j. RaptorRes'. 36(4):256-264 2002 The Raptor ResearchFoundation,
Inc.
VERNAL
MIGRATION OF BALD EAGLES COLORADO WINTERINGALAN R. HARMATA 1
FROM AREA
A SOUTHERN
Fish & WildliJb Program, Department Ecology, of Montana
StateUniversity, Bozeman, 59717 U.S.A. MTABSTRACT.--Adult Bald
Eagles(Haliaeetus leucocephalus) = 15) winteringin the San Luis
Valley (SLV), (N Colorado were radio-taggedwith
conventionaltail-mounted transmitters between 1 January-18 March
1980 and 1981 to determine migration patterns and breeding areas.
Migrating eagleswere fbllowed primarily in a singlevehicle with two
trackers.In 1980, radio-taggedeagles(N = 4) left the wintering
groundswithin a 15-d spanin March but departures 1981 (N = 7)
rangedfi:ommid-February in to earlyApril. Eaglesinitiated
migrationon dayswith higher temperatureranges, more clouds,and
higherwinds than other daysduring winter or spring.
Subsequenttravel paralleled the northward movement of the
2Cisothermboth temporallyand spatially. Locationsand pathways
migratingeagleswere of similar in both 1980 and 1981. All four
eagleslocated on their summer range were within 102 km ofeach other
in northeastern Saskatchewan and northwestern Manitoba. Mean
distance fi:om the SLV
wintering area to breeding or summerareasof Bald Eagleswas2019
km. Adult Bald Eaglesapparently migrated alone in springwith mated
males leavingfirst. Migration flightsbegan between 1015-1045 H MST
and ended between1715-1745 H. Mean daily movementwas 180 km.
Migration flight speeds averagedabout 50 km/hr. Altitude of flight
ranged fi:om 30-4572 m aboveground level (AGL), butmost often was
between 1500-3050 m.
KEYWORDS: BaldEagle;, Haliaeetus leucocephalus;Colorado;
radio-tracking; Saskatchewan; vernalmigration; wintering. MIGRACION
PRIMAVERAL DE LAS XGUILAS CALVAS DESDE UN 3.REA DE INVERNACION
ALSUR DE COLORADO
RESUMEN.--Individuos adultosde figuila calva (Haliaeetus
leucocephalus) = 15) invernando en el valle (N de San Luis (VSL)
Colorado, fheron provistos con radios transmisores convencionales
montadosen la cola, entre el 1 de enero-18 de marzo de 1980 y 1981
para determinarlos patronesde migracitn y las fireasde
reproduccitn. Las tguilas migracitn fheron seguidas primera
instanciaen un vehiculo en en sencillo con dos rastreadores. En
]980, las figuilascon radios (N = 4) abandonaron los camposde
invernacitn en un intervalode 15 dias en marzo, pero las
partidasen 1981 (N = 7) fueron entremediados de fEbrero y
principios de abril. Las tguilasiniciaron la migracitn en dias con
rangosde temperaturasmas altos,mas nubes,y vientosmas altos que
otros dias durante el invierno o primavera. E1subsiguiente vuelofue
paraleloal movimientohaciael norte de la isoterma2Ctanto
temporalcomo espacialmente.Las localizaciones vias de paso de las
figuilasmigratoriasfheron similaresen 1980 y y 1981. Las cuatro
tguilaslocalizadas su rango de verano estuvierondentro de 102 km
una de otra cn en el nororiente de Saskatchewan noroccidente de
Manitoba. La distanciamedia desde el firea de y invernacitn del VSL
alas fireasde reproduccitn o de verano de las figuilascalvas fhe de
2019 km. Las figuilascalvasadultasaparentementemigraron solasen
prinaveramientrasque susmachospareja sal-
ieron primero.Losvuelos migraci6n de comenzaron entre 1015-1045H
MST y terminaronentre 17151745 H. E1 movimientomedio diario rue 180
kin. Las velocidades losvuelosdurante la migraci6n de tuvieronun
promediode 50 km/h. La altitud de vuelo tuvoun rango entre 30-4572
m.s.n.m.pero lamayoria a menudo estuvoentre 1500-3050 m.
