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Influence of Latitude on the Winter Abundance of Red-tailed
Hawks (Buteo jamaicensis) and American Kestrels (Falco
sparverius) in Illinois
Author(s): Anna Groves Vic Berardi , Paul Sweet , and Janice Sweet Angelo P.
Capparella R. Given Harper
Source: Journal of Raptor Research, 47(4):410-415. 2013.
Published By: The Raptor Research Foundation
DOI: http://dx.doi.org/10.3356/JRR-12-00007.1
URL: http://www.bioone.org/doi/full/10.3356/JRR-12-00007.1
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SHORT COMMUNICATION
J. Raptor Res. 47(4):410–415
E2013 The Raptor Research Foundation, Inc.
INFLUENCE OF LATITUDE ON THE WINTER ABUNDANCE OF RED-TAILED HAWKS (BUTEO JAMAICENSIS)AND
AMERICAN KESTRELS (FALCO SPARVERIUS)IN ILLINOIS
ANNA GROVES
Environmental Studies Program, Illinois Wesleyan University, 201 Beecher Street, Bloomington, IL 61701 U.S.A.
VIC BERARDI,PAUL SWEET,AND JANICE SWEET
Illinois Beach State Park Hawk Watch, Zion, IL 60099 U.S.A.
ANGELO P. CAPPARELLA
School of Biological Sciences, Illinois State University, Normal, IL 61790 U.S.A.
R. GIVEN HARPER
1
Department of Biology, Illinois Wesleyan University, 201 Beecher Street, Bloomington, IL 61701 U.S.A.
KEY WORDS:Red-tailed Hawk; Buteo jamaicensis; American
Kestrel; Falco sparverius; abundance;Illinois;latitude;winter
abundance.
Red-tailed Hawks (Buteo jamaicensis) and American Kes-
trels (Falco sparverius) are among the most common overwin-
tering raptors in Illinois (Bohlen 1989, Kleen et al. 2004,
Preston and Beane 2009); however, no comprehensive study
has documented their winter abundance and distribution in
Illinois for more than 50 yr (Graber and Golden 1960, Gra-
ber and Graber 1963). Because Illinois has a long north-
south extent (603.5 km) spanning approximately 5.5 de-
grees of latitude, temperature and snowfall throughout the
state vary considerably. Winter survival of Red-tailed Hawks
and American Kestrels can be negatively affected by cold
temperatures and heavy snow cover (Mills 1975, Root 1988,
Lish and Burge 1995). Snow cover may also influence prey
availability and decreases in the abundance and vulnerability
of prey have been associated with reduced observations of
both species (e.g., Baker and Brooks 1981, Preston 1990).
Through a series of automobile surveys, we evaluated the
temporal stability of a distributional pattern first reported
more than 100 yr ago and reconfirmed some 50 yr ago that
the winter abundance of both species decreases with lati-
tude, being higher in the central regions of Illinois than in
the northern regions (Forbes and Gross 1923, Graber and
Golden 1960, Graber and Graber 1963). Substantial habitat
changes have occurred within these regions during the past
50 yr (Walk et al. 2010) and unlike the previous surveys, our
survey protocol was designed specifically to detect raptors.
Because changes have been projected for North America’s
climate in the coming decades (e.g., USGCRP 2009), this
study will provide useful data for predicting the future win-
ter distributions of these two species in the midwestern U.S.
We also present additional observations of age classes of
Red-tailed Hawks and sexes of American Kestrels, which
were not documented in the earlier studies.
STUDY AREA
All automobile survey routes were within the Northern
and Central Divisions of Illinois (Gault et al. 1922) and
spanned approximately 2u179of latitude (Fig. 1). The lati-
tude and longitude for the Northern Division of Illinois
were: northernmost 542u309N; southernmost 540u559N;
westernmost 591u059W; easternmost 587u319W (Fig. 1).
