Neotropical Migrants Exhibit Variable Body-Size Changes
Over Time and Space
Michael D. Collins, George E. Relyea, Erica C. Blustein, and Steven M. Badami
Northeastern Naturalist, Volume 24, Issue 1 (2017): 82–96
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M.D. Collins, G.E. Relyea, E.C. Blustein, and S.M. Badami
22001177 NORTHEASTERN NATURALIST V2o4l.( 12)4:,8 N2–o9. 61
Neotropical Migrants Exhibit Variable Body-Size Changes
Over Time and Space
Michael D. Collins1,*, George E. Relyea2, Erica C. Blustein1, and
Steven M. Badami1
Abstract - Recent changes in the Earth’s climate have been linked to changes in phenology,
geographic distributions, and morphology of species, and warming temperatures associated
with climate change have been predicted to result in decreases in avian body sizes. We
examined changes in wing length and fat-free mass of 34,844 fall migrants from 31 neotropical
migratory species captured at Patuxent Wildlife Research Center in Maryland between
1980 and 2012. Body size changes varied across species, but wing length and fat-free mass
increased significantly over time in the pooled sample of all species. Magnitudes of change
were small and similar to other studies, with mean wing length increasing 0.55% and mean
fat-free mass increasing 1.30% across all species. General morphological changes at our
site differed from those at a banding station located 235 km away. Across species, changes
in wing length were weakly correlated between stations, and changes in fat-free mass were
uncorrelated. Populations of some species showed opposite morphological changes, demonstrating
that morphological changes can vary regionally. Over short time scales, factors
other than climate might drive observed changes in body size of neotropical migrants, and
alternative hypotheses for body size changes should be considered.
Introduction
Abundant evidence has documented rapid changes in the Earth’s climate (Field
et al. 2014, Hansen et al. 2006, Jones et al. 2001, Karl and Trenberth 2003), and
climate change has been linked to observed changes in phenology (Crick 2004, Macmynowski
et al. 2007, Miller-Rushing et al. 2008, Torti and Dunn 2005, Végvári et
al. 2010), morphology (Gardner et al. 2011, Goodman et al. 2012, Van Buskirk et al.
2010, Yom-Tov et al. 2006), geographical distributions (Graves 1991, Parmesan and
Yohe 2003, Thomas 2010, Tingley et al. 2009) and population size (Jiguet et al. 2010,
Ozgul et al. 2010). Body sizes of birds and other endotherms have been predicted
to decrease with rising temperatures based on Bergmann’s (1847) rule (Daufresne
et al. 2009, Kirchman and Schneider 2014, Van Buskirk et al. 2010). Bergmann’s
rule states that body sizes of endotherms increase with latitude. Because latitude
is negatively correlated with temperature, climate change is predicted to result in
smaller body sizes. This prediction is based largely on the heat-conservation hypothesis,
which argues that larger endotherms would have an advantage in colder
environments because a larger body would reduce the loss of heat energy (Bergmann
1847, Mayr 1956). However, Bergmann’s rule is also seen in some poikilotherm
1Department of Biology, Rhodes College, 2000 North Parkway, Memphis, TN 38112.
2School of Public Health, University of Memphis, Memphis, TN 38152. *Corresponding
author - collinsm@rhodes.edu.
Manuscript Editor: Jeremy Kirchman
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83
vertebrates (Caruso et al. 2014, Olalla-Tárraga et al. 2006) and in some invertebrates
(Arnett and Gotelli 1999, Atkinson 1994, Cushman et al. 1993, Ray 2005). Most bird
species adhere to Bergmann’s rule (Ashton 2002, Blackburn and Gaston 1996), but
how widespread the pattern is and its underlying cause remain unresolved (Blackburn
et al. 1999, Meiri 2011, Olson et al. 2009, Watt et al. 2010).