[Traducci6n de C6sarMarquez]
Relatively stable wintering populations of Bald Eagles
(Haliaeetusleucocephalus) existed in the San Luis Valley (SLV) of
southern Colorado throughE-mail address: [email protected]
out the DDT era (Ryder 1965, AlamosaNational Wildlif Refuge
reports 1954-83). In the 1970s,a mean of 185 (SD = 66.6) Bald
Eagleswascountedin the SLV annually (N = 5; Craig 1981). However,
origins of eagles wintering in the SLV were unknown.
DECEMBER 2002
BALr> EAGLE VERNAL MIGRATION
257
Between 7January-18 March 1980 and 1981, 15 adult Between
December-April 1977 and 1978, winBald Eagleswere captured and
radio-taggedin the SLV tering Bald Eagles (N = 36) in the SLV were
All were captured by a modified "Lockhart" method marked with
yellow patagial wing markers to de- (Miner 1975) with and without
live Bald Eagle and Goldtermine their geographicorigins or breeding
areas en Eagle (Aquila chrysaetos) birds. Trap siteswere lure
(Harmata and Stahlecker1993). Colormarking re- chosen on the basis
of frequency and duration of the vealed fidelity of individual Bald
Eaglesto this win- presenceof two adult Bald Eaglesof
distinctlydissimilar size, presumablymated, within 1.6 km. In 1981,
I estitering area was high, but by January 1981 only mated the SI.V
winter population at 170 Bald Eagles three sightingsof
yellow-markedeaglesoccurred (Harmata 1984).outside the SLV and none
were in a documented
Mated status of radio-tagged Bald Eagles was determined by
frequency and duration of time spent in the 1978, after being
tracked locally for nearly six presence of another adult eagle of
distinctly dissimilar weeks,an adult Bald Eagle wearing a
conventional, size,observedcopulation (N = 3) in the SLV, or
association with a nest site on the breeding grounds (N = 2).
tail-mounted transmitter left her SLV wintering Eagles were
considered unmated if diurnal movements area. She was tracked for
two days over 300 km in the SLV were clearly independent of other
eaglesand through the most rugged portion of the Rocky they were
not observed to roost away from communal Mountains, being lost only
due to lack of logistical roostswith just one other eagle, asmated
birds often did Climatologicaldata associated with daysthat Bald
Eaplanning, not our ability to maintain contact. This gles left the
SLV were analyzedby stepwise discriminate serendipitousevent
revealed the potential of de- analysis(Dixon 1981) to
investigatemeteorologicalcontermining migration routes and breeding
areas of ditions associated with initiation of vernal migration.
ChBald Eagles usingconventional radio-tracking. This
matologicaldata recorded near the geographical center
nesting area or during summer (Harmata and Stahlecker 1993). The
primary objective of the marking program had not been realized.
Clearly, other methodswere required to achieveobjectives in a
timely and cost-efficient manner. Prior to the 1980s, technology
for remotely tracking long-range movementsof individual birds
(i.e., satellite platform transmitter terminals or PTTs) was not
available. However, on 28 February
Gender of radio-tagged eagles assigned methods was by presented
by Garcelon et al. (1985). Three were confirmed by behavior during
copulation. All Bald Eagles were radio-tagged with two-stage radio
transmitters mounted proximo-ventrallyon the tail. Transmitter
frequencieswere between 148.500 and 148.950 mHz. Unit life
expectancy was -->5 mo. Transmitter, antenna, and mounting tab
weighed 50-57 g. Telemetry receiving equipment included fixed
channel and programmablereceivers.
paper discusses results of subsequentlong-range tracking of Bald
Eagleswith conventional telemetry from one seasonalrange to another
prior to widespread use of PTTs on eagles (e.g., Grubb et al. 1994,
Brodeur et al. 1996, Meyburg et al. 2001). Objectivesof this study
were: (1) to determine breeding areas of adult Bald Eagles
wintering in southern Colorado and (2) to gather information
regarding factors associated with initiation of vernal migration,
routes, duration, stopover habitats used, and other factors
affecting the successful completion of migration.
of the SLV (Alamosa, Colorado) were obtained from Na-
tional Oceanographicand AtmosphericAdministration,
Asheville,North Carolina Monthly SummarySheets. Data for daysthat
eaglesleft the SLV were compared to data for daysrandomly
selectedbetween I January-15 April 1980 and 1981 that they did not.
