vernal migration of bald eagles from a southern colorado wintering

j. RaptorRes'.36(4):256-264
¸ 2002 The Raptor ResearchFoundation, Inc.
VERNAL
MIGRATION
OF BALD EAGLES
COLORADO
WINTERING
FROM
AREA
A SOUTHERN
ALAN R. HARMATA 1
Fish & WildliJ•b
Program,Department
ofEcology,
Montana StateUniversity,
Bozeman,
MT 59717 U.S.A.
ABSTRACT.--Adult
Bald Eagles(Haliaeetusleucocephalus)
(N = 15) winteringin the San Luis Valley (SLV),
Colorado were radio-taggedwith conventionaltail-mounted transmittersbetween 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 departuresin 1981 (N = 7) rangedfi:ommid-Februaryto
earlyApril. Eaglesinitiated migrationon dayswith higher temperatureranges,more clouds,and higher
winds than other daysduring winter or spring. Subsequenttravel paralleled the northward movement
of the 2øCisothermboth temporallyand spatially.Locationsand pathwaysof migratingeagleswere
similar in both 1980 and 1981. All four eagleslocated on their summer range were within 102 km of
each
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), but
most
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 AL
SUR
DE
COLORADO
RESUMEN.--Individuos
adultosde figuila calva (Haliaeetusleucocephalus)
(N = 15) invernando en el valle
de San Luis (VSL) Colorado, fheron provistoscon radios transmisoresconvencionalesmontadosen la
cola, entre el 1 de enero-18 de marzo de 1980 y 1981 para determinarlos patronesde migracitn y las
fireasde reproduccitn. Las •tguilasen migracitn fheron seguidasen primera instanciaen un vehiculo
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 entre
mediados 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.
E1subsiguientevuelofue paraleloal movimientohaciael norte de la isoterma2øCtanto temporalcomo
espacialmente.Las localizacionesy vias de paso de las figuilasmigratoriasfheron similaresen 1980 y
1981. Las cuatro •tguilaslocalizadascn su rango de verano estuvierondentro de 102 km una de otra
en el nororiente de Saskatchewany noroccidente de Manitoba. La distanciamedia desde el firea de
invernacitn del VSL alas fireasde reproduccitn o de verano de las figuilascalvasfhe de 2019 km. Las
figuilascalvasadultasaparentementemigraron solasen pri•naveramientrasque susmachospareja sal-
ieron primero.Losvuelosde migraci6ncomenzaronentre 1015-1045H MST y terminaronentre 17151745 H. E1 movimientomedio diario rue 180 kin. Las velocidadesde losvuelosdurante la migraci6n
tuvieronun promediode 50 km/h. La altitud de vuelo tuvoun rango entre 30-4572 m.s.n.m.pero la
mayoria 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 through-
out the DDT era (Ryder 1965, AlamosaNational
Wildlif• Refuge reports 1954-83). In the 1970s,a
mean of 185 (SD = 66.6) Bald Eagleswascounted
in the SLV annually (N = 5; Craig 1981). However,
origins of eagles wintering in the SLV were un-
E-mail address:[email protected]
known.
256
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)
lure birds. Trap siteswere
(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
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-efficientmanner.
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-taggedeagleswasassignedbymethods
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 programmable
receivers.
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 associatedwith daysthat Bald Eaplanning, not our ability to maintain contact. This
gles left the SLV were analyzedby stepwisediscriminate
serendipitousevent revealed the potential of de- analysis(Dixon 1981) to investigatemeteorologicalcontermining migration routes and breeding areas of ditions associatedwith initiation of vernal migration. ChBald Eaglesusingconventionalradio-tracking.This matologicaldata recorded near the geographicalcenter
paper discussesresults 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 associatedwith 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 yagiwasattachedto a 360øtraversingmount,
allowing for directional tracking while the vehicle was
moving.Manpowerand logisticlimitationsplusvariability
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. Encompassing6475 at a time. One tracker drove while the other operated
km9, the SLV is approximatelythe size of the state of the receiving equipment. Both shared navigational duDelaware.Mean elevationof the nearly-levelvalleyfloor 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.A two or three element yagi an-
258
HARMATA
FEMALES
O
O
MALES
ß
O
i
....