Based on Bergmann’s rule and the mechanistic heat-conservation hypothesis,
Daufresne et al. (2009) hypothesized that decreasing body sizes would be a third
universal ecological response to global warming, with the first 2 responses being
geographic range shifts toward higher latitudes and elevations and changes in
phenology (seasonality). Over time scales of several millennia, clear patterns exist
between temperature and body sizes. Body sizes of mammals, for example, oscillate,
becoming smaller during warmer interglacials and increasing during colder
periods (Davis 1981). This pattern, however, is not entirely clear over shorter time
scales, and studies on the effect of recent climate change on body sizes of birds have
produced conflicting results. In a study of migrating birds in western Pennsylvania,
Van Buskirk et al. (2010) found that changes in wing length and fat-free mass (mass
when fat score is zero) differed across species and have steadily decreased since
1961 and concluded that these changes were consistent with a response to warmer
climates. In contrast, Salewski et al. (2010) found that morphological changes of 12
European passerines did not show consistent patterns. Salewski et al. (2014) found
variable body size trends in 11 bird species in Germany and showed that observed
changes were not related to temperature. Teplitsky and Millien (2014) reviewed the
literature on body size and climate change and found only mixed evidence that body
sizes have decreased, with 60% of avian cases and 7% of mammalian cases showing
decreases. Some studies have found increases in avian body sizes (Goodman et
al. 2012). Thus, observed changes in avian body size in response to recent climate
change have been variable and inconsistent (Gardner et al. 2011). Because many
factors can influence body size (Calder 1984, Peters 1983) and the relationship
between climate and body size can be complicated (Chown 2012; Huey et al. 2012;
Ozgul et al. 2009, 2010), heterogeneous responses of avian body sizes to climate
change should not be surprising (Millien et al. 2006).
Our study aims (1) to examine how body sizes of neotropical migrants have
changed in Laurel, MD, between 1980 and 2012; (2) to determine which particular
species show significant changes in body size; and (3) to compare our findings to
those from another banding station in the eastern United States to examine whether
changes in body size show variation at regional spatial scales.
Field-Site Description
Our banding station was located at the USGS Patuxent Wildlife Research Center
(PWRC) in Laurel, MD (elevation 50 m; 39.05°N, 76.81°W). Habitat near the banding
site included a transmission line in 2–3-m–tall dense shrubbery dominated by
native shrub species with scattered grassy areas. The site has seen very little vegetation
change since 1980 (D. Bystrak, USGS PWRC, Laurel, MD, pers. comm.)
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2017 Vol. 24, No. 1
Methods
Between 1980 and 2012 (excluding 2004–2006), we captured birds in 12-m,
30-mm–gauge mist nets in the fall (August through November). We generally deployed
26 nets 25 minutes before sunrise for 3–4 hours on the days we banded. We
identified individuals to species and aged and sexed birds using skulling (examination
of the extent of bone pneumatization in the skull) and molt limits (see Pyle
1997). We measured wing length (as unflattened wing chord ± 1 mm), mass (± 0.1
g), and fat score (on a scale of 0–4) and used wing length and fat-free mass as our
measures of body size. Wing length is the most common measure of avian body size
(Ashton 2002), and fat-free mass, the estimated mass when fat is zero, is obtained
with covariance analysis by including fat score as a covariate. Over the course of
the study, 10 banders measured the vast majority of birds, and 2 individuals measured
about 2/3 of all captured birds.
Statistical analyses
We captured 87,832 individuals of 121 species. Here, we examine only neotropical
migrants and exclude species that winter primarily in North America (residents
and short-distance migrants). We excluded from our analyses species with fewer
than 300 captured individuals, repeated captures of an individual within a season,
and individuals of unknown age; 34,844 individuals of 31 species met our criteria
for inclusion.
We used generalized linear mixed models (GLMMs; West et al. 2006) with the
“GLIMMIX” procedure in SAS 9.3 (SAS Institute, Inc. 2011) and examined the
main effect of year to determine overall or universal (sensu Daufresne et al. 2009)
trends in body size. We analyzed wing length and body mass separately, and our
response variables were ln(wing length) and ln(mass) of individual birds. Fixed
effects were year and capture date (Julian day) as continuous variables and age
and sex as categorical variables. We included species as a random effect. To test
whether species or ages differed in their slopes, we compared models with and
without the heterogeneity in slopes (i.e., with and without species on the RANDOM
statement) with likelihood ratio tests (West et al. 2006). For analyses of mass, we
also included fat score and time of day as fixed continuous vari ables.
For each of the 31 species included in the GLMM, we used the “MIXED” procedure
in SAS to estimate the species’ change in wing length and change in mass over
years. We included age, sex, year, and Julian day as covariates and the age*year interaction.
For analyses of mass, we also included time of day and fat score.
Results
Body-size changes
For all species combined, wing length increased between 1980 and 2012 (F1, 34060
= 46.06, P < 0.001, Table 1). While highly significant, the magnitude of the change
in wing length (after back-transforming) was small at 0.55% ± 0.08% (mean ± SE)
over the course of the study. Change in wing length differed significantly across
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species (χ2 = 119664.2, df = 1, P < 0.001) and ranged from -2.03% to +2.00%.