Variables selected for comparisons among days were maximum, mean,
and range of temperature, percent of clear sky and mean wind speed.
Migrating eagleswere followed primarily from a single 4 X 4 vehicle
with two human trackers and a dog. An omnidirectional antenna and
two element "H" yagi receiving antenna were mounted on the roof of
the chase vehicle.The yagiwasattached a 360traversing to mount,
allowing for directional tracking while the vehicle was
moving.Manpowerand logistic limitationsplusvariability STUDY AREA
AND METHODS in departure dates,routes, and travel speedsof Bald
EaThe SLV is the largestand most southernof four large gles
prevented ground tracking of more than one eagle intermountain
basinsin Colorado. Encompassing 6475 at a time. One tracker drove
while the other operated km9, the SLV is approximately the size of
the state of the receiving equipment. Both shared navigational
duDelaware.Mean elevationof the nearly-level valleyfloor ties. Due
to often high chasespeeds(up to 150 km/hr) is 2286 masl. High
(>3050 m) mountain ranges border and off-road
"adventures,"visualcontactwith migrating the valley on eastand
west,merging at the northern end. eagles could not be maintained,
so migration behavior The Rio Grande and Conejosriversflow through
the SLV often could not be recordedcontinuously, accurately, or and
numerousnatural warm springsand wetlandsthat safely.Route,
direction, and speed of the chasevehicle, seldom freeze have made
the SLV attractive to waterfowl therefore, often was
selectedprimarily to maintain maxand Bald Eagles,probablyfor
millennia. Water develop- imum audio signal strength. When contact
with a mentsand agriculturein the 20th centuryhaveprobably grating
eagle waslost, an aerial searchwasimplemented improved the
attractiveness. using local air services. two or three element yagi
anA
258
HARMATA
VOL. 36, NO. 4
FEMALES
O
O
o
MALES
O
O
i
....
I
....
i
....
I
....
i
....
I
....
i
....
I
....
I
....
I
....
i
....
-40
-35
-30
-25
-20
-15
-10
-5
0
5
10
15
DAYS FROM VERNAL EQUINOX
Figure 1. Daysfrom vernal equinox that radio-tagged Bald
Eaglesdeparted (N = 12) their San LuisValley,Colorado, winter
rangesto initiate vernal migration,1978 (N = 1), 1980 (N = 4), 1981
(N = 7). Open circlesindicateeagles determined to be unmated,
closedcirclesindicate eaglesthat were mated (see text), and
squareindicateseagle ofundetermined mated status.
tenna was taped to the wing strut (high wing) or step (low wing)
of light aircraft and the area surveyedin transectstylewith
intermittent, "lazy" circlesat high altitude. Frequencieswere
scanned to searchfor the target eagle and other potential migrating
birds. When contactwas reestablishedand eagle's statusdiscerned as
stationary, vehicle trackingresumed.Flight altitudeswere estimated
based on eagle's position relative to the search aircraft and
trees, geograph)5and structuressuch as buildings and radio towers
while ground tracking. Eagles were monitored in their summering
groundswith Beaverair'craft, snow machines, and snow shoes.
RESULTS AND DISCUSSION
Initiation of Migration. Over half (58%) of radio-taggedBald
Eagleswith known departure dates (N = 12), initiated vernal
migration within one week after the vernal equinox (Fig. 1). In
1980, radio-tagged eagles (N = 4) all left the wintering grounds
within a 15-d span in March (Table 1). Range of known departure
dates (N = 8) spanned 50 d in 1981,with the first
confirmeddepartureof
Table 1. Residencyand mated statusof radio-taggedadult Bald
Eagleswith known departure dates from winter rangesin the San Luis
Valley (SLV), Colorado.MINIMUMRESIDENCY
EAGLEDESIGNATION SEX CAPTURED
DATEDEPARTED SLV
MATEDS[XI'US '
INSLV(DAYS)
378 180 280 380 480 181281 381
F F M M M FM F
21Jan. 1978 18Jan. 1980 23Jan. 1980 23Jan. 1980 27 Jan. 1980
7Jan. 19819Jan. 1981 9 Jan. 1981
28 Feb. 1978 21Man 1980 24Man 1980b 21Man 1980 8Man 1980 11 Feb.
198120 Feb. 1981 27Man 1981
U M M M U UU M M U?'
