, within-individual
variability in migration timing (year 1 as compared to year 2)
at key points around the annual cycle (start and stop dates,
and approximate midway points). We show that broad, intra-
individual variability (up to 24 days earlier or 23 days later in
the second year of tracking), was a feature of both spring and fall
migration. Our results therefore suggest individual plasticity as a
potential mechanism to account for population-level advances in
spring arrival date (8–20 days) reported for purple martin (Arab
and Courter, 2015). The degree of plasticity we show in individual
martins also exceed mean population-level advancements (1
week) for spring migration reported for North American and
European migratory landbird systems, including several species
of long-distance aerial insectivores (Lehikoinen et al., 2019).
It has been difficult to reconcile the decadal-scale advances
in spring migration timing at the population level, with
intra-individual data that show high consistency of migration
timing (e.g., Gill et al., 2014). It is debated whether strong
selection for advanced timing and rapid micro-evolution could
be responsible for population-level change (Knudsen et al.,
2011) because advances via these mechanisms are generally
predicted to take much longer. For example, using quantitative
genetic models an observed 14-days advance in laying date
in great tits (Parus major) was estimated to require more
than two centuries to attain via micro-evolution (Charmantier
and Gienapp, 2014). The predicted time period is considerably
longer than the scale of the 10–100-years advances reported
for multiple landbird species across North American and
European migration systems (Lehikoinen et al., 2019). If micro-
evolutionary responses cannot occur this quickly (Charmantier
and Gienapp, 2014), and individuals do not exhibit a high
level of phenotypic plasticity in spring timing, then how do
we explain population-level advances in timing over short
timescales? Our results show, that at least in purple martins,
individual variation is a potential explanation for the kinds of
advances reported in spring timing (Arab and Courter, 2015).
We found within individual variation of up to 24 days whereas
population level advancements for this species over 100 years
are between 8 and 20 days (Arab and Courter, 2015). Our
results contrast those for Icelandic godwit, where consistency in
individual timing precluded individual plasticity of adult birds
as a viable explanation for population-level advances in timing
(Gill et al., 2014). Further studies are required across species
and systems to determine whether individual plasticity is a
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Fraser et al. Individual Variability in Songbird Migration
FIGURE 2 | The difference in migration timing between year one and year two for individual purple martins tracked between North American breeding and South
American overwintering sites. A positive value indicates earlier timing in the second year of tracking. Sample sizes for each timing category are as follows: depart fall =
32, cross fall =10, arrive fall =32, depart spring =32, cross spring =28, arrive spring =33. Data show timing at migration start and end points, as well as the timing
of crossing the Tropic of Cancer (23.4N), in both spring and fall. Long-term data for mean breeding arrival date in spring across the range (from Arab and Courter,
2015) are included to enable comparison of these longer-term advances in timing over 100 years with short-term differences at the individual level (this study).
potential explanation for observed advances in timing. It would
also be valuable for future studies to investigate the influence
of additional factors on individual plasticity, such as nocturnal
vs. diurnal migratory strategies, foraging guild, short vs. long-
distance migration (Lehikoinen et al., 2019), nest strategy, and
within-species patterns.
We found moderate repeatability of spring migration timing
(range: r=0.32–0.39) that is lower than reported in other studies
of nocturnally migrating songbirds (e.g., range: r=0.49–0.71,
Stanley et al., 2012) and many long-distance migrants generally
(Both et al., 2016). We found the highest repeatability of timing
at spring departure from the non-breeding grounds as has been
shown for red-backed shrikes, Icelandic whimbrels, and black-
tailed godwits; perhaps owing to the strong role of endogenous
cues in migration initiation (Gwinner, 1996; Pedersen et al.,
2018; Carneiro et al., 2019; Senner et al., 2019). Sex and age
were also important factors influencing spring departure timing,
but differences between the sexes diminished by the time of
arrival at breeding areas, whereas older birds were consistently
earlier than younger ones. Relatively high repeatability of spring
arrival date may reflect strong selection on timing at the breeding
ground. In martins, high competition for nest cavities may
further contribute to higher repeatability of spring arrival dates
(Brown and Tarof, 2013). In contrast, fall timing was much
less repeatable (r=0.0001–0.001). Particularly low repeatability
of fall migration arrival date (r=0.0009), may reflect relaxed
selection on this trait in purple martins; a species that is non-
territorial in winter and joins large communal roosts, in contrast
to a songbird that is territorial in winter, where repeatability was
relatively high (r=0.62; Stanley et al., 2012). Intra-individual
variation in nest success may also have contributed to low fall
repeatability values in our study, if birds with failed nests depart
earlier than birds attending young from successful broods. The
overall, lower repeatability of migratory timing in our study of
a diurnal migrant as compared to results for some nocturnally
migrating songbirds (Both et al., 2016) may be influenced by
migratory strategy (diurnal vs. nocturnal). Phenological and
repeatability studies of songbirds have tended to focus more
on nocturnally migrating species and comparisons of short and
long-distance migrants (Both et al., 2016; Lehikoinen et al., 2019),
however, diurnal and nocturnal migrants may exhibit different
amounts of plasticity in timing to environmental change and
should be further investigated.
