Assessing Migration of Ruby-Throated Hummingbirds (Archilochus colubris) at Broad Spatial and Temporal Scales

Abstract

Phenological patterns in birds appear to be temperature-dependent in part, and global temperatures are undergoing change. Many studies of bird phenology are conducted at broad temporal but local spatial scales, making it difficult to assess how temperature affects bird migration across landscapes. Recently, networks of "citizen science" volunteers have emerged whose collective efforts may improve phenology studies as biases associated with such efforts are recognized and addressed. We compared mean Ruby-throated Hummingbird (Archilochus colubris) first arrival dates from Journey North (2001–2010) with data from the North American Bird Phenology Program (1880–1969). Ruby-throated Hummingbirds arrived earlier in the more recent period throughout the eastern United States; these advances, however, varied by latitude from 11.4 to 18.2 days, with less pronounced changes above 41°N. Warmer winter and spring temperatures in North American breeding grounds were correlated with earlier arrivals at lower latitudes in our recent period. Surprisingly, Ruby-throated Hummingbirds arrived later at high latitudes (42–43°N) during warmer winters and later at both mid-and high latitudes (38–39, 41–44°N) during warmer springs, which perhaps indicates extended migratory stopovers below 40°N during these years. Overall, weather variables predicted arrival dates better in the recent than in the historical period. Our results document spatial variability in how warming temperatures affect hummingbird arrivals and add credence to the hypothesis that spatial differences in arrival patterns at high versus low latitudes could exacerbate asynchrony between some birds and their food resources and modify associated ecosystem services such as pollination and insect pest suppression.

Full text

ASSESSING MIGRATION OF RUBY-THROATED HUMMINGBIRDS
(ARCHILOCHUS COLUBRIS) AT BROAD SPATIAL AND TEMPORAL SCALES
Jason R. CouRteR,1,5 Ron J. Johnson,2 William C. BRidges,3 and Kenneth g. huBBaRd4
1Department of Earth and Environmental Sciences, Taylor University, Upland , Indiana 46989, USA;
2School of Agricultural, Forest, and Environmental Sciences, Clemson University, Clem son, South Carolina 28634, USA;
3Department of Mathematical Sciences, Clemson University, Clemson, South Carolina 28634, USA; and
4High Plains Regional Climate Center, University of Nebraska-Lincoln, Lincoln, Nebraska 68583, USA
A.—Phenological patterns in birds appear to be temperature-dependent in part, and global temperatures are undergoing
change. Many studies of bird phenology are conducted at broad temporal but local spatial scales, making it difficult to assess how
temperature affects bird migration across landscapes. Recently, networks of “citizen science” volunteers have emerged whose collective
efforts may improve phenology studies as biases associated with such efforts are recognized and addressed. We compared mean Ruby-
throated Hummingbird (Archilochus colubris) first arrival dates from Journey North (–) with data from the North American
Bird Phenology Program (–). Ruby-throated Hummingbirds arrived earlier in the more recent period throughout the eastern
United States; these advances, however, varied by latitude from . to . days, with less pronounced changes above °N. Warmer
winter and spring temperatures in North American breeding grounds were correlated with earlier arrivals at lower latitudes in our
recent period. Surprisingly, Ruby-throated Hummingbirds arrived later at high latitudes (–°N) during warmer winters and later at
both mid- and high latitudes (–, –°N) during warmer springs, which perhaps indicates extended migratory stopovers below
°N during these years. Overall, weather variables predicted arrival dates better in the recent than in the historical period. Our results
document spatial variability in how warming temperatures affect hummingbird arrivals and add credence to the hypothesis that spatial
differences in arrival patterns at high versus low latitudes could exacerbate asynch rony between some birds and their food resources and
modif y associated ecos ystem services such a s pollination and in sect pest suppression. Received  March , acc epted  October .
Key words: Archilochus colubris, arrival, bird phenology, citizen science, climate change, ecosystem services, Ruby-throated
Hummingbird, spatial trend.
Evaluación de la Migración de Archilochus colubris a Escalas Amplias de Tiempo y Espacio
R.—Los patrones fenológicos de las aves parecen ser en parte dependientes de la temperatura y las temperaturas globales
están cambiando. Muchos estudios de fenología de aves son hechos a lo largo de escalas temporales amplias pero a escalas espaciales
locales, lo que hace difícil evaluar cómo los cambios de temperatura afectan la migración de las aves a través de diferentes paisajes.
Recientemente, han apa recido redes de “científicos ciudadanos” volunta rios, cuyos esfuerzos colectivos podrían mejorar los estudios de
fenología en la medida en que los sesgos asociados con dichos esfuerzos sean reconocidos y abordados. Comparamos las fechas medias
de llegada de Archilochus colubris de Journey North (-) con datos del North American Bird Phenology Program (-).
El arribo de A. colubris fue más temprano en periodos m) duanitudes medias y altas (-, -tro periodo reciente. . diada con los
parentales se correlacionaron con los ial en eás recientes a través del este de Estados Unidos; sin embargo, estos avances variaron con la
latitud entre . y . días, con cambios menos pronunciados por encima de °N. Temperaturas mayores en invierno y primavera en
las áreas de reproducción en Norte América estuvieron correlacionadas con llegadas más tempranas en latitudes menores en nuestro
periodo reciente. Sorpresivamente, A. colubris llegó más tarde a latitudes altas (-°N) durante inviernos más cálidos, y más tarde
a latitudes medias y altas (-, -°N) durante primaveras más cálidas, lo que tal vez indicaría paradas migratorias extendidas a
menos de °N durante esos años. En general, las variables climáticas fueron mejores predictores de las fechas de llegada en el periodo
reciente que en el periodo histórico. Nuestros resultados documentan variabilidad espacial en cómo las temperaturas más cálidas
afectan la llegada de los colibríes y dan credibilidad a la hipótesis de que las diferencias espaciales en los patrones de llegada en latitudes
altas y bajas podrían aumentar la asincronía entre algunas aves y sus recursos alimenticios, y modificar los servicios ecosistémicos
asociados como la polinización y la supresión de pestes de insectos.