The latitude and longitude for the Central Division were:
northernmost 541u449N; southernmost 539u009N; west-
ernmost 591u309W; easternmost 587u329W (Fig. 1). Walk
et al. (2010) classified the state’s land cover into three broad
categories: corn (Zea mays) and soybean (Glycine max)row
crops (i.e., cultivated), development, and deciduous forest.
In 2006–2008, the Northern Division of Illinois was 12%
forested, 28% developed, and 54% cultivated, while the Cen-
tral Division was 8% forested, 4% developed, and 85% cul-
tivated (smaller percentages of habitat types are not includ-
ed; data from Walk et al. 2010). During the surveys, most
corn fields contained corn stubble while most soybean fields
had sparse plant residue. On all routes some of the crop
fields had been plowed, with the fields consisting of exposed
soil when they were not covered by snow.
METHODS
We assessed winter abundances of both species using a
preexisting database of surveys initiated by the Illinois
1
Email address: gharper@iwu.edu
410
Beach State Park Hawk Watch in the Northern and Central
Divisions of Illinois (i.e., no surveys were conducted in the
Southern Division). We analyzed 143 raptor surveys from
18 different survey routes (Fig. 1). Volunteer surveyors
with raptor identification experience chose the routes
(49.1–99.6 km in length) based on their knowledge of
local roadways that had a mix of potential habitat and
tractable traffic. Surveys were completed via automobile
once per month (December–February) from 2004–2005
to 2008–2009, between approximately 1000 H and 1600
H, on days when inclement weather would not decrease
raptor detectability (e.g., heavy precipitation and/or con-
stant high winds). This was the same monthly time period
of winter raptor surveys used in previous studies (Graber
and Graber 1963). Approximately 80% of surveys were
completed by 1420 H, the maximum cutoff for avoiding
time-of-day effects on Red-tailed Hawk abundance (Bunn
et al. 1995; they found no such effects on American Kes-
trels). Most surveys spanned both the morning and after-
noon time periods, and during several months surveys
were not conducted on some routes. Surveyors drove per-
sonal vehicles, maintained an appropriate speed for spot-
ting raptors (approximately 24–56 km/hr) and stopped as
necessary to identify birds using binoculars and/or spot-
ting scopes. At least two surveyors participated in each
survey. Surveyors recorded the species, age class (for
Red-tailed Hawks only), and sex (for American Kestrels
only) of the raptors that were observed. To estimate the
potential effects of weather on raptor abundance, we com-
pared monthly temperature and snowfall data from major
weather stations (Illinois State Climatologist’s Office 2012)
in northern (Chicago O’Hare International Airport,
41u58.759N) and central Illinois (Greater Peoria Airport,
40u40.259N).
Christmas Bird Count (CBC) data were obtained from the
online CBC database (National Audubon Society 2010).
CBCs were conducted annually across the state between 14
December and 5 January by volunteers assigned a specified
route through a 24-km diameter circle. CBC surveys were
conducted via multiple methods (e.g., on foot, by automo-
bile) and every bird seen or heard was counted. The eBird
data, which were collected via point counts, transects, and
area searches conducted by bird watchers, were obtained
from an online database (eBird 2012). For both CBC and
eBird datasets, data were obtained for the same counties in
which winter raptor survey routes were conducted.
We analyzed data with R 2.15.1 software (R Development
Core Team 2012). Abundance data met normality assump-
tions after square-root transformation (Shapiro-Wilk test, a
50.01) and were tested using one-way ANOVA (a50.05).
We tested the relationship between abundance (mean in-
dividuals per 1000 km driven) and survey route latitude
with a linear regression. The latitudinal midpoint of each
route was calculated using the latitudes of the northern-
most and southernmost points on each route using Arc-
Map 10 (ESRI 2011). We tested differences in temperature
and snowfall measured at the Chicago and Peoria, Illinois,
weather stations among years with two-way ANOVAs.