We found higher repeatability in spring than fall for
crossing of the Tropic of Cancer (23.4N), which is generally
associated with crossing of the Gulf of Mexico in martins, as
most individuals make a >800 km open-water crossing of this
“barrier” during spring and fall migration (Fraser et al., 2013a,b).
Lower repeatability in fall may indicate greater, population-level
synchronization of the timing of crossing in this season. In
wood thrushes, crossing of the gulf showed low repeatability
during both spring and fall (r=0.12, Stanley et al., 2012).
In spring, the timing of gulf crossing in martins is not largely
impacted by weather conditions (Abdulle and Fraser, 2018),
thus we infer that higher consistency in individual timing at
this barrier is a result of a carry-over effect of spring departure
timing, rather than an influence of conditions at this stage
of migration.
We speculate that the generally larger, intra-individual
variation (up to 24 days) that we found for martins as compared
to other songbirds (Stanley et al., 2012; Both et al., 2016; Pedersen
et al., 2018), may reflect the nature of migration and social
behavior in martins, and possibly other swallows. The intra-
individual variability we found for purple martins is more similar
to broad, within-individual variation recently shown for some
shorebirds (Senner et al., 2019; Verhoeven et al., 2019), than
to data reported for nocturnally migrating songbirds. Purple
martins are diurnal migrants that roost in large flocks during
stopovers across migration (Brown and Tarof, 2013), and may
use island-like habitats for stopover (Fraser et al., 2017; Fournier
et al., 2019). Large social aggregations and suitable stopover
habitat are unevenly distributed across a migratory landscape,
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Fraser et al. Individual Variability in Songbird Migration
thus martin stopover decisions may be influenced by these social
factors which could contribute to variation in their individual
timing. In contrast, diurnal songbirds that migrate singly during
the night may not require social stopover cues to the same
degree, which may favor more independent and consistent
migration schedules. While it has been demonstrated that short-
distance migrants may exhibit swifter phenological shifts in
response to environmental change than long-distance migrants
(Hurlbert and Liang, 2012; Kullberg et al., 2015; Takuji et al.,
2017), whether diurnal, long-distance migrants exhibit greater
phenotypic plasticity than nocturnal ones would be valuable
to determine.
Our data did not provide the opportunity to examine
variation in repeatability across populations breeding at different
latitudes, but such within-species investigations are an important
frontier. Such research would be particularly important for
purple martins and other aerial insectivores, where strong
north-south patterns of population decline are reported (Nebel
et al., 2010), and where relative limitations in behavioral
plasticity between populations could be playing a role. More
northern breeding populations may exhibit larger advances
in spring arrival dates than more southern ones (Arab and
Courter, 2015), which should be investigated in concert with
individual-level patterns.
In an era of rapid, global environmental change, it is critical
that we address the degree to which migratory birds can mount
phenotypic responses to change. In this first investigation using
a diurnal migrant and the largest repeat-tracking data set for a
songbird, we show that phenotypic plasticity in migration timing
is a potential mechanism to explain decadal-scale, population-
level advancement in spring migration timing. It remains to
be determined whether the degree of individual plasticity we
show is connected to, or cued by, temperature and whether any
advances in migration timing are sufficient to match advances
in seasonal phenology of lower trophic levels. Future research
should also investigate the role of environmental cues and
other mechanisms contributing to within-individual variation in
migration timing.
All datasets generated for this study are included in
the manuscript and/or the Supplementary Files.
This study was conducted in accordance with the
recommendations of the Ornithological Council’s Guidelines
to the Use of Wild Birds in Research’ and was approved by
the University of Manitoba and York University Animal Care
Committees (2009- 2W, F14-009/1-3).
KF, AS, and JS conducted fieldwork. KF, AS, EG, and CG analyzed
the data. KF, AS, EG, JS, and CG wrote the manuscript.
For field assistance and support we thank Nanette Mickle, Paul
Mammenga, Tim Shaheen, Kelly Applegate, Michael North,
Larry Leonard, Edward Cheskey, Megan McIntosh, Pat Kramer,
Cassandra Silverio, Lee Bakewell, Richard Doll, Myrna Pearman,
Alisha Ritchie, Bridget Stutchbury, and John Tautin. Funding
and support were provided by University of Manitoba, NSERC,
Nature Canada, Central Lakes College, Minnesota Audubon,
Minnesota Ornithologists’ Union, Brainerd Lakes Audubon
Society, Mille Lacs Band of Ojibwe, and Ellis Bird Farm.
The Supplementary Material for this article can be found
online at:
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Conflict of Interest Statement: The authors declare that the research was
conducted in the absence of any commercial or financial relationships that could
be construed as a potential conflict of interest.
Copyright © 2019 Fraser, Shave, de Greef, Siegrist and Garroway. This is an open-
access article distributed under the terms of the Creative Commons Attribution
License (CC BY). The use, distribution or reproduction in other forums is permitted,
provided the original author(s) and the copyright owner(s) are credited and that the
original publication in this journal is cited, in accordance with accepted academic
practice. No use, distribution or reproduction is permitted which does not comply
with these terms.