Wing length increased significantly in 9 species (Geothlypis trichas [Common
Yellowthroat], Mniotilta varia [Black-and-white Warbler], Seiurus aurocapilla
[Ovenbird], Setophaga caerulescens [Black-throated Blue Warbler], Catharus
fuscescens [Veery], Catharus minimus [Gray-cheeked Thrush], Catharus ustulatus
[Swanson’s Thrush], and Vireo olivaceus [Red-eyed Vireo]) and decreased
significantly in 3 (Setophaga discolor [Prairie Warbler], Empidonax flaviventris
[Yellow-bellied Flycatcher], and Empidonax minimus [Least Flycatcher]) (Table 2).
Change in wing length did not differ between Hatch Year (HY) and After Hatch
Year (AHY) age classes (χ2 = 2.0, df = 1, P = 0.26).
For all species combined, fat-free mass increased 1.30% ± 0.20% between 1980
and 2012 (F1, 32369 = 42.37, P < 0.001, Table 1). Species varied significantly in
change in fat-free mass over time (χ2 = 116447.94, df = 1, P < 0.001), ranging from
-2.87% to +3.69% between 1980 and 2012. Fat-free mass increased significantly
in 6 species (Common Yellowthroat, Black-and-white Warbler, Ovenbird, Prairie
Warbler, Veery, and Red-eyed Vireo) and decreased in only Setophaga virens
(Black-throated Green Warbler) (Table 2). Across species, change in wing length
and change in fat-free body mass were positively correlated (r = 0.49, n = 31, P =
0.005; Fig. 1).
Spatial variation in body-size changes
For all species combined, change in wing length over time at our site in Maryland
was weakly correlated with change in wing length from 1961 to 2006 at a
Table 1. Summaries of generalized linear mixed models (GLMMs) to examine morphological changes
(log-transformed wing length and log-transformed fat-free mass) for 31 neotropical migratory species
from 1980-2012. Estimates are coefficients. Negative coefficients indicate declining size and positive
coefficients indicate increasing size. SE is standard error.
Source of variation Estimate SE F value P
Wing length
Year 0.000171 0.000025 46.06 less than 0.001
Julian day 0.000136 0.000011 165.93 less than 0.001
Age AHY 0.022810 0.000371 3777.15 less than 0.001
HY 0.000000
Sex Female -0.021540 0.000519 7984.38 less than 0.001
Male 0.034030 0.000518
Unknown 0.000000
Fat-free mass
Year 0.000405 0.000062 42.37 less than 0.001
Time 0.000061 3.50 E-6 300.95 less than 0.001
Julian day 0.000340 0.000026 177.63 less than 0.001
Age AHY 0.018890 0.000852 491.52 less than 0.001
HY 0.000000
Sex Female -0.017030 0.001193 1050.47 less than 0.001
Male 0.029150 0.001189
Unknown 0.000000
Fat 4537.02 less than 0.001
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Table 2. Changes in log-transformed wing length and log-transformed fat-free mass (change x10000/year). Sample size is given by n. Estimates are coefficients;
negative coefficients indicate declining size and positive coefficients indicate increasing size. SE is standard error. * indicates P < 0.05, † indicates
P < 0.01, and ‡ indicates P < 0.001.