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107
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e Auk 130(1):107−117, 2013
 e American Ornithologists’ Union, 2013.
Printed in USA.
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, Number , page s −. ISSN -, elec tronic ISSN - .   by e America n Ornithologi sts’ Union. A ll rights res erved. Please d irect all
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5E-mail: jason_courter@taylor.edu

108 — Co uRteR et al. — auK, Vol. 130
passage time was inversely related to temperature. Hüppop and
Winkel () used first arrival dates of Pied Flycatchers (Fice-
dula hypoleuca) at six sites along a migratory pathway in Europe
to show that migration was strongly influenced by temperatures
en route. One of the broadest-scale studies to date used observa-
tions from an extensive net work of volunteer observers at >,
sites around Spain to predict changes in arrival dates for common
migratory species from  to  in relation to weather vari-
ables (Gordo and Sanz ). In general, however, studies of this
magnitude are difficult because of the enormous network of ob-
servers required to pinpoint annual “first-events” that often span
thousands of kilometers.
A counterpart for assessing historical, broad-scale changes
in migration in North America had been largely unavailable un-
til a recent effort by the U.S. Geological Survey revitalized the
North American Bird Phenology Program (NABPP; see Acknowl-
edgments). From  to , the NABPP coordinated efforts of
hundreds of naturalist volunteers to report annual first bird sight-
ings in North America using standardized observation protocols
to better understand migration patterns and bird distributions
(Merriam , J. Zelt pers. comm.). Efforts are currently un-
der way to scan and digitize this largely unanalyzed (except for
Droege et al. , Zelt et al. ) database and make records
available to the public through the USA National Phenology Net-
work (see Acknowledgments; Dickinson et al. ). At the same
time, “citizen scientists” are reporting spring events such as dates
of bird arrival, insect emergence, and plant flowering dates that
have enabled others to describe spring arrival in birds (Wilson
) and migrator y pathways of Monarch Butterflies (Danaus
plexippus; Howard and Davis ). Such data could improve
phenology studies if biases associated with citizen data-collection
techniques are recognized and addressed (Miller-Rushing et al.
, Dickinson et al. ).
Hummingbirds are charismatic, abundant Neotropical mi-
grants that have fascinated naturalists for centuries (Robinson
etal. ), and detailed observations of the Ruby-throated Hum-
mingbird (Archilochus colubris; hereafter “ruby-throat”) have
been made in both recent and historical periods. Ruby-throats
are easily identified and, given that they are the only regularly
occurring hummingbird in eastern North America, are suitable
subjects for long-term monitoring programs. Ruby-throats regu-
larly winter in Central America between northern Panama and
southern Mexico, and most migrate across the Gulf of Mexico,
arriving at their breeding grounds in eastern North Amer-
ica between February and May (Robinson et al. ). During
migrat ion, ruby-throats feed prima rily on nectar a nd small inse cts
(Robinson et al . ) and occasiona lly on tree sap associated wit h
wells of Yellow-bellied Sapsuckers (Sphyrapicus varius; Mil ler and
Nero ). Recent studies have indicated that ruby-throats are
arriv ing earlier at their breed ing grounds tha n in previous periods
in Maine (Wilson et al. ), Massachusetts (Butler , Led-
neva et al. ), South Dakota (Swanson and Palmer ), and
New York (Butler ).
Given the recent trend of earlier ruby-throat arrivals, the
extensive geographic database of obser vations now available, and
a general understanding that climate influences bird migration at
multiple scales, we assessed spatial differences in arrival dates of
ruby-throats from  to  in eastern North America in rela-
tion to climate variables. We also examined potential mechanisms
B   used to assess the effects of climate change on
wildlife species because they are charismatic and easy to iden-
tify, and monitoring programs have been in place for more than
a century (Crick ; Møller et al. , ; Wilson ;
Newson et al. ; Knudsen et al. ). e results of recent
studies su ggest that many species a re returning e arlier than in pre-
vious periods largely because of changes in global climate (Cotton
, Miller-Rushing et al. ), such as changes in mean annual
temperature (Ledneva et al. ), winter temperature (Cotton
, Swanson and Palmer , Hurlbert and Lia ng ), spring
temperature (Murphy-Klassen et a l. ), and large-sc ale climate
indices such as the North Atlantic Oscillation Index (Hüppop and
Hüppop , Vähätalo et al. ). Changing arrival dates have
also been correlated to nonclimate factors, such as an increase in
the popularity of backyard bird feeding (Robb et al. ), chang-
ing sizes of bird populations (Miller-Rushing et al. ), and
landcover changes in wintering grounds, breeding grounds, and
migratory pathways (Moore et al. , Parrish ).
In addition to serving as sentinels of climate change, birds
provide important ecosystem services to farmers and the gen-
eral public (Şekercioğlu , Whelan et al. , Wenny et al.
). Birds function as insect predators (Mols and Visser ),
pollinators (Clout and Hay ), scavengers (Şekercioğlu et al.