Frontiers in Ecology and Evolution | 7September 2019 | Volume 7 | Article 324
... Additionally, urban habitats may provide relatively stable refugia for urban exploiters to replace or subsidize habitat made unsuitable by climate change. Purple martins (Progne subis), for example, are increasing their use of urban areas for nesting and migratory staging (Bridge et al., 2016) and displaying phenotypically plastic migration timing in association with rising global temperatures (Fraser et al., 2019). The simultaneous effects of land use change and climate change can also be balancing; for example, in Danish avian communities, agricultural land use change decreases species abundances, while warmer winters increase species abundances (Bowler et al., 2018). ...
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The concept of ecological resilience is widely used to assess how species and ecosystems respond to external stressors but is applied infrequently at the level of the community or to chronic, ongoing disturbances. In this review, we first discuss the concept of ecological resilience and methods for quantifying resilience in ecological studies. We then synthesize existing evidence for the resilience of avian communities to climate change and urbanization, two chronic disturbances that are driving global biodiversity loss, and conclude with recommendations for future directions. We only briefly discuss the theoretical framework behind ecological resilience and species-specific responses to these two major disturbances, because numerous reviews already exist on these topics. Current research suggests strong heterogeneity in the responses and resilience of bird communities to urbanization and climate change, although community disassembly and reassembly is high following both disturbances. To advance our understanding of community resilience to these disturbances, we recommend five areas of future study (1) the development of a standardized, comprehensive community resilience index that incorporates both adaptive capacity and measures of functional diversity, (2) measurement/modeling of both community resistance and recovery in response to disturbance, (3) multi-scale and/or multi-taxa studies that include three-way interactions between plants, animals, and climate, (4) studies that incorporate interactions between disturbances, and (5) increased understanding of interactions between ecological resilience and socio-ecological dynamics. Advancement in these areas will enhance our ability to predict and respond to the rapidly accelerating effects of climate change and urbanization.
... Some individuals can show highly consistent behavior (i.e., individuals departing at similar dates from year to year; Lourenço et al. 2011, Vardanis et al. 2011, whereas others are highly flexible in their phenology (e.g., wide ranges of departure dates across years; Hasselquist et al. 2017, Tedeschi et al. 2019. To measure the strength of individual responses within a population, behavioral ecologists quantify their consistency (Vardanis et al. 2016, Fraser et al. 2019. Such an approach allows the estimation of the adaptative capacity of both populations and individuals to cope with environmental changes (Vardanis et al. 2016, McCrary et al. 2019, Sugasawa and Higuchi 2020. ...
Interannual consistency (an indicator of the strength of adjustments) in migration phenology of Golden Eagles (Aquila chrysaetos) in North America is most strongly associated with breeding region, the season and with late season temperature on breeding and wintering grounds. Consistency was greatest in boreal spring migration and the breeding regions of eastern Canada. Using multi-year GPS tracks of 83 adults breeding in three spatially distant regions (Alaska, northeast Canada, and southeast Canada), we quantified the interannual consistency of migration phenology and wintering latitude within and among individuals tracked across multiple years and the repeatability (r) by breeding regions and seasons. By comparing regions and seasons, we found that consistency was highest (r > 0.85) for boreal spring migration in eastern Canada while Alaska had the lowest value (r < 0.15). Because seasonal consistency of migration phenology was only detected in eastern Canada, we conclude that seasonal features are not a primary constraint. While regional differences in consistency were not related to differences in migratory distances, they could be the result of genetic or habitat differences. We also found that temperatures warmer than the decadal average at the region of departure delayed the start of boreal spring migration by ~10 days and advanced boreal autumn migration by ~20 days. These results suggest that warmer temperatures would reduce residence time on breeding grounds, contrary to expectations and trends found in other studies. Wide variations in migratory strategies across a species distribution can add to the lists of challenges for conservation but may give migrants the capacity to acclimate to environmental changes.
... Variability is a hallmark of behavior and is observed across timescales (Tinbergen, 1951). On long timescales, variability has been studied in the migratory behavior of birds; birds display interindividual variability in migratory patterns, timing, and kinematics such as migratory speed (Potti, 1998;Trierweiler et al., 2014;Fraser et al., 2019;Phipps et al., 2019). On shorter timescales, many studies have looked at variability in movement kinetics, kinematics, and endpoints of reaching movements (Gordon et al., 1994;Messier and Kalaska, 1999;van Beers et al., 2004;Wu et al., 2014). ...
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Changes in locomotion mediated by odors (odor-guided locomotion) are an important mechanism by which animals discover resources important to their survival. Odor-guided locomotion, like most other behaviors, is highly variable. Variability in behavior can arise at many nodes along the circuit that performs sensorimotor transformation. We review these sources of variability in the context of the Drosophila olfactory system. While these sources of variability are important, using a model for locomotion, we show that another important contributor to behavioral variability is the stochastic nature of decision-making during locomotion as well as the persistence of these decisions: Flies choose the speed and curvature stochastically from a distribution and locomote with the same speed and curvature for extended periods. This stochasticity in locomotion will result in variability in behavior even if there is no noise in sensorimotor transformation. Overall, the noise in sensorimotor transformation is amplified by mechanisms of locomotion making odor-guided locomotion in flies highly variable.