39 63 61 58 41 3643 78
481 581881
M FM
11 Jan. 1981 15 Jan. 198115Man 1981
27Man 1981 25Man 198121 Man 1981
76 707
981
F
18Man 1981
1Apn 1981
M
15
M = mated, U = unmated (see text).
First departed on 22 March 1980, but eagle encountereda winter
slorm and returned to winter range.Undetermined.
DECEMBER 2002
BALD EAGLE VERNAI. MIGRATION
259
Table 2. Mean climatological values(+99% confidence interval)
which discriminated (P-< 0.01) between days radio-taggedBald
Eaglesinitiated northward migration from their San Luis Valley,
Colorado wintering area (Depart) and daysthey did not
(Remain).TEMPERATURE PERCENT WIND
southerly winds; similar to conditions noted by Bagg et al.
(1950) for other birds. Although Bald Eaglesare apparentlysensitive
to local conditions when migration is initiated, they appear to be
cognizant of little beyond their mmediate environs.Eagle 280
departed the SLV on22 March 1980 and traveled 145 km north
before
RANGE
SKY
SPEED
(C)
CLE
(km/hr)
Depart (N = 11)Remain (N = 21) a' Randomlyselected.
24 (5)19 (4)
50 (26)56 (20)
22 (5)16 (5)
being stoppedby severesnowsqualls.He spentthe night on a
mountain passand as the storm persistedto the north the next day,
he returned to his winter range in the valley.He spent the
remainder of that day and part of the next in closeassociation with
a female (distinctlylarger eagle) and was observed to copulate
during this period. He initiated migration again on 24 March,
leaving the SLV forthe season.
a radio-tagged eagle occurring in mid-February and the last in
early April (Table 1). Number of daysfrom the equinox that
eaglesdepartedwasnot different between genders (Mann-Whitney U =
16.50, P = 0.81). However, mated eaglesdeparted the SLV closer to
the equinox and later than unmated eagles (Mann-Whitney U = 3.00, P
= 0.03; Fig. 1). Three climatological variablesdiscriminated
between days that radio-tagged Bald Eagles left the SLV on
migration and those they did not (Table 2). Eaglesinitiated
migration on dayswith much larger range of temperatures, more
clouds, and with higher winds than other daysover the winterspring
period. Migration tended to be initiated about 5 hr after sunrise
regardlessof the immediate meteorological condition.
Photoperiodismis consideredto be the ultimate stimulusfor the onset
of migration in birds (King and Farner 1963). All radio-taggedBald
Eaglesdetermined to be mated left the SLV within 12 days of the
vernal equinox, indicating breeding adults may be sensitive equal
periodsof light and dark. to Unmated, nonbreeding, or immature
eagles may be equally sensitivebut may not be driven by pressure to
procreate. In fact, first observations subof adult eagles in
northern ranges both years were not until at least two weeks
subsequentto the arrival of the first radio-tagged adults.
Migrational movement also paralleled the northward movement of the
2C isotherm (Lincoln 1979) both temporallyand spatially, hinting
that thermal cues may also be involved in the initiation of
migration. Once the urge is kindled, proximate affectors of Bald
Eaglemigration appear to be coincidentwith incoming low pressure
systems,associatedwide range of temperatures, cyclonic air flows,
and
Migration Routes, Destination, and Navigation. In 1980, a
relatively narrow migration corridor through Colorado, Wyoming, and
Montana was used by three radio-tagged eagles (Fig. 2). Eagle 380
became sedentary in north-central Saskatchewan after 15 d of
migration, six of which were spent sitting out bad weather. After
four days of almost continual solitary soaring, he was fbund
perched close to a larger eagle where he remained fbr severalhours.
The next day the pair began construction of a new nest on Chachukew
Lake, north
of Pelican Narrows, Saskatchewan. They eventually fledged one
young in August 1980. In 1981, eagle 281 left the SLV on 20
February, and wasfollowed for two daysover 302 km through the
mountains of western Colorado (Fig. 3). Direction was primarily
northwest and 281 was the only eagle tracked that crossedthe
Continental Divide. Subsequent tracking of 281 was interrupted
becausea radio of the samefrequencyand pulserate as 281 was tracked
for a day before I discoveredit was a transmitter on a collar of a
bighorn sheep( Ovis canadensis).
