Table Of Content)
The Wilson Journal ofOrnithology 118(4):471—477, 2006
SOARING AND GLIDING FLIGHT OF MIGRATING
BROAD-WINGED HAWKS:1 BEHAVIOR IN THE NEARCTIC AND
NEOTROPICS COMPARED
VINCENT CAREAU,14 JEAN-FRAN^OIS THERRIEN,15 PABLO PORRAS,2
DON THOMAS, AND KEITH BILDSTEIN36
—
ABSTRACT. We compared migrating behavior of Broad-winged Hawks (Buteo platypterus) at two sites
along their migration corridor: Hawk Mountain Sanctuary in eastern Pennsylvania and the Kekoldi Indigenous
Reserve in Limon, Costa Rica. We counted the number of times focal birds intermittently flapped their wings
and recorded the general flight type (straight-line soaring and gliding on flexed wings versus circle-soaring on
fully extended wings). We used a logistic model to evaluate which conditions were good for soaring by calcu-
lating the probability of occurrence or absence of wing flaps. Considering that even intermittent flapping is
energetically more expensive than pure soaring and gliding flight, we restricted a second analysis to birds that
flapped during observations, and used the number offlaps to evaluate factors influencing the cost ofmigration.
Both the occurrence and extent offlapping were greater in Pennsylvania than in Costa Rica, and during periods
ofstraight-line soaring and gliding flight compared with circle-soaring. At both sites, flapping was more likely
during rainy weather and early and late in the day compared with the middle of the day. Birds in Costa Rica
flew in larger flocks than those in Pennsylvania, and birds flying in large flocks flapped less than those flying
alone or in smaller flocks. In Pennsylvania, but not in Costa Rica, the number of flaps was higher when skies
were overcast than when skies were clear or partly cloudy. In Costa Rica, but not in Pennsylvania, flapping
decreasedastemperature increased. Ourresultsindicatethatbirdsmigratinginlargeflocksdo somoreefficiently
than those flying alone and in smaller flocks, and that overall, soaring conditions are better in Costa Rica than
in Pennsylvania. We discuss how differences in instantaneous migration costs at the two sites may shift the
species’ migration strategy from one of time minimization in Pennsylvania to one of energy minimization in
Costa Rica. Received 15 November2005, accepted 8 July 2006.
Each year, more than one million Broad- mass exponent of 1.17 (Pennycuick 1972),
winged Hawks (Buteo platypterus make a large-bodied migrants are penalized compared
round-trip migration of 6,000-10,000 km with small-bodied migrants in that they need
along the Mesoamerican Land Corridor when a disproportionately larger fat reserve to ac-
traveling between their North American complish a non-stop, powered-flight migration
breeding grounds and wintering areas in Cen- of a given distance. As such, long-distance
tral and South America (Bildstein and Zalles migration represents a potentially acute ener-
2001). Because the power requirement for getic challenge for large-bodied migrants such
continuous, flapping flight has an allometric as Broad-winged Hawks (265-560 g; Good-
rich et al. 1996). In fact, measures of fat re-
serves at the onset of migration suggest that
bro1oDkeep,t,QCdeJ1bKio2loRg1i,e,CaUnnaidva.. de Sherbrooke, Sher- Broad-winged Hawks do not carry the fuel
2Asociacion ANAI, Costado Norte de Cancha de supply needed to sustain powered flight be-
Futbol del Colegio Monterrey, Vargas Araya, San Pe- tween their breeding and wintering grounds
dro, San Jose, Costa Rica. without also feeding en route (Bildstein 1999).
3Acopian Center for Conservation Learning, Hawk There are two possible solutions to this en-
Mountain Sanctuary, 410 Summer Valley Road, Or-
wigsburg, PA 17961, USA. ergetic challenge. First, large-bodied migrants
4Current address: Dept, des sciences biologiques, may complete their migration in stages, paus-
Univ. du Quebec a Montreal, 141 Av. President-Ken- ing periodically to feed and replenish fat re-
nedy, C.P. 8888 succursale Centre-ville, Montreal, QC serves en route. Second, if their flight me-
H3C 3P8, Canada; etCentre d’etudes nordiques, Univ. chanics permit, they may significantly reduce
Laval, QC G1K 7P4, Canada. the energetic costs associated with powered
d’e5tCuudrersenntordaidqdureess,s:UniDve.ptL,avadle, QbiColoGg1ieK e7tP4,CeCnatnr-e flight by relying instead on soaring and glid-
ada. ing flight. Although ducks, geese, and many
6Corresponding author; e-mail: shorebirds and landbirds exploit the first strat-
[email protected] egy (Moore 2000), Broad-winged Hawks do
471
472 THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 118, No. 4, December2006
not feed substantially when migrating, partic- and temporal windows of soaring opportunity
ularly in the tropics (Bildstein 1999), possibly act to group the birds during their migrations
because their sit-and-wait foraging strategy (Smith 1985). Alternatively, others have spec-
does not lend itself well to the high capture ulated that soaring migrants, such as Broad-
rates needed for the rapid accumulation of fat winged Hawks, actively form groups because
reserves. Instead, they rely heavily on gliding doing so allows them to gather information
and soaring flight to complete their long-dis- (e.g., Danchin et al. 2004) about the location
tance movements (Smith et al. 1986). Because and strength of individual thermals passively
basal metabolic rate (BMR) increases with provided by individuals traveling with them
mass by an allometric exponent of approxi- (Kerlinger 1989).