RESULTS
Surveyors made 1746 observations of Red-tailed Hawks
and 601 observations of American Kestrels. There was a
significant negative relationship between latitude and the
mean number of all Red-tailed Hawks per 1000 km driven
(F
1,16
55.3, P50.035, R
2
50.20; Fig. 2a), and the mean
number of all American Kestrels per 1000 km driven (F
1,16
532, P,0.001, R
2
50.64; Fig. 2b). When comparing
Red-tailed Hawks by age class, there was no significant
relationship between latitude and the mean number of
adults (F
1,16
53.77, P50.07), but there was a significant
negative relationship for juveniles (F
1,16
55.65, P50.03,
R
2
50.21). There were highly significant negative relation-
ships between latitude and mean abundance for both male
American Kestrels (F
1,16
516.0, P50.001, R
2
50.47) and
females (F
1,16
531.9, P50.0001, R
2
50.65). Mean
monthly temperatures at the Greater Peoria Airport were
higher than at the Chicago O’Hare International Airport
in 14 of 15 mo of the study period (mean difference 5
0.9uC), and mean monthly snowfall was greater in Chicago
than Peoria in 13 of the 15 mo surveyed (mean difference
513.6 cm). At Peoria, the mean monthly temperature was
22.3uC (low 5212.3uC, high 56.9uC) and the mean
monthly snowfall was 16.6 cm (minimum 50 cm, maxi-
mum 543.2 cm), while in Chicago the mean monthly
temperature was 23.2uC (low 5213.4uC, high 55.6uC)
and the mean monthly snowfall was 30.2 cm (minimum 5
1.5 cm, maximum 570.6 cm). However, none of the dif-
ferences were significant (P.0.05).
Figure 1. Automobile survey routes and the Northern,
Central, and Southern Divisions of Illinois U.S.A. Divisions
are based on Gault et al. (1922).
DECEMBER 2013 SHORT COMMUNICATION 411
There was no significant difference in the abundance of
Red-tailed Hawks (mean 5147.2 Red-tailed Hawks/
1000 km) across years (F
1,141
51.8, P50.18) or months
(F
1,141
52.3, P50.13), nor was there a significant differ-
ence for American Kestrels (mean 551.1 kestrels/
1000 km) across years (F
1,141
50.88, P50.35) or months
(F
1,141
50.87, P50.35). The data from CBC counts and
eBird (December–February) data for both species showed
trends similar to those detected in our automobile surveys
(Fig. 3). Of the 78% of Red-tailed Hawks identified by age
class in our surveys, 10% were juveniles and 90% were
adults. Of the 80% of kestrels identified to sex in our
surveys, 64% were males and 36% were females.
DISCUSSION
We found increases in the overall abundance of both
species from the Northern to the Central Divisions of Illi-
nois, although adult Red-tailed Hawks showed no signifi-
cant relationship. Lish and Burge (1995) suggested that
adult Red-tailed Hawks may winter farther north than ju-
veniles in order to remain closer to nesting territories.
Graber and Golden (1960) and Graber and Graber
(1963) suggested that their detected latitudinal gradients
in the abundance of both species in Illinois from the early
1900s to the 1950s may have been due to temperature.
Enderson (1960) found a negative correlation between
American Kestrel abundance and low temperatures and
high wind velocities in December and January in east-cen-
tral Illinois. Root’s (1988) analysis of CBC data indicated
that temperature was a major factor associated with the
abundance patterns of both species, with peak abundances
of each in the central U.S. at approximately the same lat-
itudes as the Central Division of Illinois.
Although mean monthly temperatures were generally
colder and snowfall was greater in Chicago compared to
Peoria, we found no significant differences between the
values reported by the two weather stations. Local weather
conditions along individual survey routes may have dif-
fered from those at the weather stations (e.g., blowing
and drifting snow on some routes but not on others),
and raptor abundance may have been linked with severe
weather events that were averaged out in the monthly
weather data records. It is also likely that weather data
from only two stations may not represent general weather
conditions across the two regions, as pronounced snowfall
Figure 2. (a) Mean overall Red-tailed Hawk (RTHA)
abundance per 1000 km relative to the latitudinal mid-
point of each survey route, and (b) mean overall American
Kestrel (AMKE) abundance per 1000 km relative to the
latitudinal midpoint of each survey route.