), seed dispersers (Levey et al. ), seed predators (Holmes
and Froud-Williams ), and ecosystem engineers (Valdivia-
Hoeflich et al. ). Recent evidence suggest s that changing tem-
peratures and other factors are disrupting i mportant food webs by
causing birds to arrive either too early or too late compared with
food resources (Marra et al. , Visser and Both , Saino
et al. ). Møller et al. () reported that population sizes
of migratory bird species that were unable to adjust their spring
migrations to use peak food resources declined between 
and  in Europe. Such asynchrony could be detrimental to
bird populations and, potentially, to the biological pest sup-
pression that birds provide, leading to increased pest outbreaks
(Price ). Predicting where potential asynchronies may be
most severe and how climate change may alter migration pat-
terns remains difficult because of the spatial variability of chang-
ing temperatures (Stenseth et al. , Stokke et al. , Visser
and Both ). e effects of climate change often vary region-
ally and are most pronounced in northern latitudes, especially
in North America (Easterling et al. , Hurrell and Trenberth
), providing challenges to birds that pass through multiple
climate regions during migration (Strode , Newton ).
Many studies of bird phenology have been conducted at
broad temporal but narrow spatial scales (Bradley et al. , Cot-
ton , Ledneva et al. , Murphy-Klassen et al. , Swan-
son and Palmer ). Benefits of site-based migration studies
include the ability for multiple species to be compared simulta-
neously, observer error to be reduced, and available weather data
to be collected and correlated consistently over multiple years.
Inferences, however, can be limited spatially, making it difficult
to assess the effects of temperature changes that vary widely
across landscapes (Primack et al. , Knudsen et al. ). Some
studies have used multiple observations along migratory routes
to examine how temperature influences migration (Knudsen
et al. ). For example, Marra et al. () compared the inter-
val between banding dates of long-distance migrants at stations
, km apart in the eastern United States and found that mean

JanuaRy 2013 — Changes in humm ingBi Rd migR ation — 109
for the observed changes in relation to their long-distance migra-
tion patterns and foraging habits, and spatial variation of climate
effects from wintering grounds to their more northerly breeding
areas.
Meth ods
Arrival data.—Historical ruby-throat migration data (–;
hereafter “historical”) provided by the NABPP were transcribed
from handwritten arrival cards to Microsoft Excel spreadsheets
by J.R.C. and student volunteers. Each arrival record was then re-
checked to ensure accuracy. Recent ruby-throat data (–;
hereafter “recent”), reported by citizen science volunteers th rough
hummingbirds.net and Journey North, were accessed from the
Journey North online database (see Acknowledgments). First
arrivals reported between  February and  May were double
checked for accuracy and converted to day of year (e.g.,  April =
day ), accounting for leap years. Arrivals were assigned a loca-
tion (i.e., latitude, longitude, and elevation) based on the centroid
of the reported arrival city and zip code using the ARCGIS, ver-
sion , Geocoding Function (ESRI, Redlands, California) and the
GPS Visualizer geocoding ser vice (see Acknowledgments).
Arrivals from historical and recent periods were divided into
° latitudinal bands (~ km each; Fig. ) from  to .°N to en-
compass the northward pattern of ruby-throat migration in the
eastern United States. For example, all arrival records between 
and .°N were grouped into the °N band. When summariz-
ing results, we refer to bands –°N as “lower” latitudes, bands
–°N as “middle” latitudes, and bands –°N as “higher” lati-
tudes. Arrival data north of °N and south of °N did not meet
our minimum sample size requirement (≥ observations per pe-
riod) and were omitted from analyses. Longitudinally, we included
arrival records east of °W, which is the approximate range limit
for ruby-throats (Robinson et al. ). Outliers were removed at 
standa rd deviations by period and ° latit udinal band to remove firs t
arrivals that were likely incorrectly reported by citizen volunteers.
In sum, we an alyzed , first-arriva l records (n = , from his-
torical a nd n = , from recent period; Fig. ).
Weather data.—To approximate annual weather condi-
tions in the eastern United States, we used monthly weather data
(–) from the Nationa l Oceanic and Atmospheric Admin-
istration Time Bias Corrected Divisional Temperature–Precipi-
tation–Drought Index Data Set (see Acknowledgments), reported
by climate division (designations of the U.S. National Climate
Data Center that group areas of similar elevation, temperature,
and precipitation). Weather variables previously linked to changes
in bird phenology (i.e., winter temperature, spring temperature,
and spring precipitation; Gordo ) were joined to arrival re-
cords by year and climate division using ARCGIS, version 
(ESRI). We used mean monthly temperatures in January and Feb-
ruar y for winter values and mea n monthly temperatures in March
and April for spring values. To approximate temperatures en-
countered in Central American wintering grounds, we searched
for weather stations in the Global Historical Climatology Net-
work (see Acknowledgments) located near the center of the ruby-
throat’s winter range (southern Mexico to northern Panama)
Fig. 1. Locations within our study region (33–44°N, 67–94°W) where Ruby-throated Hummingbird arrivals were reported by the North American Bird
Phenology Program (1880–1969; blue) and Journey North (2001–2010; red). Numbers represent approximate degrees north latitude. First arrivals in
our study were grouped by period and 1° latitudinal band.

110 — Co uRteR et al. — auK, Vol. 130
that reported long-term monthly temperature records from 
to . In general, such stations were scarce. Only one (Aerop.
Interna, GHCN Station no. , .°N, –.°W,
Yucatan, Mexico) met our criteria and was therefore used to ap-
proximate temperatures on the ruby-throat’s wintering grounds.