... Turtle doves are susceptible to previous and ongoing changes, such as increased cultivation of the Sahel and Sudan zone, overgrazing and cutting of trees, overuse of pesticides, suppression of woodland within farmland, and the homogenization of cropland (Lutz 2007;Fisher et al. 2018;Mansouri et al. 2020). Moreover, similar to other migratory species, turtle doves are particularly at-risk due to global climate change, as they must adapt their breeding and migration timing to asynchronous changes in suitable conditions across broad, spatiotemporal scales (Fraser et al. 2019). ...
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Conservation of migratory birds requires knowledge of breeding and nonbreeding ranges and the connections between them. European turtle doves ( Streptopelia turtur ) are Palearctic-African long-distance migrants with wintering areas in the Sub-Saharan belt that are classed as vulnerable due to strong population declines. However, detailed non-breeding locations of individuals from different migratory flyways are unknown. To identify wintering regions of turtle doves, we measured stable isotopes of feathers grown on the wintering grounds and used a dual-isotope (hydrogen ( δ ² H f ) and carbon ( δ ¹³ C f )) probabilistic assignment to analyse origins of individuals migrating through the western and central/eastern flyways. The most probable wintering areas for turtle dove samples from both flyways were in the western and central Sub-Sahara. However, we found differences in δ ² H f and δ ¹³ C f values between turtle doves following different migratory routes (western vs central/eastern flyway). This result suggests a higher likelihood of origins in the central Sub-Sahara for central and eastern migrants, while turtle doves using the western flyway originated primarily in the western Sub-Sahara, highlighting the importance of both regions for the future conservation of turtle doves from European breeding populations. The establishment of migratory connectivity of populations requires sampling from birds from the European as well as Asian continent; however, we provide important results that can be used to test hypotheses regarding population declines resulting from factors experienced over the full annual cycle for some populations.
... Between years, changes in migratory timing for several species have also been documented [e.g. 12,14,17] and improvements in migratory performance have been evidenced as individuals age [18]. ...
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Background Birds that forage while covering distance during migration should adjust traveling behaviors as the availability of foraging habitat changes. Particularly, the behavior of those species that depend on bodies of water to find food yet manage to migrate over changing landscapes may be limited by the substantial variation in feeding opportunities along the route. Methods Using GPS tracking data, we studied how traveling behaviors vary with available foraging habitat during the long-distance migration of Caspian terns (Hydroprogne caspia), a bird with a specialized diet based on fish that needs bodies of water to forage. We measured individual variation in five traveling behaviors related to foraging along the route and used linear mixed effects models to test the following variables as predictors of traveling behaviors: proportion of overlap with water bodies, weather conditions, days at previous stopover and days of migration. Also, we tested if during traveling days flight height and speed varied with time of day and if birds were in areas with greater proportion of water bodies compared to what would be expected by chance from the landscape. Results We found variation in migratory traveling behaviors that was mainly related to the proportion of overlap with water bodies and experienced tailwinds. Suggesting a mixed migratory strategy with fly-and-foraging, Caspian terns reduced travel speed, flew fewer hours of the day, had lower flight heights and increased diurnal over nocturnal migratory flight hours as the proportion of overlap with water bodies increased. Birds had lower flight speeds and higher flight heights during the day, were in foraging habitats with greater proportions of water than expected by chance but avoided foraging detours. Instead, route tortuosity was associated with lower wind support and cloudier skies. Conclusions Our findings show how birds may adjust individual behavior as foraging habitat availability changes during migration and contribute to the growing knowledge on mixed migratory strategies of stopover use and fly-and-forage.
... [19,39]), songbirds (e.g. [40,41]) and seabirds (e.g. [42][43][44]) have reported repeatabilities as low as 0.03 for the duration of migration in Scopoli`s shearwater (Calonectris diomedea) [42], or as high as 0.99 for stopover sites (longitude and latitude) in oriental honey buzzards [35]. ...
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Background Understanding the evolution of migration requires knowledge of the patterns, sources, and consequences of variation in migratory behaviour, a need exacerbated by the fact that many migratory species show rapid population declines and require knowledge-based conservation measures. We therefore need detailed knowledge on the spatial and temporal distribution of individuals across their annual cycle, and quantify how the spatial and temporal components of migratory behaviour vary within and among individuals. Methods We tracked 138 migratory journeys undertaken by 64 adult common terns ( Sterna hirundo ) from a breeding colony in northwest Germany to identify the annual spatiotemporal distribution of these birds and to evaluate the individual repeatability of eleven traits describing their migratory behaviour. Results Birds left the breeding colony early September, then moved south along the East Atlantic Flyway. Wintering areas were reached mid-September and located at the west and south coasts of West Africa as well as the coasts of Namibia and South Africa. Birds left their wintering areas late March and reached the breeding colony mid-April. The timing, total duration and total distance of migration, as well as the location of individual wintering areas, were moderately to highly repeatable within individuals (repeatability indexes: 0.36–0.75, 0.65–0.66, 0.93–0.94, and 0.98–1.00, respectively), and repeatability estimates were not strongly affected by population-level inter-annual variation in migratory behaviour. Conclusions We found large between-individual variation in common tern annual spatiotemporal distribution and strong individual repeatability of several aspects of their migratory behaviour.