Figure 3. (a) Mean 6SD Red-tailed Hawk (RTHA) abun-
dance per 1000 km across survey years, compared to mean
CBC and eBird data (per observer hour), and (b) mean 6
SD American Kestrel (AMKE) abundance per 1000 km
across survey years, compared to mean CBC and eBird data
(per observer hour).
412 SHORT COMMUNICATION VOL. 47, NO.4
and temperature gradients existed from the Northern
through the Southern Divisions of Illinois throughout the
past 30–50 yr (Changnon et al. 2004, Illinois State Climatol-
ogist’s Office 2012). There have not been significant chang-
es in average winter temperatures in Illinois during the past
century (Illinois State Climatologist’s Office 2012), al-
though this does not account for yearly variability and ex-
treme weather events. However, it is predicted that under
global climate change, Illinois will experience milder, wet-
ter winters with the climate similar to the current southern
Mississippi River valley by mid-century (USGCRP 2009).
Such changes will likely lead to a northward shift of winter
abundance for both species if temperature is the dominant
weather-related controlling factor.
The winter abundance of both Red-tailed Hawks and
American Kestrels remained steady throughout the study.
Craighead and Craighead (1956) also found that winter
Red-tailed Hawk populations in Michigan were stable in
January and February. Migration apparently had little in-
fluence on the observed abundances in our study, despite
the fact that Bohlen (1989) reported that Red-tailed Hawk
spring migration in Illinois began in mid- to late February,
and Enderson (1960) reported that the spring American
Kestrel migration in east-central Illinois began in early
February. We observed much higher abundances than
those recorded by Schnell (1967) via automobile surveys
in northern Illinois in the winters of 1964–1965 and 1965–
1966. He documented a December–February mean of 13.8
Red-tailed Hawks/1000 km (compared to our 147.2/
1000 km) and 3.2 American Kestrels/1000 km (compared
to our 51.1/1000 km). These increases agree with substan-
tial increases in CBC sightings for both species in that time
period (e.g., National Audubon Society 2010, McCay et al.
2001), which are likely due to their adaptability to human-
dominated landscapes and widespread increases in wood-
land-agricultural edge habitat (McCay et al. 2001). Graber
and Golden (1960) and Graber and Graber (1963) found
that winter populations of both species declined in Illinois
from the early 1900s to the 1950s. It is possible that envi-
ronmental contaminants (e.g., organochlorine pesticides)
may have decreased the abundance of American Kestrels
in Illinois in the 1950s (Walk et al. 2010) when the last
comprehensive winter raptor surveys were conducted.
Our observation that adult Red-tailed Hawks on our sur-
veys greatly outnumbered juveniles was not unexpected as
juvenile Red-tailed Hawks have lower survival rates than
adults (Henny and Wight 1972, Santana and Temple
1988). Greater numbers of wintering adult than juvenile
Red-tailed Hawks have also been reported by Gates (1972)
and by Lish and Burge (1995). A factor that may have
influenced our male-biased sex ratio of kestrels is that
males have been observed wintering farther north than
females, which affects the sex ratio of the species at the
northern boundaries of their range (approximately at the
northern border of Illinois; Smallwood and Bird 2002).