We used mean February temperatures to approximate tempera-
tures on wintering grounds because February is typically the last
full month in which ruby-throats overwinter prior to their depar-
ture to North America (Robinson et al. ).
Statistical analyses.—We compared mean arrival dates by
latitudinal band using standard least-squares regression with
period as a predictor. We initially examined mean arrival dates
by decade and noted that arrivals in our recent period were sig-
nificantly earlier than mean arrival dates in each of the previous
decades. erefore, to simplify our output, we grouped arrival
dates into a pre- and post-climate-change period based on noted
similarities of arrival dates within periods and a general consen-
sus that a climatic change point occurred in the mid-s, after
which many phenological events began to advance (Walther
et al. , Gordo and Sanz ). To adjust for micro-sc ale differ-
ences within bands, we included latitude, longitude, and elevation
in our models, along with possible interaction terms. To examine
remaining variability in arrival date, we then explored differences
among the environmental variables associated with arrival dates
(winter and spring temperature on breeding grounds, precipita-
tion on breedi ng grounds, and temperat ure on wintering grou nds)
by latitudi nal band and period and noted that envi ronmental vari-
able means differed between periods.
Given the mean d ifferences in both arr ival dates and environ-
mental variables, we used stepwise variable selection techniques
to identify sets of environmental variables that were related to ar-
rival date at each latitudinal band. Initial analyses indicated that
relationships between environmental variables and bird arrivals
were inconsistent between periods and that there was a high cor-
relation among environmental variables. erefore, we analyzed
the relationship between arrival date and weather variables sep-
arately, for each period and band combination, using standard
least-squares regression. All statistical analyses were conducted
using JMP, version . (SAS Institute ).
Migratory rates were calculated by subtracting mean arrival
times at adjacent latitudinal bands and dividing by  km (the ap-
proximate leng th of ° of latitude). Total migrator y passage time wa s
calculated by subtracting mean arrival dates at °N from those at
.°N for each period. To compare arrival dates graphically, we
generated a smoothed raster map from point data for each period
using inverse distance weighting (IDW) in ARCGIS, a procedure
that assig ns values to raster c ells on the basis of k nown va lues of sur-
rounding c ells. For our IDW models, we calc ulated mean arr ivals by
period and climate division and included all divisions between 
and °N that had a minimum of  arrival points per period; this
included  climate divisions from the historical and  climate
divisions from the recent period. Although variability was higher
for mean arrival dates between  and °N and between  and
°N in our historical period, we chose to include these data in this
analysis for comparative purposes. We assigned each mean arrival
date a latitude and longitude based on the centroid of the climate
division it represented. For our graphical analysis, we considered a
-cell search radius and delineated arrivals using an -day inter val.
Result s
Mean first arrival dates differed dramatically between periods at
all latitudes (Fig. ), with ruby-throats arriving .–. days ear-
lier in the recent period (Table ). Moreover, differences in fi rst ar-
rival date varied by latitude (Fig. ). At lower and middle latitudes,
ruby-throats arrived ~ days earlier in the recent period, but at
higher latitudes they arrived ~. days earlier (Table ). Hum-
mingbirds, on average, took . days to travel between  and
°N during the historical period (= . km day–) and . days
(= . km day–) to travel between  and °N in recent times.
Fig. 2. A depiction of mean first arrival dates of Ruby-throated Hummingbirds in eastern North America, 1880–1969 and 2001–2010. Arrival dates
were advanced at all latitudes. This figure was generated using inverse-distance weighted (IDW) interpolation in ARCGIS, version 10.

JanuaRy 2013 — Changes in humm ingBi Rd migR ation — 111
Migratory rate (inversely related to passage days; Fig. ) increased
at higher latitudes in both periods.
Climate variables associated with arrival differed between
periods, with warmer winters and warmer and wetter springs
reported in recent times at higher latitudes (Table ). In general,
winter and spring temperatures were highly correlated in both pe-
riods (r = ., df =  and , P < .). On average, February
temperatures on Central American wintering grounds were .
± .°C (SE) warmer for arrivals in recent times (P < .) than
in the historical period. Several weather variables predicted ar-
rival dates at various latitudes dur ing the recent period (Table A).
Most notably, birds arrived earlier in warmer winters and springs
at lower latitudes, but later in warmer winters and springs at
higher latitudes. Wetter springs were correlated with earlier a rriv-
als at  and °N, but with later arrivals at  and °N (Table ).
In general, birds arrived earlier when February wintering-ground
temperatures were warmer. Weather variables during the histor-
ical period were less predictive of avian arrivals; although some
trends were similar to the recent period, on ly  of  possible vari-
ables were significant at our  latitudes (Table B).
taBle 1. First arrival dates of Ruby-throated Hummingbirds in North America reported by latitude for the historical (1880–1969) and recent (2001–
2010) periods. Differences in mean arrivals were compared using t-tests.