Species Wing Fat-free mass
Family/ common name code Scientific name n Est. SE Est. SE
Cardinalidae
Indigo Bunting INBU Passerina cyanea (L.) 407 2.76 1.69 -2.90 4.07
Scarlet Tanager SCTA Piranga olivacea (Gmelin) 313 1.33 2.22 2.53 4.48
Parulidae
Canada Warbler CAWA Cardellina canadensis (L.) 860 -1.40 1.01 4.70 2.58
Common Yellowthroat COYE Geothlypis trichas (L.) 4443 4.90‡ 0.58 8.74‡ 1.28
Black-and-white Warbler BAWW Mniotilta varia (L.) 939 2.95† 0.94 5.91* 2.41
Connecticut Warbler CONW Oporornis agilis (Wilson) 404 2.04 1.97 6.76 4.50
Tennessee Warbler TEWA Oreothlypis peregrina (Wilson) 1427 -0.60 0.68 0.88 1.64
Nashville Warbler NAWA Oreothlypis ruficapilla (Wilson) 347 3.60* 1.72 2.57 4.31
Ovenbird OVEN Seiurus aurocapilla (L.) 1962 2.49‡ 0.75 4.72† 1.71
Northern Parula NOPA Setophaga americana (L.) 399 -2.20 1.61 -5.20 3.58
Black-throated Blue Warbler BTBW Setophaga caerulescens (Gmelin) 1525 2.15† 0.72 3.43 1.87
Bay-breasted Warbler BBWA Setophaga castanea (Wilson) 573 -0.80 1.66 1.22 3.26
Hooded Warbler HOWA Setophaga citrina (Boddaert) 539 3.15 1.61 3.86 3.44
Prairie Warbler PRAW Setophaga discolor (Vieillot) 361 -5.70* 2.30 11.31* 4.81
Magnolia Warbler MAWA Setophaga magnolia (Wilson) 4274 -0.20 0.40 0.90 0.97
Chestnut-sided Warbler CSWA Setophaga pensylvanica (L.) 738 -1.00 1.18 2.70 2.79
American Redstart AMRE Setophaga ruticilla (L.) 1679 -1.00 0.78 -3.70 1.99
Blackpoll Warbler BLPW Setophaga striata (Forster) 418 -1.10 1.71 3.34 3.93
Black-throated Green Warbler BTNW Setophaga virens (Gmelin) 805 0.80 0.93 -4.70* 2.29
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Table 2, continued.
Species Wing Fat-free mass
Family/ common name code Scientific name n Est. SE Est. SE
Polioptilidae
Blue-gray Gnatcatcher BGGN Polioptila caerulea (L.) 314 -3.80 3.01 -6.30 5.51
Turdidae
Veery VEER Catharus fuscescens (Stephens) 752 4.14† 1.41 9.90‡ 2.90
Gray-cheeked Thrush GCTH Catharus minimus (Lafresnaye) 533 6.20‡ 1.80 7.21 3.90
Swainson's Thrush SWTH Catharus ustulatus (Nuttall) 2151 2.67‡ 0.64 -0.30 1.61
Wood Thrush WOTH Hylocichla mustelina (Gmelin) 455 2.64 1.83 2.95 3.97
Tyrannidae
Eastern Wood-Pewee EAWP Contopus virens (L.) 294 -1.60 2.63 0.42 6.12
Yellow-bellied Flycatcher YBFL Empidonax flaviventris (Baird and Baird) 400 -5.00* 2.09 1.89 5.23
Least Flycatcher LEFL Empidonax minimus (Baird and Baird) 310 -6.40† 2.21 -9.10 6.38
Traill’s Flycatcher TRFL Empidonax sp. 695 -2.20 1.49 -1.10 3.20
Acadian Flycatcher ACFL Empidonax virescens (Vieillot) 407 -1.90 2.42 -4.90 4.41
Vireonidae
White-eyed Vireo WEVI Vireo griseus (Boddaert) 504 1.71 1.51 3.93 2.96
Red-eyed Vireo REVI Vireo olivaceus (L.) 5616 2.91‡ 0.34 8.61‡ 1.13
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station in western Pennsylvania, 235 km away (r = 0.37, n = 30, P = 0.043; Fig. 2).
Change in fat-free mass was not correlated between banding stations (r = 0.27, n =
30, P = 0.16; Fig. 3).
Discussion
We documented changes in wing length and fat-free mass across 31 neotropical
migratory bird species between 1980 and 2012 at the USGS Patuxent Wildlife Research
Center (PWRC) in Laurel, MD. While highly statistically significant, overall
general changes in body size were small, amounting to a 0.55% mean increase in
wing length and a 1.30% mean increase in fat-free mass over the course of the
study. Changes in both measures of body size varied between species, and species-
Figure 1. Annual change (x10000) of ln(fat-free mass) and ln(wing length) between 1980
and 2012 for 31 neotropical migratory bird species (r = 0.49, n = 31, P = 0.005). Estimates
are for the separate models for each species. Species codes are defined in Table 2.
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specific changes sometimes swamped the general trend. For example, despite a
general increase in wing length and fat-free mass across species, 3 species showed
significant decreases in wing length, and 1 exhibited a significant decline in fat-free
mass. Wing length and fat-free mass increased significantly in 9 and 6 species, respectively.