... Without NaCl supplementation, only one general stress protein yugI_2 was observed. It is reported that chaperones help to maintain homeostasis and appropriate protein folding under thermal and hyperosmotic stress by preventing the accumulation of misfolded protein units (Fraser et al. 2019). ...
The present work explores the adaptive behavior of well-known ureolytic bacterial strain namely Sporosarcina pasteurii for microbial-induced calcite precipitation (MICP) under saline environment. MICP activity was observed up to 10% NaCl supplementation and confirmed by several characterization techniques viz. scanning electron microscopy (SEM), X-ray diffraction (XRD), and thermogravimetric analysis (TGA). The urease activity did not show appreciable decline till 5% NaCl supplementation but beyond that drastic reduction was observed though MICP process continued. These results indicated a shift in the biological pathway for MICP process at higher salt concentrations. This was confirmed by estimation of ammonium ion concentration which was approximately 4 µg/ml with 10% NaCl supplementation as compared to 8.5 µg/ml at 0% NaCl supplementation. To provide an insight on the nature of protein expressed, combination of liquid chromatography-mass spectrometry (nano-LC-MS) approaches along with an exponentially modified protein abundance index (emPAI) methodology were explored. The findings suggest the presence of significant number of osmoregulatory proteins substantiating halophilic adaptation of S. pasteurii and bicarbonate transport mediated precipitation at higher levels of salinity. Further, co-precipitation of CaCl2 and MgSO4 was also confirmed with SEM and energy dispersive spectroscopy (EDS) mapping under higher saline environment thus substantiating the efficacy of the strain.
... Whereas early research posited that characteristics of individual movements remained fixed (Farner, 1950), contemporary studies find that many animals alter F I G U R E 1 Key examples of efforts to restore lost migrations across four major vertebrate groups the timing, direction, and duration of yearly migrations in response to environmental fluctuations. Such flexibility occurs across diverse vertebrates including fish (Meager et al., 2018), birds (Fraser et al., 2019), mammals (Xu et al., 2021), and herpetofauna (Jourdan-Pineau et al., 2012). Perhaps more surprisingly, all taxonomic groups include some individuals that go so far as to alter-nate between migratory and nonmigratory behavior (e.g., striped bass [Morone saxatilis], Secor et al., 2020;wood storks [Mycteria americana], Picardi et al., 2020; spotted salamanders [Abystoma talpoideum], Kinkead & Otis, 2007;and elk, Eggeman et al., 2016). ...
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Global declines in wildlife migrations have prompted new initiatives to conserve remaining migratory behaviors. However, many migrations have already been lost. Important attempts have been made to recover extirpated migrations, and our understanding of restoration remains narrowly confined to these particular species and landscapes. Here, we examine diverse restoration efforts through the unifying lens of behavioral ecology to draw broader inferences regarding the feasibility and effectiveness of restoring lost migrations. First, we synthesize recent research advances that illuminate key roles of exploration, learning, and adaptation in migratory behavior. Then, we review case studies to identify common themes of restoration success across four major vertebrate groups: fish, birds, mammals, and herpetofauna. We describe three broad strategies to effectively restore lost migrations: reestablishing migratory populations, recovering migratory habitats, and reviving migratory behavior itself. To guide conservation and research efforts, we link these strategies with specific management techniques , and we explore the biological mechanisms underpinning the success of each. Our work reveals a previously underappreciated potential for restoring lost migrations in terrestrial and freshwater vertebrates, and it provides guidance on whether and how conservation practitioners, researchers, and policymakers can work to restore the valuable migrations we have lost.
... Migrations between distant residency regions commonly occur in response to maintaining optimal thermal envelopes (Kessel et al., 2014;Payne et al., 2016Payne et al., , 2018, but are often associated with seeking out highly productive areas (i.e., high prey availability) (e.g., Jorgensen et al., 2010;Barnett et al., 2011) and areas for reproduction (Chapman et al., 2015). While population-level movement may appear predictable, certain species can show variability in migration timing among individuals and across years as a result of the dynamic environment they inhabit and their individual physiological needs (Brodersen et al., 2012;Fraser et al., 2019;Bauer et al., 2020). Defining animal residency and migration routes and variation in timing of movements consequently is essential to accurately delineate core space use for wildlife management. ...