I
-40
VOL. 36, NO. 4
....
-35
i
O
....
I
-30
oß
....
-25
i
-20
....
I
....
ß
i
-15
ß
....
I
-10
....
-5
I
....
I
0
....
5
i
....
10
15
DAYS FROM VERNAL EQUINOX
Figure 1. Daysfrom vernal equinox that radio-taggedBald 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 of
undetermined
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.
MINIMUM
RESIDENCY
EAGLE
DESIGNATION
DATE
SEX
CAPTURED
DEPARTEDSLV
MATED
INSLV
S•[XI'US
•'
(DAYS)
378
180
280
380
480
181
F
F
M
M
M
F
21Jan. 1978
18Jan. 1980
23Jan. 1980
23Jan. 1980
27 Jan. 1980
7Jan. 1981
28 Feb. 1978
21Man 1980
24Man 1980b
21Man 1980
8Man 1980
11 Feb. 1981
U
M
M
M
U
U
39
63
61
58
41
36
281
381
M
F
9Jan. 1981
9 Jan. 1981
20 Feb. 1981
27Man 1981
43
78
481
581
M
F
11 Jan. 1981
15 Jan. 1981
27Man 1981
25Man 1981
U
M
M
U
881
M
15Man
21 Man
?'
7
981
F
18Man 1981
M
15
1981
1981
1Apn 1981
M = mated, U = unmated (see text).
First departed on 22 March 1980, but eagle encountereda winter slorm and returned to winter range.
Undetermined.
76
70
DECEMBER 2002
BALD EAGLE VERNAI. MIGRATION
Table 2. Mean climatologicalvalues(+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
southerly winds; similar to conditions noted by
Bagg et al. (1950) for other birds.
Although Bald Eaglesare apparentlysensitiveto
local conditions when migration is initiated, they
appear to be cognizant of little beyond their •mmediate environs.Eagle 280 departed the SLV on
22 March
PERCENT
WIND
RANGE
SKY
(øC)
CLE•
(km/hr)
SPEED
Depart (N = 11)
24 (5)
50 (26)
22 (5)
Remain (N = 21) a
19 (4)
56 (20)
16 (5)
'• Randomlyselected.
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 sensitiveto equal periodsof light and dark.
Unmated, nonbreeding, or immature eagles may
be equally sensitivebut may not be driven by pressure to procreate. In fact, first observationsof subadult 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 2øC 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
259
1980 and traveled
145 km north
before
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 for
the season.
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 gradually
shifted
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.He continued north along the
Coteau for about 100 kin. Approaching the Canadian Shield, 481 made a sudden change in course
260
HARMATA
Figure 2. Partial migrationroutesof radio-taggedadult
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).
VOL. 36, NO. 4
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 locationsof BaldEaglesbandedin the SLV (January1977)
are indicatedby (•). The October 1981 recoverylocation
of a Bald Eagle radio-taggedin the SLV in March 1981
is indicated by 781.
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 eaglesmay
ciated with a nest site on Kamuchawie
Lake in west-
have been a function of transmitter failure (one
ern Manitoba near the Saskatchewanborder (Fig.
wasknown to fail in the SLV), premature shedding
3).