Species in the same family sometimes showed similar changes in body
size (Table 2). Two of the 3 species with significant decreases in wing length were
flycatchers (Tyrannidae), and the other 3 species of flycatcher showed decreasing
but nonsignificant changes in wing length. In thrushes (Turdidae), wing lengths increased
significantly in 3 of 4 species, and the fourth species showed a positive but
nonsignificant trend. When examined individually, many migratory species did not
exhibit significant changes in body size: 19 species showed no significant change
Figure 2. Across species, annual change (x10000) in ln(wing length) in our study from 1980
to 2012 and a study in western Pennsylvania from 1961 to 2006 are weakly correlated (r =
0.37, n = 30, P = 0.043). We excluded Northern Parula because this species was not caught
in the fall in Pennsylvania. Species codes are defined in Table 2.
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in wing length, and 24 species showed no significant change in fat-free mass. Most
of the individuals captured in our study likely belonged to northerly populations
and were caught during migration. Consequently, our samples likely consist of
individuals from different breeding populations. It is possible that changes in body
size have occurred at finer spatial scales, but that opposing patterns result in no net
effect at broader scales. At the spatial scale examined here, neotropical migrants
have largely shown individualistic changes in body size at PWRC between 1980
and 2012, and we find no evidence for widespread declines in body size as a universal
response to climate change as posited by Daufresne et al. (2009).
Our findings that wing length and fat-free mass have generally increased contrast
with those of Van Buskirk et al. (2010). Van Buskirk et al. (2010) found widespread
declines in wing length and fat-free mass of passerines in western Pennsylvania
Figure 3. Across species, annual change (x10000) in ln(fat-free mass) is not correlated between
banding stations (r = 0.27, n = 30, P = 0.16). Gray-cheeked Thrush (6.2, 6.51) is not
shown. Species codes are defined in Table 2.
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between 1961 and 2006 and noted that these changes were consistent with a response
to a warming climate. In contrast, Goodman et al. (2012) documented
increases in wing length and in fat-free mass between 1983 and 2009 in California,
and Collins et al. (2017) found increases in wing length but not in fat-free mass for
20 resident and short-distant migrant passerine species at PWRC. Goodman et al.
(2012) hypothesized that increases in body size reflected increases in climatic variability
or primary productivity. Bumpus (1899) proposed that more severe weather
at higher latitudes might drive Bergmann’s rule by selecting for larger individuals
with increased fasting endurance. This starvation resistance hypothesis has been
supported by studies that have demonstrated that severe weather events can favor
larger body sizes (Ashton 2002, Brown and Brown 1999, Jaramillo and Rising
1995). Climate change is predicted to increase the frequency and severity of some
extreme weather events, such as heat waves and the number of heavy precipitation
events, (Easterling et al. 2000, Meehl and Tebaldi 2004, Min et al. 2011, Stouffer
and Wetherald 2007) while decreasing other events, such as cold-temperature extremes.
Consequently, this hypothesis predicts that climate change may result in
either larger or smaller body sizes.
Our study, Van Buskirk et al. (2010), Goodman et al. (2012), and Collins et al.
(2017) all found that changes in body size differed between species, and magnitudes
of species change were similarly small in all 3 studies: -0.09% to +0.11% per
year in our study, -0.08 to +0.02% per year in Van Buskirk et al. (2010), -0.03 to
+0.08% per year in Goodman et al. (2012), and -0.13 to +0.16% per year in Collins
et al. (2017). Across species, change in wing length was correlated with change in
fat-free mass at our site (Fig. 1). One species, Prairie Warbler, showed a significant
decrease in wing length but a significant increase in fat-free mass. Our findings
agree with those of Salewski et al. (2014) and demonstrate that observed body size
changes depend on the species and morphological trait examined.
That we documented general increases in body size while Van Buskirk et al.
(2010) found widespread declines is particularly surprising given the proximity of
study sites and the similarity of the 2 studies. Only 235 km separate our banding
station in Maryland from theirs in western Pennsylvania. Both studies used wing
length and fat-free mass as measures of body size and examined a similar set of
species over comparable times and durations (32 years vs. 46). In both studies,
large sample sizes allowed inclusion of covariates such as age, sex, and date of
capture into statistical models. Of the 31 species examined in our study, Van Buskirk
et al. (2010) analyzed fall banding records for all species except Setophaga
americana (Northern Parula). Both studies found significant change over time
for all species combined, but when comparing the changes in individual species,
the change in wing length in our study was only weakly correlated with change
in wing length in western Pennsylvania (Fig. 2). In addition, 6 species (Common
Yellowthroat, Catharus minimus [Gray-cheeked Thrush], Oreothlypis ruficapilla
[Nashville Warbler], Ovenbird, Red-eyed Vireo, and Catharus ustulatus
[Swainson’s Thrush]) that showed significant decreases in wing length in western
Pennsylvania increased significantly in our study. Similarly, changes in fat-free
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mass were not correlated between banding stations (Fig. 3), and in 4 species
(Common Yellowthroat, Ovenbird, Red-eyed Vireo, and Veery), fat-free mass
increased in our study but decreased in western Pennsylvania. Together, these
findings demonstrate that changes in body size over time are species-specific, that
these relative and absolute changes vary across space, and that populations of a
species can exhibit opposite changes in body size over regional spatial scales.