Our abundance values for Red-tailed Hawks were lower
than those recorded by Lish and Burge (1995) in Okla-
homa (October–March, 378.4 Red-tailed Hawks/1000 km)
and by Garner and Bednarz (2000) in Arkansas (December–
March, 502 Red-tailed Hawks/1000 km). For American
Kestrels, observed abundances were much lower than Pearl-
stine et al. (2006) reported in south Florida (October–April,
310 kestrels/1000 km). These studies across the U.S. agree
with our observed trends that show increased winter abun-
dance of both species with decreasing latitude from north-
ern to central Illinois. There has been temporal stability in
this pattern during the past 100 yr (Forbes and Gross 1923,
Graber and Golden 1960, Graber and Graber 1963) despite
substantial changes in both habitat and agricultural practic-
es in the last 50 yr that likely reduced habitat availability and
quality. The abundance of hay land (and likely pastures)
declined substantially over this period and agricultural prac-
tices shifted throughout the state as the alternate growing of
corn and soybeans displaced a corn-oats (Avena spp.)-alfalfa
(Medicago lucerne) rotation (Walk et al. 2010). Such a shift to
intensive row crop practices likely reduced rodent popula-
tions (e.g., Cummings and Vessey 1994), which suggests that
prey abundance was not the major determinant of Red-
tailed Hawk and American Kestrel abundance. Currently,
agricultural practices in Illinois are undergoing profound
changes through the expansion of row crops (especially
corn) at the expense of Conservation Reserve Program
and other grasslands (Meehan et al. 2010), and the use of
genetically modified crops that may actually allow for more
intensive use of pesticides (Benbrook 2012). Predictions of
the impacts of these landscape changes on winter raptor
abundance are difficult to make but such changes highlight
the importance of baseline data through standardized rap-
tor counts.
INFLUENCIA DE LA LATITUD EN LA ABUNDANCIA
INVERNAL DE BUTEO JAMAICENSIS YFALCO SPARVE-
RIUS EN ILLINOIS
RESUMEN.—Utilizamos cinco an˜os de datos de 18 rutas
censadas para determinar la estabilidad temporal de un
patro´ n reportado por primera vez hace 100 an˜ os, y recon-
firmado hace 50 an˜os, de que la abundancia invernal de
Buteo jamaicensis yFalco sparverius disminuye al aumentar la
latitud, siendo ma´s elevada en las regiones centrales de
Illinos que en las regiones del norte. Voluntarios entrena-
dos llevaron a cabo muestreos (n5143) mensuales con-
duciendo por rutas seleccionadas de diciembre a febrero,
desde 2004–2005 hasta 2008–2009. Encontramos incre-
mentos significativos en la abundancia de ambas especies
desde las regiones del norte hacia las regiones centrales de
Illinois. No se evidenciaron efectos significativos del an˜o o
del mes en la abundancia deB. jamaicensis (media general 5
147.2 individuos de B. jamaicensis/1000 km) y en la abun-
dancia de F. sparverius (media general 551.1 individuos de
F. sparverius/1000 km). Del 78% de los individuos de B.
jamaicensis identificados por clase de edad, 10% fueron ju-
veniles y 90% fueron adultos. Del 80% de los individuos de
F. sparverius identificados por sexo, 64% fueron machos y
DECEMBER 2013 SHORT COMMUNICATION 413
36% fueron hembras. Nuestros hallazgos indican que ha
habido una estabilidad temporal de 100 an˜os en el patro´n
de incremento de la abundancia invernal de ambas especies
desde el norte hacia el centro de Illinois, a pesar de cambios
substanciales tanto en el ha´bitat como en las pra´ cticas agrı´-
colas durante los u´ ltimos 50 an˜os.
[Traduccio´ n del equipo editorial]
ACKNOWLEDGMENTS
We thank the additional individuals responsible for con-
ducting the winter raptor surveys: Joan Bruchman, Eric
Delbecq, Roger Digges, Thad and Jan Edmonds, Norman
Emerick, Bob and Karen Fisher, Matt Fraker, Richard Gib-
son, Jim Kriss, Karen Lund, Dick Riner, Darryl Sham-
baugh, Anne Straight, Chris Whittier, Matthew Winks,
and Jerry Zamirowski. Will Jaeckle, Ed Pandolfino, and
anonymous reviewers provided useful comments on the
manuscript. RGH thanks Wendell and Virginia Kingsolver
for their support.
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