of the transmitter (one transmitter was found still
Aerial surveys
coveringca. 210000 km2 of north- attached to broken tail feathersbelow a perch tree
in the SLV), mortality, incomplete surveycoverage,
or summeringareaswere located outsideof north-
ern Saskatchewanwere 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 weak
potentially-detectableeagles(N = 13) were located
within an area roughly the size of YellowstoneNational Park (ca. 9300 km2) suggests
that mostadult
Bald Eagleswintering in the SLV originated from
and intermittent,
this area
which indicated
either a radio
ern Saskatchewan. However, the fact that 31% of
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 within
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. How-
102 km of each other
ever, it is not unreasonable to assume that an adult
in northeastern
Saskatche-
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 2,5the 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-
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 systemshas been
illustrated for homing pigeons (Columba livza)
(Able 1980) and in all probability, severalbackup
navigational systemsare 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 YellowstoneRiver
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 physiographicfeaturessuchasdeep canyons, other adult eagles.He roosted with other eagles
rivers, and north-south oriented topography three times, but two sitesobviouslywere 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. Subsequentobserthews 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 oba phenomenonalsonoted by Gerrard and Gerrard served. Occasional associationswith other eagles
(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 establishedpairswould reduce the possibilityof
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%
262
HARMATA
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 compensatorycapacitiesundoubtedly 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 stretchesof
rivers were still frozen. The only areasof open water were rapids or narrows on rivers or between
lakes.Radio-taggedeaglesspentmost of their time
there, presumablyforaging for fish. Other eagles
observedduring aerial surveyswere associatedwith
ubiquitousholesin lake ice and viscerapiles of fish
left by native commercial fishing operations. Foraging eagles were also seen on or near caribou
(Rangifertarandus)and moose (Alcesalces)carcasses killed by natives or wolves (Canis lupus).A few
eagleswere seen with snowshoehare (Lepusamer•canus) remains.
Regional Relationships.McClelland et al. (1994)
noted that Bald Eaglesradio-taggedin Glacier National Park, Montana, in autumn wintered west of
the
Continental
Divide
and
summered
in
VOL. 36, No. 4
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
necessaryfor roosting. In late March 1982, Swenson (1983) counted 232 Bald Eaglesalong the YellowstoneRiver between the mouth of the Bighorn
River and Miles City, Montana. A site where eagle
380
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 YellowstoneRiver 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) estimateda population
of 14000 Bald Eagles in Saskatchewan.Some eagles from north-central Saskatchewanwere captured in autumn at Hauser Lake (Restani et al.
2000) and Glacier National Park (McClelland et al.
1982), while others passedthrough eastern Mon-
the
tana during migration (Gerrard et al. 1978, Har-
MacKenzie River watershed of northern Alberta,
northwest Saskatchewan, and Northwest Territo-
mata et al. 1985). Both Hauser Lake and Glacier
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) suggestedthat 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 breeding
areas in Saskatchewan and Manitoba,
all in the
Churchill River watershed.Jenkins et al. (1982) followed two adult Bald Eaglesradio-taggedin Wyoming during winter. One trapped on the west side
of
the
Continental
Divide
was
fbllowed
to
the
MacKenzie River watershed,while one trapped on
the
east
Churchill
side
of
the
Divide
River watershed,
was followed
to
the
similar to those from
the SLV. These data suggesta "Churchill-East
Slope" Migration Flywayexists,distinct from the
"Mackenzie-Intermountain" Flyway proposed by
McClelland et al. (1994).
National Park Bald Eagle concentrationsare now
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
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-investigatorduring
DECEMBER 2002
BAI,D EAGLE VERNAl, MIGRATION
263
preliminary marking studies.P. Harmata (then age 5)
Bald Eagles Haliaeetusleucocephalus
fi'om Arizona and
helped track the first migrating Bald Eagle in 1978. G.
Michigau with satellite telemetry. Pages 347-358 zn
Montopoli wasinstrumental for successthlcompletion of
B.-U. Mcyburg and R.D. Chancellor fEDS.I, Raptor
the first year's migration tracking. M. Lockhart provided
conservationtoday. World Working Group on Birds
details of eagle capture techniques and field assistance.
of Prey. Pica Press,London, U.K.
S. Werner and E. Spettigue participated in winter and
migration tracking.L. Stevensonof PelicanNarrows,Sas- It^RMATA,A.R. 1984. Bald Eaglesof the San Luis Valley,
Colorado; their winter ecologyand spring migrauon.
katchewan, donated flight expertise and air time. FinanPh.D. dissertation, Montana State University, Bozecial assistancewas provided by R. Koteen; Fred Jcnse,
man, MT U.S.A.