Several explanations can account for differing body sizes changes in neotropical
migrants between studies. Most of these neotropical migratory species have
large breeding populations north of our banding stations. In our study, even for
species that breed locally, most captured individuals likely belonged to northerly
populations and were caught during migration. It is possible that the 2 banding
stations caught birds from different breeding areas and that climate change differed
between areas. It is also plausible that morphological responses to climate
change differed between breeding populations. Context dependence could cause
the influence of warming temperatures on body sizes to differ between breeding
populations (Yom-Tov and Geffen 2011) and lead to variable or contrasting trends
in body size over time. Interactions between climatic variables are one possible
mechanism of context dependence. For example, summer temperatures might
affect fat-free mass indirectly, through its influence on productivity and food
availability (Yom-Tov and Geffen 2011), and warming temperatures might increase
productivity and avian body sizes in wetter areas but cause water stress and
decrease productivity and body sizes in drier locations. Identification of breeding
areas would permit analyses of climate data and examination of morphological
responses to climate change to test these hypotheses for differing trends in body
sizes of neotropical migants. One could use stable isotopes to identify breeding
areas (Rubenstein and Hobson 2004), but this approach would require feather or
tissue samples, which we did not collect.
Another hypothesis for the differing changes in body size of neotropical migrants
between the 2 studies is that observed changes in body size were not driven by climate
change in at least 1 site. Patterns between climate and body size over long
periods of millenia are unambiguous: body sizes become smaller during periods
with warmer climates (Davis 1981, Kurtén 1968). But, this pattern might not hold
over shorter time scales. Salewski et al. (2014) found that morphological changes
in 11 bird species in Germany over the last century were not related to temperature,
and Collins et al. (2017) found that species-specific body-size changes in resident
birds and short-distance migrants at PWRC were not driven by mean summer or
mean winter temperatures. Although magnitudes of body-size change that we recorded
in neotropical migrants were comparable to or greater than those reported
from other studies (Gardner et al. 2014, Goodman et al. 2012, Van Buskirk et al.
2010), observed changes were small, ranging between -0.09% and 0.11% per year.
Many physiological and ecological processes are influenced by body size (Calder
1984, Peters 1983), and many selective pressures can contribute to changes in body
size. For example, warming temperatures associated with climate change have
decreased migration distances (Visser et al. 2009), and shorter migration distances
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might select for shorter wing lengths. Moreover, a change in one morphological
trait can influence other morphological traits. Decreased mass, for example, might
select for reduced wing length due to allometric responses and selective pressures
associated with aerodynamics (Yom-Tov et al. 2006). Changes in body size reflect
the combined selective forces of these factors, so over shorter periods with only
moderate increases in temperature, other forces might drive changes in body size.
If so, then climate would drive changes in body size only when climate change is
more extreme or prolonged.
Our work adds to a growing literature on the effect of recent climate change on
avian body sizes (Goodman et al. 2012; McCoy 2012; Salewski et al. 2010, 2014;
Van Buskirk et al. 2010) and demonstrates that morphological changes in neotropical
migratory birds were highly variable since 1980. We find no evidence for widespread
declines in wing length or fat-free mass. Species exhibited inconsistent and variable
changes in body size across space, and some species displayed opposite morphological
changes between banding stations in Maryland and western Pennsylvania. While
variable and conflicting body size trends might arise between sites due to regional
differences in climate change or to differing effects of climate change on avian body
sizes through context dependence, factors other than climate might drive observed
changes in body size of neotropical migrants, and alternative hypotheses for bodysize
changes over short time scales should be considered.
Acknowledgments
We thank all station staff and assistants, particularly Danny Bystrak and Deanna Dawson,
who ran the PWRC banding station for many years. V. Ellis and H. Horne commented
on the manuscript. Rhodes College provided financial support, and USGS provided logistical
support. We thank Jeremy Kirchman and 4 anonymous reviewers for helpful comments
on the manuscript.
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