First arrivals 1880–1969 First arrivals 2001–2010 Difference
Latitude nDOY aSE nDOY aSE
Days
earlier SE P
33 83 104.9 1.16 1,138 89.3 0.27 15. 6 1.19 <0.001
34 75 112.3 1.2 2 1,778 94 .1 0.20 18 .2 1.24 <0.001
35 169 112 . 6 0.70 2,475 99.5 0.16 13.1 0.72 <0.001
36 118 11 7. 3 0.92 1,996 102.2 0.18 15.1 0.93 <0.001
37 129 121.8 0.75 1,974 10 6. 5 0 .17 15.3 0 .76 <0.001
38 191 125.7 0.70 2,694 111.1 0.15 14.6 0.71 <0.001
39 298 128.8 0.51 3,308 115 . 5 0.14 13. 3 0.53 <0.001
40 569 135. 0 0.42 3,057 118 . 7 0.14 16. 3 0.44 <0.001
41 898 135. 2 0.28 4,225 121.5 0 .11 13.7 0.30 <0.001
42 1,0 0 9 135.9 0.23 4,007 124.2 0 .11 11.7 0.25 <0.001
43 564 137. 6 0.26 2,618 125.9 0 .12 11. 7 0.29 <0.001
44 488 138.7 0.31 1,76 8 1 27. 3 0.14 11.4 0.34 <0.001
a Arrival dates expressed as day of year (DOY) and corrected for leap years; for example, 95 = 5 April.
Fig. 3. Migration advancement in Ruby-throated Hummingbirds, 1880–
1969 and 2001–2010, by 1° latitudinal band. Linear regression line
shows that changes in first arrival dates are less pronounced in northern
latitudes.
Fig. 4. Number of passage days spent between 1° latitude intervals dur-
ing spring migration by first-arriving Ruby-throated Hummingbirds. Lin-
ear regression lines indicate that migration rates increased (i.e., fewer
passage days) in northern latitudes in both 1880–1969 and 2001–2010.
disc ussi on
Understanding how species and ecosystems respond across spatial
and tempora l scales is one of the cha llenges facing cli mate-change re-
search (Primack et al. ). e innate urgency of birds to complete
northward migration in time for breeding activities to occur when
food and other resources are plentiful is constrained by availability
of suitable temper atures and sufficient food at a v ariety of latitude s en
route (Hüppop and Winkel , Tøttrup et al. ). Our findings
demonstrate that Ruby-throated Hummingbirds arrive at breeding
areas throughout the eastern United States . to . days earlier
than they did historically (Fig. ), a result generally consistent with
site-specific reports at various latitudes. For example, we report an
.-day advancement in ruby-throat migration at °N, whereas
Ledneva et al. () reported an .-day advancement in Mid-
dleborough, Massachusetts (.°N, .°W), from  to ;

112 — Co uRteR et al. — auK, Vol. 130
Butler () reported a .-day shift in Worcester, Massachusetts
(.°N, –.°W), from  to . Butler () also reported
a modest -day shift (P = .) toward earlier arrivals at Cayuga
Lake Basin, New York (.°N, –.°W), but arrival periods were
grouped di fferently (i.e., – and –) than in our s tudy.
At °N, we report an .-day advancement, whereas Wilson et al.
() found a -day advancement in Maine (~°N, °W; compar-
ing intervals – and –) and Swanson and Palmer
() found an .-day advancement in South Dakota (~°N,
°W; between  and ). Swanson and Palmer () found
no evidence that ruby-throats arrived earlier in Minnesota between
 and  and, although Minnesota (~°N, °W) is outside
our study region, this result is somewhat consistent with our finding
that advancement in arrival dates declines at higher latitudes (Fig. ).
Effects of climate on hummingbird arrivals.—Our findings are
consistent with a growing body of evidence that winters and springs
are warming in recent years, especially at higher latitudes (i.e., above
°N; Karl and Trenberth , Loarie et al. ; Table ). Earlier
hummingbird arrivals in our study were correlated with weather
variables in both periods (Table ), consistent with a general trend
reported across bird taxa (Gordo , Lehikoinen and Sparks ).
Photoperiod has long been regarded as the primary cue that trig-
gers migration in birds (Farner ), with weather variables such as
temperature and precipitation helping to fine tune migration timing
(Tøttrup et al. , Knudsen et al. ). Interestingly, our results
showed that weather variables affected arrival dates to a greater ex-
tent in recent times, with  of  metrics significant in the recent
period, compared with only  of  in the historical period (Table ),
which may suggest that local-scale weather or climate-related cues
are emerging as factors of increasing importance to ruby-throats,
both in Nor th America and on Cent ral American w intering ground s.
During our recent period (–), birds arrived earlier to
most latitudes when February temperatures were higher in their
wintering grounds prior to departure (Table ). Few studies have
used temperature on the wintering ground to predict migratory ar-
rival to North America, because long-term data from tropical areas
in the wester n hemisphere are limite d (Gordo ). Evidence from
Europe, however, suggests that migrants return earlier when win-
ters are warmer in Af rica (Boyd , Cotton , Balbontín et al.
). Our results also show that recent arrivals are earlier when
winters a nd springs are warmer i n North America, but onl y at lower
latitudes (Table ), which suggests that migration of Ruby-throated
Hummingbirds is likely constrained by weather or foraging condi-
tions en route (Marra et al. , Tøttrup et a l. ).
Ruby-throats migrated north at a rate of . km day– during
the recent period, a rate similar to the . km day– (or  miles
day–) reported by the popular citizen-science website humming-
birds.net. O ur results suggest t hat migration occur red faster histor-
ically (. km day–), meaning that hummingbirds currently take
~ additional days to travel between  and °N. It is somewhat
surprising that the migratory rate has slowed in recent times, even
though the migrator y period occurs much earlier in the spring (Fig.
), given recent increases in ruby-throat populations and the likeli-
hood that competition for food may be intensified. An increase in
the provision of sugar water along migration routes in recent times
may partially explain this delay. If so, periodic stops along the mi-
gratory rout e to refuel at feeders could help reduce morta lity during
migration and allow hummingbirds to arrive in breeding areas in
better condition and to better compete for nesting territories.