Eagle 981 left the SLV on 1 April and was followed to about 65
km north of Casper,Wyoming,where she was lost in a snow storm
because roads
became impassable. Eagle 481 was detected approaching Fort Peck
Reservoir in east-centralMontana on 5 April. Afier leaving Fort
Peck Reservoir, 481 flew due north about 35 km then
graduallyshifted to a northwest course. This route took him
directly to the Missouri Coteau, a ridge line running
longitudinally for over 150 km in southwestern Saskatchewan.
continued north along the He Coteau for about 100 kin. Approaching
the Canadian Shield, 481 made a sudden change in course
260
HARMATA
VOL. 36, NO. 4
Figure 2. Partial migrationroutesof radio-tagged adult Bald
Eaglesfrom winter rangesin the San Luis Valley, Colorado in 1978
and 1980. Numbers indicate eagle designation and year (Table 1).
Summeringarea of eagle 380 is indicated by () (confirmed nest
location).
from north to northeast, paralleling the direction of many
elongatedlakesand riversin the Canadian Shield country. Eagle 481
was subsequentlyfollowed to Reindeer Lake, Saskatchewan, arriving
on malfunction, a dead eagle, or a bird in incubation 10 April. On
22 April, 481 was found by air asso- posture.Lack of contactwith
other SLV eaglesmayciated with a nest site on Kamuchawie Lake in
west-
Figure 3. Partial migration routesof radio-taggedadult Bald
Eagles from winter ranges in the San Luis Valley (SLV), Colorado in
1981. Numbers indicate eagle designation (Table 1).
Summeringareasare indicated by () (nest locations). Respective1981
and 1982 recovery locations BaldEagles of bandedin the SLV
(January1977) are indicatedby (). The October 1981 recoverylocation
of a Bald Eagle radio-tagged the SLV in March 1981 in is indicated
by 781.
have been a function of transmitter failure (one
ern Manitoba near the Saskatchewanborder (Fig.3).
wasknown to fail in the SLV), premature sheddingof the
transmitter (one transmitter was found still
Aerial surveys covering 210000 km2 of north- attached to broken
tail feathersbelow a perch tree ca.ern Saskatchewan were conducted
to detect signals of other SLV Bald Eagles in mid- to late April in
both 1980 and 1981. During aerial surveys, eagle 181 waslocated on
the Pagato River, south of Reindeer Lake, Saskatchewan, and eagle
581 was detected briefly in the area of Trade Lake, south of the
Churchill River (Fig. 3). The signal was weakand intermittent,
which indicated either a radio
in the SLV), mortality, incomplete surveycoverage, or
summeringareaswere located outsideof northern Saskatchewan.
However, the fact that 31% of
potentially-detectable eagles(N = 13) were located within an
area roughly the size of Yellowstone National Park (ca. 9300 km2)
suggests mostadult that Bald Eagleswintering in the SLV originated
fromthis area of Canada.
DECEMBER 2002
BALD EAGLE VERNAL MIGRATION
261
Mean distance (as the eagle flies) from the SLV wintering area
to breeding or summer areas of Bald Eagleswas 2019 km (N = 4, SD =
50). All eagleslocated on their summer rangeswere within102 km of
each other in northeastern Saskatche-
the remainder of migration clearly compensated for the one day
blown off course (Fig. 2). Unless the eagle had been exposed to the
area on previous migrations, true navigation is indicated. However,
it is not unreasonable to assume that an adult
eagle over five years old, may indeed be familiar with a great
portion of western North America as a result of vagariesof
previousmigrations. Regardless,redundancy in navigational systems
has been illustrated for homing pigeons (Columba livza) (Able 1980)
and in all probability, severalbackup navigational systems are
available to Bald Eagles, especiallyexperienced adults. ed the
journey in only about 40% of the time. Migration Behavior.Mated
adult Bald EaglesapHowever,the Arizona eaglewasa subadult (3rd yr)
parently migrated alone in the spring. Eagle 380 and presumablywas
not driven by the impetus to was seen roosting solitarilyat all but
one stopover nest, as were adult SLV eaglestracked. location (six
other eagleson the Yellowstone River Geography,celestialcues, and
weather may all near Hysham, Montana). Eagle 481 did not miplay a
role during migration of Bald Eagles.Prom- grate with a mate, but
often moved northward with inent physiographic featuressuchasdeep
canyons, other adult eagles.He roosted with other eagles rivers,
and north-south oriented topography three times, but two
sitesobviously were not comseemed to assistin visual navigation
during flight munal or traditional because of lack of similarity
(Griffin 1943). These features could have been im- to typical Bald
Eagle roost sites (Kiester and Anprinted in the memory of
eaglesduring their ini- thony 1983). Paired roostingwas probably a
result tial migrations and experience dictated direction of
facilitatory behaviorinfluenced by poor weather during subsequent
flights (kinesis theory; Mat- and lack of daylightremaining.