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Understanding how mobile, marine predators use three-dimensional space over time is central to inform management and conservation actions. Combining tracking technologies can yield powerful datasets over multiple spatio-temporal scales to provide critical information for these purposes. For the white shark ( Carcharodon carcharias ), detailed movement and migration information over ontogeny, including inter- and intra-annual variation in timing of movement phases, is largely unknown in the western North Atlantic (WNA), a relatively understudied area for this species. To address this need, we tracked 48 large juvenile to adult white sharks between 2012 and 2020, using a combination of satellite-linked and acoustic telemetry. Overall, WNA white sharks showed repeatable and predictable patterns in horizontal movements, although there was variation in these movements related to sex and size. While most sharks undertook an annual migratory cycle with the majority of time spent over the continental shelf, some individuals, particularly adult females, made extensive forays into the open ocean as far east as beyond the Mid-Atlantic Ridge. Moreover, increased off-shelf use occurred with body size even though migration and residency phases were conserved. Summer residency areas included coastal Massachusetts and portions of Atlantic Canada, with individuals showing fidelity to specific regions over multiple years. An autumn/winter migration occurred with sharks moving rapidly south to overwintering residency areas in the southeastern United States Atlantic and Gulf of Mexico, where they remained until the following spring/summer. While broad residency and migration periods were consistent, migratory timing varied among years and among individuals within years. White sharks monitored with pop-up satellite-linked archival tags made extensive use of the water column (0–872 m) and experienced a broad range of temperatures (−0.9 – 30.5°C), with evidence for differential vertical use based on migration and residency phases. Overall, results show dynamic inter- and intra-annual three-dimensional patterns of movements conserved within discrete phases. These results demonstrate the value of using multiple tag types to track long-term movements of large mobile species. Our findings expand knowledge of the movements and migration of the WNA white shark population and comprise critically important information to inform sound management strategies for the species.
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Changes in phenology and distribution are being widely reported for many migratory species in response to shifting environmental conditions. Understanding these changes and the situations in which they occur can be aided by understanding consistent individual differences in phenology and distribution and the situations in which consistency varies in strength or detectability. Studies tracking the same individuals over consecutive years are increasingly reporting migratory timings to be a repeatable trait, suggesting that flexible individual responses to environmental conditions may contribute little to population‐level changes in phenology and distribution. However, how this varies across species and sexes, across the annual cycle and in relation to study (tracking method, study design) and/or ecosystem characteristics is not yet clear. Here, we take advantage of the growing number of publications in movement ecology to perform a phylogenetic multilevel meta‐analysis of repeatability estimates for avian migratory timings to investigate these questions. Of 2,433 reviewed studies, 54 contained suitable information for meta‐analysis, resulting in 177 effect sizes from 47 species. Individual repeatability of avian migratory timings averaged 0.414 (95% confidence interval: 0.3–0.5) across landbirds, waterbirds and seabirds, suggesting consistent individual differences in migratory timings is a common feature of migratory systems. Timing of departure from the non‐breeding grounds was more repeatable than timings of arrival at or departure from breeding grounds, suggesting that conditions encountered on migratory journeys and outcome of breeding attempts can influence individual variation. Population‐level shifts in phenology could arise through individual timings changing with environmental conditions and/or through shifts in the numbers of individuals with different timings. Our findings suggest that, in addition to identifying the conditions associated with individual variation in phenology, exploring the causes of between‐individual variation will be key in predicting future rates and directions of changes in migratory timings. We therefore encourage researchers to report the within‐ and between‐ individual variance components underpinning the reported repeatability estimates to aid interpretation of migration behaviour. In addition, the lack of studies in the tropics means that levels of repeatability in less strongly seasonal environments are not yet clear.
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Direct tracking methods in combination with remote sensing data allow examination of habitat use by birds during migration. Species that roost communally during migration, such as some swallows, form large aggregations that can attract both avian and terrestrial predators. However, the extent to which they might use patchy habitats that could reduce predation risk during migration is unknown. We tested the hypothesis that Purple Martins (Progne subis) use forest islands (patches of suitable forest habitat surrounded by unsuitable habitat) as roost sites during migration between breeding sites in North America and overwintering sites in South America. We used high‐precision (< 10 m), archival GPS units deployed and retrieved during the 2015 and 2016 breeding seasons, respectively, at 12 colonies located across eastern North America. We found that Purple Martins roosted in forest islands more often than expected based on availability during both spring and fall migration. Despite an apparent association with urban habitats by Purple Martins based on observational and radar data in North America during the fall, the roost locations we identified during spring and fall migration were not more closely associated with urban areas than random locations. The use of forest islands during both spring and fall migration suggest that Purple Martins may use these habitats to reduce predation risk during migration. Our results suggest that some species of birds may use similar habitats as stopover sites during migration and that patches of forest habitat may be important conservation targets for Purple Martins and other species. Identifying habitat use during migration represents an important advance in support of full annual‐cycle conservation of Purple Martins and other migratory species with declining populations. Rastreo directo preciso y sensores remotos revelan el uso de islas de bosques a través de las migraciones de primavera y otoño por parte de Progne subis Métodos de rastreo directo en combinación con información de sensores remotos permiten examinar el uso de hábitat por las aves durante la migración. Especies que usan dormideros comunales durante la migración, como es el caso de algunas golondrinas, forman congregaciones grandes que pueden atraer aves depredadoras así como depredadores terrestres. Sin embargo, el grado en el cual pueden usar los hábitats fragmentados que pueden reducir el riesgo de depredación durante la migración