Our data also show that warmer winter temperatures advance
migrat ion below °N but delay hummingbi rd migration above °N
(Table A). It is possible that a failure to me et winter chill ing require-
ments of plants, due to recent warmer winters in the eastern United
States, may delay bud brea k for some plant species (Morin et al . ,
Harrington et al. , Cook et al. ) below °N (Zhang et al.
), meaning t hat migratory bird s, such as hummi ngbirds, may ex-
tend their stopover periods to obtain sufficient food to complete mi-
gration (Strode ) or in response to another plant phenology cue.
We report a migratory delay (i.e., an increase in the number of pas-
sage days; Fig. ) between °N and °N in the recent period, which
appears to be consistent with this hypothesis. Spring temperatures
were also correlated with later arrivals at mid- and high latitudes, but
taBle 2. Differences (Diff.) in climate variables in the region between 33 and 45°N and from 67 to 94°W, between historical (1880–1969) and recent
(2001–2010) periods.
Winter temperature (°C) aSpring temperature (°C) bSpring precipitation (cm) c
Latitude Diff. dSE PTren d eDiff. dSE PTrend eDiff. dSE PTrend e
33 –0.69 0.23 0.003 Colder 0.14 0 .15 0.37 – 4.98 1.42 <0.001 Dryer
34 –1.01 0.22 <0.001 Colder 0.19 0.14 0.17 –3.21 1.3 0 0.01 Dryer
35 0.26 0 .15 0.08 0.83 0.09 <0.001 Warmer – 4.22 0.72 <0.001 Dryer
36 0.48 0.20 0.02 Warmer 1.14 0.13 <0.001 Warmer – 4.65 1.17 <0.001 Dryer
37 –0.08 0.20 0.69 0.53 0.12 <0.001 Warmer 0.37 1.17 0.75
38 0.63 0.18 <0.001 Warmer 1.41 0.10 <0.001 Warmer 0.92 0.79 0.24
39 0.36 0.14 0.01 War mer 1.28 0.08 <00001 Warmer 0.67 0.55 0.22
40 0.26 0 .12 0.04 Warmer 1.14 0.07 <0.001 Warmer 1.29 0.42 0.002 Wetter
41 0.43 0.09 <0.001 Warmer 1. 31 0.06 <0.001 Warmer 4.76 0.36 <0.001 Wetter
42 0.90 0.08 <0.001 Warmer 1.25 0.05 <0.001 Warmer 4.04 0.29 <0.001 Wetter
43 1.31 0 .12 <0.001 Warmer 1. 27 0.08 <0.001 Warmer 1.9 8 0. 37 <0.001 Wetter
44 2.82 0.16 <0.001 Warmer 1.69 0 .11 <0.001 Warmer 2.62 0.42 <0.001 Wetter
a Mean January and February temperatures on North American breeding grounds.
b Mean March and April temperatures on North American breeding grounds.
c Mean sum of February–April precipitation in North American breeding grounds.
d Differences calculated by subtracting 1880–1969 climate means from 2001–2010 climate means.
e Summary of how recent climate data (2001–2010) compare with historical climate data (1880–1969).

JanuaRy 2013 — Changes in humm ingBi Rd migR ation — 113
taBle 3. Significant predictors (P < 0.05) of Ruby-throated Hummingbird arrival dates in (A) recent (2001–2010) and (B) historical (1880–1969) peri-
ods. We used regression models to identify the environmental variables that predicted arrival date at each latitudinal band. Latitude, longitude, and
elevation were included as covariates to adjust for possible regional effects within latitudinal bands.
Winter temperature (°C) aSpring temperature (°C) bSpring precipitation (cm) cWintering grounds temp. (°C) d
Latitude
Slope
(SE) PDescription
Slope
(SE) PDescription
Slope
(SE) PDescription
Slope
(SE) PDescription
(A) Recent dat a (2001–2010)
33 –0.92
(0 .18)
<0.001 ↑Temp, Earlier –1.3 6
(0. 28)
<0.001 ↑Temp, Earlier –0.13
(0.03)
<0.001 ↑Precip, Earlier –0.81
(0.25 )
0.001 ↑Temp, Earlier
34 –0.64
(0 .14)
<0.001 ↑Temp, Earlier –0.22
(0.23)
0.33 – 0.06
(0.02)
0.02 ↑Precip, Earlier – 0. 21
(0.20)
0.29
35 –0. 53
(0.10)
<0.001 ↑Temp, Earlier –0.40
(0.17 )
0.02 ↑Temp, Earlier –0.03
(0.02)
0.20 0.02
(0 .16)
0.88
36 –0.25
(0 .12)
0.03 ↑Temp, Earlier 0.19
(0 .18)
0.28 0.03
(0.02)
0.10 0 .17
(0 .18)
0.32
37 –0.54
(0.10)
<0.001 ↑Temp, Earlier –0.02
(0.17 )
0.92 0.04
(0.02)
0.008 ↑Precip, Later –0.06
(0.17 )
0.72
38 –0.55
(0.08 )
<0.001 ↑Temp, Earlier 0.42
(0 .14)
0.003 ↑Temp, Later 0.03
(0.02)
0.12 – 0.41
(0.15 )
0.008 ↑Temp, Earlier
39 –0. 36
(0.07)
<0.001 ↑Temp, Earlier 0.33
(0 .14)
0.01 ↑Temp, Later 0.02