Subsequent obserthews 1963). Imprinting of migration routes from
vations of migration flight confirmed he moved wintering
areas,which compliment survivalduring alone. Once in the boreal
forest of the Canadian the first year, would be more adaptive than
im- Shield country, local eagles appeared to "meet" him in flight
and escort him through their terriprinting during first
southwardmigration. Eaglesdid not migrate on daysof total overcast,
tories, but no overt agonistic encounters were obwith other eagles
a phenomenonalsonoted by Gerrard and Gerrard served. Occasional
associations (1982). The altitude of the cloud layer may have
appeared incidental for all radio-taggedeaglesand an effect, but
overcastlayers over 90 m were not were of short duration. Solitary
migration behavior experienced during tracking. Sun compassorien-
in established pairswould reduce the possibility of tation (Kramer
1952, 1957) with time compensa- both members being lost in a local
catastrophe. tion, commonly referred to as sun-azimuthorienDaily
migration flights consistentlybegan beH MST and ended between
tation (Welty 1982) may, therefore, be important tween 1015-1045
for orientation of adult Bald Eaglesduring migra- 1715-1745 H. Mean
daily movementwas 180 km, tion. but ranged from 144-435 km (N = 5)
in 1980 and True navigation (selection of a compassdirec- 33-248 km
(N = 5) in 1981. Speed of migration tion toward a known goal in
unfamiliar territory; flights recorded averaged50 km/hr (N = 7,
22Able 1980) may alsobe a component of Bald Eagle 144 km/hr in
1980; N = 9, 20-105 km/hr in 1981). migration. Migrating
eaglesappear to avoid strong Altitude of flights recorded ranged
from 30-4572 winds during migration because strong winds ap- m AGL,
but most often was between 1500-3050 m. parently influence
direction of flight. Eagles did Total distances,speed, and daily
duration of minot move, except locally, during dayswhen winds
gratory flights indicate that under optimal weather in excessof 35
km/hr occurred prior to 0900 H. conditions, Bald Eagles can reach
their breeding Eastwindsof 60-80 km/hr began about two hours
grounds within 6 d after leaving the SLV. Pennyafter initiation of
eagle 380's flight on one migra- cuick (1975) indicated a 2000 km
migration for a tion day and his flight path deviatedwell west of
bird the size of a Bald Eagle would be near maxidirect line to the
eventualgoal. Flight direction for mum attainable without eating,
assuminga 25%
wan or northwestern Manitoba (Figs.2 and 3). Duration of travel
betweenwinter and summer range for SLV Bald Eagleswith known
departure and arrival dates (N = 2) was 15 d. Although SLV Bald
Eaglestraveled about 2,5 the distanceto their summer ranges
compared to one Bald Eagle tracked by satellite from Arizona to the
Northwest Territories (3032 km, Grubb et al. 1994), they
complet-
262
HARMATA
VOL. 36, No. 4
massloss.During this study,no radio-taggedeagles were
observedfeeding during migration and mean migration distance was
2020 km. Captive eagles commonly fast more than two weekswith no
apparent deleterious effects (Brown and Areadon 1968, pets.
observ.) and wild raptors can lose up to 30% of body masswithout
problems (Newton 1979). These compensatory capacities undoubtedly
allowed adult Bald Eaglesto reach their breeding grounds with
sufficient energy reserve for breeding. All radio-tagged
eaglesarrived in their summer range at a time when lakesand most
stretches of rivers were still frozen. The only areasof open water
were rapids or narrows on rivers or between lakes.Radio-tagged
eaglesspentmost of their time there, presumablyforaging for fish.