Dept. of Veteran'sAffairs;J. Lincer and W. Clark of the
National Wildlifk Federation; R. Phmkctt of the National
--,
.i.E. TOEPFER,ANDJ.M. GERP,
Am). 1985. Fall miAudubon Society;D. Flath of Montana Fish,Wildlilk, and
gration of Bald Eagles produced in northern SasParks; I,. Jahn of the Wildlife Management Institute and
katchewan. BlueJay43:56-62.
American PetroleumInstitute;T. Ingram of EagleValley --•ND
D.W. STAHLECI4ER.
1993. Fidelity of Bald
Environmentalists;S. Rainey and C. Merrit of American
Eagles to wintering grounds in southern Colorado
Wilderness Alliance; M. Malone, Montana State Univerand northern New Mexico. J. Field Ornithol. 64:129sity (MSU) Research CreativityProgram; R. Eng, MSU
Agricultural Experiment Station;and R. Moore, MSU Biology Department. T. Grubb, M. Raune, M. Rcstani,and
D. Stahlccker made helpful comments on earlier drafts
of the manuscript."Sarge" wasthe dog, a great companion and a loyal friend.
134.
thNz, T.C. 1974. Seasonalactivity,numbers and d•str•bution of' Canada Geese (Branta canadensis)in the
lower YellowstoneValley, Montana. M.S. thesis,Montana State University,Bozeman, MT U.S.A.
JEN•UNS,
M.A., T.P. MCENEANEY,
I,. HANEmmY,raNDJ.R
LITERATURE CITED
SQUIRES.
1982. Bald Eagle (Haliaeetusleucocephalus)
esseutial habitat on and near Bureau of Laud ManageABLE, K.P. 1980. Mechanisms of orientation, navigation
ment lands in Wyoming. Dral• final report--FY 1981
and homing. Pages284-373 in S.A. Gauthreaux,Jr.
& 1982. Wintering Bald Eagles.U.S. Dept. of Interior,
[ED.], Animal migration, orientation and navigation.
Fish & Wildl. Service, Denver, CO U.S.A.
Academic Press, New York, NY U.S.A.
BAGG,A.M., W.W.H. GUNN, D.S. MILLER,J.T. NICHOLS,W.
SMITH, AND F.P. WOLFORT}I.1950. Barometric pressure-patterns and springbird migration. Wilso•Bull.
KIES'IER,G.P., JR. AND R.G. ANTHONY.1983. Characteristics of Bald Eagle communal roosts in the Klamath
Basin, Oregon and California.J. WiMl. Manage.47
1072-1079.
62:5-19.
BRODEUR, S., R. D•;•.CAmE, D.M. BIRD, AND M. FULLER.
1996. Complete migration cycle of Golden Eagles
breeding in northern Quebec. Condor98:293-299.
BROWN,L.H. AND D. AMADON.1968. Eagles,hawks, and
falcons of' the world. Vol. 2. McGraw-Hill Book Co.,
New York, NY U.S.A.
CP,
AtG,J.R. 1981. Bald and Golden Eagle winter population surveys.Wildlife ResearchReport: Part 1. Federal
Aid Job Final Report. Colorado Division of Wildlife,
Denver, CO U.S.A.
K•NG,J.R. ANDD.S. FARNER.1963. The relationship of fat
deposition to Zugunruhe. Condor65:200-223.
KP,AMER,G. 1952. Experiments on bird orientation. lbzs
94:265-285.
--.
1957. Experimentson bird orientation and their
interpretation. ibis 99:196-227.
I•EIGHTON,F.A.,J.M. GEPmAm),
P. GERP,:•P,D,
D.W.A. WH•TFIELD,ANDW.J. MAHER.1979. An aerial censusof Bald
Eaglesin Saskatchewan.