(0.02)
0.18 – 0.39
(0 .14)
0.006 ↑Temp, Earlier
40 –0.07
(0.07 )
0.35 0.01
(0.13 )
0.95 0.09
(0.02)
<0.001 ↑Precip, Later – 0.29
(0 .14)
0.04 ↑Temp, Earlier
41 0.02
(0.05)
0.66 0.33
(0.09)
<0.001 ↑Temp, Later 0.02
(0.01)
0.18 – 0.48
( 0.11)
<0.001 ↑Temp, Earlier
42 0.23
(0.05)
<0.001 ↑Temp, Later 0.50
(0.09)
<0.001 ↑Temp, Later –0.03
(0.01)
0.08 –0.73
(0 .12)
<0.001 ↑Temp, Earlier
43 0.19
(0.05)
<0.001 ↑Temp, Later 0.29
(0.08)
<0.001 ↑Temp, Later –0.02
(0.02)
0.33 – 0.53
(0 .12)
<0.001 ↑Temp, Earlier
44 0.04
(0.05)
0.40 0.30
(0.08)
<0.001 ↑Temp, Later –0.02
(0.02)
0.37 – 0.75
(0.13 )
<0.001 ↑Temp, Earlier
(B) Hi storical dat a (1880–1969)
Latitude
Slope
(SE) PDescription
Slope
(SE) PDescription
Slope
(SE) PDescription Slope (SE) PDescription
33 0.90
(0.63)
0.16 1.03
(0.88)
0.25 – 0. 21
(0.10)
0.04 ↑Precip, Earlier – 0.98
(1.0 3)
0.34
34 0.72
(0.48 )
0.14 – 0 .18
(0.93 )
0.85 -0.19
(0.10)
0.07 1.4 0
(0.98)
0.16
35 –0.28
(0. 33)
0.39 – 0.20
(0.48 )
0.68 0.03
(0.08)
0.70 – 0.10
(0.63)
0.87
36 –0.18
(0. 33)
0.60 0.27
(0.44 )
0.54 –0.06
(0.10)
0.52 0.16
(0.80)
0.84
37 –0.08
(0.40)
0.84 0.32
(0.54)
0.55 0.14
(0 .12)
0.25 – 0.01
(0. 67)
0.99
38 0.08
(0. 28)
0.77 – 0.10
(0.44 )
0.81 0.08
( 0.11)
0.44 –0.02
(0.65)
0.98
39 0.07
(0. 27)
0.80 0.34
(0.38 )
0.37 – 0.07
(0.09)
0.43 – 0. 15
(0. 59)
0.80
40 0.38
(0.13 )
0.004 ↑Temp, Later 0.16
(0 .19)
0.39 – 0.06
(0.05)
0.28 -0.09
(0. 28)
0.75
41 0.17
( 0.11)
0.13 – 0 .19
(0 .16)
0.23 –0.03
(0.05)
0.49 0.35
(0.22)
0.11
42 0.04
(0.09)
0.65 0.15
(0.13 )
0.25 – 0.01
(0.04)
0.79 0.20
(0 .19)
0.27
43 0.06
(0.10)
0.55 – 0.25
(0 .14)
0.07 0.02
(0.04)
0.66 – 0. 17
(0.23)
0.46
44 0.26
( 0.11)
0.02 ↑Temp, Later – 0 .16
(0 .14)
0.27 0.03
(0.04)
0.42 –0.50
(0.23)
0.04 ↑Temp, Earlier
a Mean January and February temperatures on North American breeding grounds.
b Mean March and April temperatures on North American breeding grounds.
c Mean sum of February–April precipitation on North American breeding grounds.
d Mean February temperature in Yucatan, Mexico (20.98°N, –89.65°W), used to approximate temperatures in wintering grounds.

114 — Co uRteR et al. — auK, Vol. 130
this m ay be becaus e spring and winter temper atures were h ighly cor-
related in our s tudy and the mechani sm that best explai ns the migra-
tory delay is the warming winter temperature. Another possibility is
that some birds delay migration in years with high productivity and
extend stopovers to take advantage of improved foraging conditions
(Tøttrup et al. , Robson and Barriocanal ). Regardless of
the mechanism(s) governing these interactions, ruby-throats appear
to arrive later in relation to spring conditions at northern latitudes,
which may indicate a mismatch between hummingbird arrival and
initial availabilit y of food. Our results demonstrate the importance
of considering latitude and possible reasons for stopover when inter-
preting migratory studies t hat assess phenology.
Using first arrival dates and a growing hummingbird popu-
lation.—We have obviated a common criticism that first arrival
dates are affected by differences in observer effort across space
(Gordo and Sanz , Dickinson et al. ) by comparing mean
first arrival dates of ruby-throats (based on ≥ observations per
band; Table ), instead of using first arrival dates of individuals.
Other biases of using first arrival dates were impossible to address
in our study, such as the tendency for early migrants to be influ-
enced more by climate change (Vähätalo et a l. , Tøttrup et al.
) and the tendency for fi rst arrival dates to advance more than
mean or median migration dates (Leh ikoinen et al. , Rubolini
et al. , Miller-Rushing et al. ). Even so, we are confident
that our resu lts illustrate biolog ically meani ngful spatia l and tem-
poral patterns and note that a study of this spatial and temporal
magnitude (Fig. ) would be nearly impossible to conduct without
using first arrival dates.