mately 0.75, soaring and gliding flightbecome As Broad-winged Hawks travel south in au-
increasingly cost-efficient as mass increases tumn, it is likely that they adjust their flight
(Hedenstrom 1993). Indeed, it has been esti- behavior to accommodate changes in the
mated that 100 g of fat would fuel powered abundance and strength of the thermals they
flight for only about 5 days in Broad-winged encounter. At the onset of migration in late
Hawks, but it would sustain soaring flight in summer in the temperate zone, the sun’s
the species for an estimated 20 days (Smith et height in the sky and overall solar intensity
al. 1986). begin to decline (Bildstein 1999); the stron-
Soaring flight is based on the conversion of gest and greatest abundance ofthermals tends
the energy in atmospheric aircurrents into pri- to occur episodically during the several days
marily potential energy (Pennycuick 1972). In of fair weather that typically follow the pas-
North America, soaring Broad-winged Hawks sage ofcold fronts (Allen et al. 1996). Farther
gain altitude while circling in thermals and ri- south in the tropics, the sun’s height in the sky
ding deflection updrafts with their wings and and solar intensity remain relatively more
tails fully spread, and then gliding on flexed constant during the migration period and ther-
wings along their preferred direction of travel mal strength appears to vary primarily as a
as they convert the altitude gained into dis- function of local cloud cover (Smith 1980).
tance traveled while seeking the next thermal It has been suggested that the movements
or updraft along their migratory route (Kerlin- of soaring migrants are less constrained in the
ger 1989). In Central America, where the spe- tropics than in the temperate zone and that
cies also alternately soars and glides among their flight patterns differ in the two regions
small thermals, it also straight-line soars and (Bildstein and Saborio 2000). For example,
glides in the much larger tropical thermals and Fuller et al. (1998) reported that the migration
“thermal streets” (sensu Smith 1985) found speed of satellite-tracked Swainson’s Hawks
in that region. (Buteo swainsoni) soaring and gliding be-
Because the distribution, abundance, and tween breeding grounds in western North
strength of thermals and updrafts are affected America and wintering areas in Argentinawas
by topography, vegetation cover, vertical tem- 42% greater in the tropics than in the temper-
perature gradient of the atmosphere, and in- ate zone. Here, we compare the flightbehavior
tensity of solar radiation, soaring flight im- of Broad-winged Hawks at temperate and
poses constraints on the spatial and temporal tropical sites to test three main predictions: (1)
organization of migration (Kerlinger 1989). because soaring conditions are better in the
Soaring migrants are able to migrate efficient- tropics, birds would begin flying earlier in the
ly only when sufficient solarradiation and low day and flap less there than in the temperate
cloud cover favor the production of thermals, zone; (2) birds within a given site would flap
thus concentrating individuals in specific sea- less at highertemperatures and less cloud cov-
sonal and daily time windows. Also, in the er; and (3) birds would flap less when trav-
temperate zone, thermals often occur in small, eling in large flocks than when traveling alone
localized pockets, which sometimes force or in smaller flocks.
soaring birds to fly close to each other when
METHODS
using the same thermal. This has led some to
suggest that flocking behavior occurs passive- We observed migrating Broad-winged
ly among soaring migrants, as limited spatial Hawks in the temperate zone at Hawk Moun-
X
Careau et al. • BROAD-WINGED HAWK MIGRATION 473
tain Sanctuary in the Central Appalachian partly cloudy skies versus complete overcast)
Mountains ofeastern Pennsylvania(40° 58' N, at hourly intervals. We also noted time ofday
74° 59' W; 464 m ASL) on 10-28 September as time after sunrise (06:45 EST in Pennsyl-
2002, during peak passage at that site. Hawk vania and 05:25 CST in Costa Rica) and then
Mountain straddles the 300-km-long Kittatin- divided the day into three periods (early, mid,
ny Ridge, which acts as a leading line for rap- and late) to simplify analyses. At both sites,
tor migrants in the region (Bildstein 1999). In the early period included the first 4 hr after
the tropics, we observed migrating Broad- sunrise, the mid-period the next 4 hr, and the
winged Hawks from a 10-m tower at the Kek- late period the next 3 hr. We did not record
oldi Indigenous Reserve, southeast of Puerto flight behavior later in the day.