J. Wildl. Manage.43:61-69.
LINCOI.N,F.C. 1979. Migration of birds. In S.R. Peterson
D•XON, W.J. 1981. BMDP. StatisticalSoftware. Univ. of
California Press,Berkeley, CA U.S.A.
and P.A. Anastasi fEDs. I, Fish & Wildlife Service C•r-
GARCEI•ON, D.K., M.S. MARTELL, P.T. RFDIG, AND L.C.
U.S.A.
cular 16. U.S. Dept. of Interior, Washington, DC
BUOEN.1985. Morphometric, karyotypic, and laparo- Mxr'rnEwS, G.V.T 1963. The orientation of pigeons as
affected by the learning of landmarksand by the disscopictechniquesfor determining sex in Bald Eagles.
tance of displacement.Anita. Behar.11:310-317.
J. Wildl. Manage.49:595-599.
GEP,
P,AP,
D, J.M., D.W.A. WmTFmLD,P. GEaP,
AUD,P.N. GEP,- MCCLELLAND, B.R., L.S. YOUNG, D.S. SHEA, P.T MCCLELI•ND, H.L. ALLEN, PuNDE.B. SPETTIGUE. 1982. The
m•V,
t3, ANDWJ. MAHER.1978. Migratory movements
Bald Eagle concentration in Glacier National Park,
and plumage of subadult SaskatchewanBald Eagles.
Can. Fidd-Nat. 92:375-382.
Montana: origin, growth and variation in numbers.
Living Bird 19:133-155.
--AND
P.N. GEmaARD.
1982. The spring migration of
L.S. YOUNG,P.T. MCCLELLAND,
J.G. CRENSHAW,
Bald Eaglesin the vicinity of Saskamon.Bluejay 40: --,
56-60.
H.L. ALLEN,AND D.S. SHEA.1994. Migration ecology
GRIFFIN,D.R. 1943. Homing experimentswith Herring
of Bald Eaglesfrom autumn concentrationsin Glacier
National Park, Montana. Wildl. Monogr.125.
Gulls and Com•non Terns. Bird-banding14:7-23.
GRUBB, T.G., W.W. BOWERMAN,AND P.W. HOWEY. 1994.
MEYt•URG,B-U., D.H. Et.L•S,C. MEY•U•C.,J.M. MENDELSON,ANDW. SCHt•;LLER.
2001. Satellite tracking of two
Tracking local and seasonalmovementsof wintering
264
HARMATA
Lesser Spotted Eagles, Aquila pomarina, migrating
from Namibia.
Ostrich 72:35-40.
M•NE•t,N.R. 1975. Montana Golden Eagle removal and
translocationproject. Pages155-162 in Proc. of the
Second Great Plains Wildlife Damage Control Workshop. U.S. Fish & Wildlife Service,Denver,CO U.S.A.
NEWTON,I. 1979. Populationecologyof raptors.Buteo
Books, Vermillion,
SD U.S.A.
PFNNYCUICK,
C.J.1975.Mechanicsof flight. Pages1-75 in
D.S. Farner and J.R. King [EDS.],Avian biology.Vol.
V. Academic Press, New York, NY U.S.A.
RESTANI,M., A.R. HARMATA,AND E.M. MADDEN. 2000. Nu-
merical and functionalresponses
of migrant Bald Ea-
VOl. 36, NO. 4
gles exploiting a seasonallyconcentrated food source.
Wilson Bull. 102:561-568.
RYDER, R.A. 1965. A checklist of the birds of the Rio
Grande drainage of southern Colorado. Colorado
State Univ. Press, Ft. Collins, CO U.S.A.
SWENSON,
J.E. 1983. Is the northern interior Bald Eagle
population in North America increasing?Pages2334 in D.M. Bird, Biology and management of Bald
Eaglesand Ospreys.Harpell Press,Ste.Anne de Bellevue, Quebec, Canada.
WELTY,
J.C. 1982. The life of birds, 3rd Ed. W.B. Saunders
Co., Philadelphia, PA U.S.A.
Received21 December 2001; accepted6 July 2002