hasta el momento es desconocido. Pusimos a prueba la hipótesis que Progne subis usa islas de bosques (parches adecuados de hábitat boscoso rodeado por hábitat inadecuado) como dormideros durante la migración entre los sitios de reproducción en Norte América y los sitios de invierno en Sur América. Usamos unidades de GPS de alta precisión (< 10 m) los cuales fueron instalados y recuperados durante las temporadas reproductivas del 2015 y 2016, respectivamente, en 12 colonias ubicadas a través del este de Norte América. Encontramos que Progne subis duerme en islas de bosques con mayor frecuencia que lo esperado basado en la disponibilidad durante las migraciones de primavera y otoño. A pesar que existe una asociación aparente con hábitats urbanos por parte de Progne subis basado en datos de observaciones y de radar en Norte América durante el otoño, las localizaciones de los dormideros que identificamos durante las migraciones de primavera y otoño no estuvieron mas estrechamente asociadas con áreas urbanas que localidades seleccionadas por azar. El uso de las islas boscosas durante las migraciones de otoño y primavera, sugiere que Progne subis puede usar estos hábitats para reducir el riesgo de depredación durante la migración. Nuestros resultados sugieren que algunas especies de aves pueden usar hábitats similares como sitios de parada durante la migración y que los fragmentos de bosque pueden ser objetivos importantes para la conservación de Progne subis y otras especies. La identificación del uso del hábitat durante la migración representa un avance importante en el apoyo para la conservación del ciclo anual completo de Progne subis y otras especies migratorias con poblaciones en disminución.
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The timing of annual events is key for organisms that exploit seasonal resources, as deviations from optimal timing might result in considerable fitness costs. Under strong time selection, individuals likely have fewer suitable strategies available than when selection is more relaxed, hence both consistency and flexibility might be advantageous depending on the life history or annual cycle stage. For migrants using both the arctic and the tropics during their annual cycle, the faster warming at higher latitudes than elsewhere in the range may lead to mismatches with local environmental conditions. Additionally, while individuals might already be limited in responding to changes at each stage, the potential degree of a given response will likely also be limited by responses at previous stages of the annual cycle. Contrary to other migratory waders breeding in Iceland, Icelandic whimbrels Numenius phaeopus islandicus have not changed arrival dates during the past 30 years, suggesting high individual consistency in spring arrival timing and a potential limitation in responding to a changing environment. After repeatedly tracking 12 individual Icelandic whimbrels at least twice throughout their annual cycle between 2012 and 2018, we investigated individual consistency of spring arrival date and other annual stages and migration strategy, and explored differences between sexes and seasons. Individuals were more consistent on timing of spring than autumn migration, and the most consistent stage was departure from the wintering sites. Timing of laying was the stage that varied the most, and no overall significant difference between sexes was observed, except on spring stopover duration. While lower consistency in laying dates might allow individuals to track the advancement of spring, consistency at departure from the wintering sites, stopover duration, and arrival into Iceland might limit the degree of advancement. Transgenerational changes in the migratory behavior of other wader species allows population level responses to a changing phenology, but seems unlikely for Icelandic whimbrels, given the stable dates of spring arrival in this population. Under continuing advancement of spring onset, it is thus important to acquire information on the timing of spring arrival of recruits and on the ontogeny of migration to understand how migratory schedules are defined and might influence responses of long-distance migrants to environmental change.
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Few studies have been able to directly measure the seasonal survival rates of migratory species or determine how variable the timing of migration is within individuals and across populations over multiple years. As such, it remains unclear how likely migration is to affect the population dynamics of migratory species and how capable migrants may be of responding to changing environmental conditions within their lifetimes. To address these questions, we used three types of tracking devices to track individual black-tailed godwits from the nominate subspecies (Limosa limosa limosa) throughout their annual cycles for up to 5 consecutive years. We found that godwits exhibit considerable inter- and intra-individual variation in their migratory behavior across years. We also found that godwits had generally high survival rates during migration, although survival was reduced during northward flights across the Sahara Desert. These patterns differ from those observed in most other migratory species, suggesting that migration may only be truly dangerous when crossing geographic barriers that lack emergency stopover sites and that the levels of phenotypic flexibility exhibited by some populations may enable them to rapidly respond to changing environmental conditions.
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Variation in migratory behavior is the result of different individual strategies and fluctuations in individual performances. A first step toward understanding these differences in migratory behavior among individuals is, therefore, to assess the relative contributions of inter- and intra-individual differences to this variation. We did this using light-level geolocators deployed on the breeding grounds to follow continental black-tailed godwits (Limosa limosa limosa) throughout their south- and northward migrations over multiple years. Based on repeated tracks from 36 individuals, we found two general patterns in godwit migratory behavior: First, migratory timing in black-tailed godwits varies mostly because individual godwits migrate at different times of the year. Second, individuals also exhibit considerable variation in timing within their respective migratory windows. Although the absolute amount of inter-individual variation in timing decreased over the course of northward migration, individual godwits still arrived at their breeding grounds across a span of more than 5 weeks. These differences in migratory timing among individuals are larger than those currently observed in other migratory bird species and suggest that the selective forces that limit the variation in migratory timing in other species are relaxed or absent in godwits. Furthermore, we could not attribute these individual differences to the sex or wintering location of an individual. We suggest that different developmental trajectories enabled by developmental plasticity likely result in these generally consistent, life-long annual routines. To investigate this possibility and to gain an understanding of the different selection pressures that could be acting during migration and throughout a godwit's life, future studies should track juvenile godwits and other migratory birds from birth to adulthood while also manipulating their spatiotemporal environment during development.