We also point out the popu lation size of ruby-throats ha s more
than doubled in the eastern United States since , according
to data from the North American Breeding Bird Sur vey (Sauer et
al. ). We chose not to include population size in our analyses
because we lacked a reliable estimate of hummingbird populations
from  to . Swanson and Palmer () reported that first
arriv al dates advanced in  of  specie s with increasin g populations
(and in  of  species with stable populations) from  to 
in Minnesota and South Dakota. Although increasing populations
are often correlated with higher detection probabilities among
citizen volunteers (Tryjanowski and Spa rks , Tryjanowsk i et al.
, Miller-Rushing et al. ), we find it unlikely that popula-
tion changes, alone, sufficiently explain the dramatic migratory ad-
vancement that we report here.
Backyard bird feeding, expanding winter ranges, and other
data limitations.—An impor tant consideration when i nterpreting
our result s is the increase in popu larity of backyard bird feeding in
the United States in past decades (Robb et al. ). Although we
are confident that data reporters in our historical period (–
) were competent naturalists, it is likely that fewer historical
observations were made at feeders, perhaps decreasing the likeli-
hood that early-arriving birds were immediately detected. Many
of our recent arrivals were also reported online (compared with
historical arrival records that were submitted by mail), perhaps
encouraging some observers to be more vigilant when ruby-
throats were reported nearby (L. Chambers pers. comm.), and
perhaps increasing the effort among competitive observers seek-
ing to report the first hummingbird arrival in a particular area
(Schaffner ). Unfortunately, the data that we used did not
include detailed observer information that would have allowed
demographic comparisons to be made between observers from
different periods, such as differences in observer age, income, and
gender (Cooper and Smith ), factors that may have contrib-
uted to the discretionary time observers had to look for birds. In
addition, impor tant demographic data about hummingbird popu-
lations (e.g., age classes of birds, sex ratios, and whether birds were
local breeders or migrat ing birds) that likely varied by latitude a nd
period were unmeasurable in our study and could have influenced
the changes in hummingbird migration that we report.
It is also possible that the winter ranges of hummingbirds
could be advancing northward into the southern United States
as bird feeders and warming winter temperatures provide more
predictable food resources (Parmesan and Yohe ). A more
northerly winter range could potentially decrease the distance
and time that a hummingbird needs to migrate and cause birds to
arrive earlier to their breeding grounds (Robb et al. , Visser
et al. ), although birds would still face similar environmental
constraints in migrating northward. It is even possible that some
ruby-throats have changed their migratory routes altogether (i.e.,
migrating over land through Mexico and Texas rather than over
the Gulf of Mexico; Zelt et al. ). Although we were not able to
account for this possibility, we defined our study area as north of
°N, which almost certainly eliminated the chance for wintering
birds to be repor ted as first arrivals (Hauser and Cur rie , Rob-
inson et al. ).
We have demonstrated a major phenological shif t in the past
century for the ruby-throat that is most pronounced at lower lati-
tudes and is largely related to climate. Extended migratory stop-
overs in mid-latitudes during warmer winters, when spring is
earlier in the north, may present a double effect on synchrony
between birds and their breeding habitats. Taken together, our
results demonstrate advanced migration arrival dates but with
spatial variation for Ruby-throated Hummingbirds and suggest
that local-scale weather-related cues, in both North American
breeding and Central American wintering grounds, are emerg-
ing as factors of increasing importance to bird phenology. Large-
scale comparative stud ies such as this could help conservationists
and policy makers identify where ecosystem services provided by
birds (e.g., pollination and pest suppression) are most likely to be
impeded and help inform management decisions.
Ackn owle dgM ent s
We thank L. Chambers from hummingbirds.net and E. How-
ard and Journey North, with funding from the Annenberg
Foundation, for collecting and compiling thousands of recent
first-arrival reports. We are grateful to the countless contribu-
tors from Journey North and hummingbirds.net for more than
a decade of careful observation, without which a study of this
magnitude would not be possible. We thank J. Zelt and S. Droege
for their commitment to providing and protecting historical mi-
gration records through the North American Bird Phenology
Program (U.S. Geological Survey, Patuxent Wildlife Research
Center) and for their expertise and support, which underscores
the vision of the hundreds of early naturalists who fa ithfully con-
tributed arrival cards from  to . We thank R. F. Baldwin
from Clemson University, W. W. Bowerman from the Univer-
sity of Maryland, and A. Arab from Georgetown University for

JanuaRy 2013 — Changes in humm ingBi Rd migR ation — 115
help with our study design and analysis. We thank Clemson Uni-
versity students K. Auman, C. Boaman, J. Burroughs, J. H. Col-
lins, B. Crawford, E. Kaiser, M. Kynoch, B. Lang, E. Purcell, D.
Stone, M.-K. Spillane, C. Stucyk, and S. Taylor for transcrib-
ing arrival cards and helping with our analysis, and P. Leonard
and S. Essewein for help with ARCGIS. We thank C. Rogers and
two anonymous reviewers for helpful comments to improve the
manuscript. is study was funded primarily by Clemson Uni-
versity, with additional support from a Carolina Bird Club grant.
e USA National Phenology Network is online at www.usanpn.
org. e North American Bird Phenology Program is online at
www.pwrc.usgs.gov/bpp. e Journey North database is at www.
journeynorth.org. e GPS Visualizer geocoding service is avail-
able at www.gpsvisualizer.com. e National Oceanic and At-
mospheric Administration Time Bias Corrected Divisional
Temperature–Precipitation–Drought Index Data Set is available
at lwf.ncdc.noaa.gov/oa/climate/onlineprod/drought/offline/
readme.html.e Global Historical Climatology Network is at
www.ncdc.noaa.gov/ghcnm/.
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