Other eagles observedduring aerial surveys were associated with
ubiquitousholesin lake ice and viscera piles of fish left by native
commercial fishing operations. Foraging eagles were also seen on or
near caribou (Rangifertarandus)and moose (Alces alces) carcasses
killed by natives or wolves (Canis lupus).A few eagleswere seen
with snowshoe hare (Lepusamercanus) remains.
Stopoverareas used during vernal migration of SLV Bald
Eagleswere generallywidely distributed. A tree of adequate size,
secure from human disturbance in any type habitat, was all that
seemed necessary roosting. In late March 1982, Swenfor son (1983)
counted 232 Bald Eaglesalong the YellowstoneRiver between the mouth
of the Bighorn River and Miles City, Montana. A site where eagle380
roosted is in the middle of this stretch and
within an area where the river seldom freezes, con-
tained the most highly braided portion of channel, most
heavilywooded islands,and highest Canada goose (Branta canadensis)
populations of three sections of river studied by Hinz (1974). Use
of this sectionof the Yellowstone River by adultsmay be dictated
primarily by tradition and availabilityof water, becausemigrating
adults were not known to feed during this study. Areas in eastern
Montana may be equally important to asmany or more migrating
eaglesasthe more highly-publicized areas, where ephemeral
concentrations of eagles occur in western Montana.Leightonet al.
(1979) estimated population a of 14000 Bald Eagles in Saskatchewan.
Some eagles from north-central Saskatchewan were captured in autumn
at Hauser Lake (Restani et al. 2000) and Glacier National Park
(McClelland et al. 1982), while others passedthrough eastern
Mon-
Regional Relationships. McClelland et al. (1994) noted that Bald
Eaglesradio-taggedin Glacier National Park, Montana, in autumn
wintered west ofthe Continental Divide and summered in the
tana during migration (Gerrard et al. 1978, Harmata et al.
1985). Both Hauser Lake and Glacier
MacKenzie River watershed of northern Alberta, northwest
Saskatchewan, and Northwest Territo-
ries. The summer range of a Bald Eagle that wintered in Arizona
(west of the Divide) also was in the MacKenzie River watershed
(Grubb et al.
1994). McClelland et al. (1994) suggested that wintering areas
may be related to the watershed of origin and Bald Eaglesbe managed
by application of a "Migration FlywayConcept." Adult Bald
Eaglesradio-taggedand banded in the SLV wintering area (east of the
Divide) were tracked to breedingareas in Saskatchewan and Manitoba,
all in the
National Park Bald Eagle concentrations now are defunct due to a
collapsed, exotic food base (kokanee salmon, Oncorhynchus nerka),
but eastern Montana habitatsstill support large numbersof native
prey (lagomorphs,ungulates,waterfowl). However, lack of a
concentrated food base, diffusion of
Churchill River watershed. Jenkins et al. (1982) followed two
adult Bald Eaglesradio-taggedin Wyoming during winter. One trapped
on the west sideof the the east Continental side of the Divide
Divide was fbllowed to to the the
MacKenzie River watershed,while one trapped onwas followed
Churchill
River watershed,
similar to those from
the SLV. These data suggesta "Churchill-East Slope" Migration
Flywayexists,distinct from the "Mackenzie-Intermountain" Flyway
proposed byMcClelland et al. (1994).
roost sites,solitary habits of migrating eagles,plus dispersion
of departure dates from winter (this study) and summer ranges
(Harmata et al. 1985), prohibit any accurateestimateof numbersof
Bald Eaglespassingthrough easternMontana. Relatively low numbers of
eagles present at any particular time at some stopover areas in
eastern Montana may belie the true importance of these areas to
migrating eagles.Turnover of individualsappeared to be daily, over
months. Therefore, western prairie statesmay provide important
migratory habitat for a large proportion of the continental
population of Bald Eaglesover long periods.ACKNOWLEDGMEN
J. Stoddart initially suggestedBald Eagle research in the SI,V.
Dale Stahleckerservedas co-investigator during
DECEMBER 2002
BAI,D EAGLE VERNAl, MIGRATION
263
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Received21 December 2001; accepted6 July 2002