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The influence of weather on the departure decisions and routes of migratory birds can now be further investigated with the use of direct tracking methods. We tested hypotheses for migration departure decisions and flight trajectories by determining the influence of wind speed and direction at the Yucatan peninsula in spring on departure date, migratory route, and longitude of arrival at the northern Gulf coast of a trans-hemispheric migratory songbird, purple martin (Progne subis). Birds were equipped with geolocators at their breeding colony and 36 were recaptured upon return after spring migration. While southerly tailwinds with low wind speeds prevailed at the Yucatan during the period of passage, we found that daily wind speed and direction were still important predictors of departure date. However, wind conditions at departure did not predict longitude of arrival at the US gulf coast after crossing the gulf. Birds appeared to favour the shortest distance across the Gulf of Mexico, aided by consistent tailwinds, but may have corrected for wind drift so as to land at a longitude near 88°, reflecting the shortest distance across from the Yucatan staging areas. Considering their use prior to departure, high quality roost sites at the Yucatan peninsula would be important conservation targets for this declining aerial insectivore.
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Following ongoing technological advances, an increasing amount of full-year tracking data on individual migratory movements is becoming available. This opens up the opportunity to study how migration develops within individuals in consecutive years and the extent to which the migratory program is constrained. Such knowledge is essential for understanding the degree of individual flexibility during the annual cycle, which may help identifying potential bottlenecks, where the range of individual decisions is restricted. In this study, we investigate repeatability in time of a long-distance migratory songbird, the red-backed shrike Lanius collurio, tracked across consecutive years (n = 7). Furthermore, we explore the population variability and dependencies between consecutive events of departure and arrival throughout the annual cycle in this species (n = 15). We find that individuals show high repeatability in timing of departure from their two main non-breeding areas in sub-Saharan Africa. In contrast, low repeatability is found in timing of arrivals to stationary sites throughout the annual cycle. Population variation in timing of departure and arrival was similar across all events, ranging from 30 to 41 days, and was highly dependent on timing of preceding events. We conclude that timing of departures is the key event potentially controlled by the individual innate migration program, while arrivals are more flexible, likely dependent on the environmental conditions experienced en route in red-backed shrikes. Still, apparent flexibility in the individual schedule may be hampered by overall constraints of the annual cycle. Significance statement The annual migration schedule of migratory animals is controlled by a combination of endogenous and exogenous factors. Understanding the temporal dynamics within and between individuals across the annual cycle is important to assess to which extent the migratory schedule is constrained in time. By using full-annual cycle tracking data of individual red-backed shrikes tracked across consecutive years, we find that individuals are highly consistent in their decision to depart from their main non-breeding areas in sub-Saharan Africa, whereas arrivals are less consistent throughout the annual cycle. Overall, the migration schedule is highly constrained across the annual cycle, with each arrival and departure event being dependent on the previous event. Our results suggest that departure decision is underlying endogenous control and that little flexibility is available throughout this complex migration system.
Climate change has been shown to shift the seasonal timing (i.e. phenology) and distribution of species. The phenological effects of climate change on living organisms have often been tested using first occurrence dates, which may be uninformative and biased. More rarely investigated is how different phases of a phenological sequence (e.g. beginning, central tendency and end) or its duration have changed over time. This type of analysis requires continuous observation throughout the phenological event over multiple years, and such data sets are rare. In this study we examined the impact of temperature on long-term change of passage timing and duration of the spring migration period in birds, and which species' traits explain species-specific variation. Data used covered 195 species from 21 European and Canadian bird observatories from which systematic daily sampling protocols were available. Migration dates were negatively associated with early spring temperature and timings had in general advanced in 57 years. Short-distance migrants advanced the beginning of their migration more than long-distance migrants when corrected for phylogenic relatedness, but such a difference was not found in other phases of migration. The advancement of migration has generally been greater for the beginning and median phases of migration relative to the end, leading to extended spring migration seasons. Duration of the migration season increased with increasing temperature. Phenological changes have also been less noticeable in Canada even when corrected for rate of change in temperature. To visualize long-term changes in phenology, we constructed the first multi-species spring migration phenology indicator to describe general changes in median migration dates in the northern hemisphere. The indicator showed an average advancement of one week during five decades across the continents (period 1959-2015). The indicator is easy to update with new data and we therefore encourage future research to investigate whether the trend towards longer periods of occurrence or emergence in spring is also evident in other migratory populations. Such phenological changes may influence detectability in monitoring schemes, and may have broader implications on population and community dynamics.