Viejo in Talamanca, Limon, Costa Rica (9° We performed all analyses using the JMP
38' N, 82°47' W; 200 m ASL) on 2-19 Oc- 5.0.1 statistical package (SAS Institute, Inc.
tober 2002, during peak passage at that site 2002). We used non-parametric Mann-Whit-
(Porras-Penaranda et al. 2004). The Caribbean ney U-tests to compare mean onset ofactivity
Sea, ~2 km to the north, and the Talamanca and flock size between Pennsylvania and Cos-
Mountains, ~5 km to the south, funnel birds ta Rica. To allow comparisons between soar-
through the region’s coastal lowlands, making ing and gliding phases of flight, we restricted
this area one of several major concentration our analyses to 30-sec sequences in which the
points along the Mesoamerican Land Corridor focal bird remained in one flight phase (soar-
(Bildstein and Zalles 2001). ing or gliding). We conducted two general
We used 7 X 35 binoculars and a 20-60 analyses. The first examined which conditions
zoom telescope to watch birds at each site be- enabled soaring and gliding flight without
tween sunrise and 18:00 EST. Individual ob- flaps. The second examined factors that influ-
servations were made on a focal individual enced the extent offlapping when it did occur.
during a 30-sec sample period beginning as For the first analysis, we divided observa-
soon as the bird was identified as a Broad- tions into those during which the bird did or
winged Hawk. The 30-sec length represented did not flap. We ran a stepwise logistic re-
a fair trade-off between gaining a representa- gression that included all independent vari-
tive record of flight behavior and losing con- ables (site, flight phase, flock size, tempera-
tact with the focal bird before the observation ture, and cloud cover) and two-way interac-
period was completed. During our observa- tions. The odds ratio (OR) measures how the
tions, we recorded the number of seconds the fitted probability is multiplied as the regressor
focal individual spent (1) circle-soaring in an changes from its minimum to its maximum for
individual thermal and (2) straight-line soar- continuous data, or from one category to the
ing and gliding between thermals and along other for nominal data (Hosmer and Leme-
thermal streets. When circle-soaring, birds as- show 1989). We used the log-likelihood ratio
cended thermals on fully outstretched wings (L-R) test to determine P-values. The second
with their tails fanned. When straight-line analysis was restricted to birds that flapped at
soaring and gliding, birds flew on semi-flexed least once while we were observing them. For
wings with theirwingtips and tails partly fold- each site, we conducted an ANCOVA on the
ed. We also recorded the number offlaps (i.e., number (logl0-transformed) offlaps, according
individual wing beats) and used it as a mea- to the flock size, flight phase, temperature, and
sure of powered flight. cloud cover. Data are presented as means ±
We determined flock size by counting or SE.
estimating the number ofbirds soaring within
RESULTS
the same thermal or soaring and gliding in the
same flight line as the focal bird. In Pennsyl- We made 1,537 30-sec observations of
vania, flocks were composed of only Broad- Broad-winged Hawks during 13 days in Penn-
winged Hawks. In Costa Rica, however. sylvania and 2,103 observations during 15
Broad-winged Hawks sometimes commingled days in Costa Rica. In Costa Rica, flocks
with Swainson’s Hawks and Turkey Vultures ranged in size from 2 to >1,000 individuals
(Cathartes aura in mixed-species flocks. We (mean = 427 ±10; median = 140). In Penn-
)
noted temperature and cloud cover (clear and sylvania, flock size never exceeded 350 indi-
474 THE WILSON JOURNAL OF ORNITHOLOGY • Vol 118. No. 4. December2006
9
Early Mid-day Late Rainyweather
Time of day and weather
FIG. 1. Mean numbers of wing flaps per 30-sec observation period (±SE) in relation to time of day and
rain condition in Pennsylvania. USA. 10-28 September 2002. and in Talamanca. Costa Rica. 2-19 October
2002. Numbers above the error bars represent sample sizes.
viduals (mean = 26 ± 1; median = 10). Over- At both sites, birds were more likely to flap
all, flock size was significantly greater in Cos- early and late in the day than at mid-day
ta Rica than in Pennsylvania (U = 1177.6, P (Pennsylvania: L-R x2= 67.1. P < 0.001; Cos-
< 0.001). The first migrant of the day was ta Rica: L-R x2 — 68.6. P < 0.001; Fig. 1).
sighted almost one hour later in Pennsylvania Flapping was greater during rainy periods at
than in Costa Rica (198 ± 53 min aftersunrise both sites, but significantly so only in Costa
versus 150 ± 11 min after sunrise, U = 6.32, Rica (Pennsylvannia: L-R x2 = 3.84. P =
P = 0.022), and the first individuals sighted 0.051: Costa Rica: L-R x2 = 78.6. P < 0.001).
each day were more likely to flap in Pennsyl- To account for these effects, we excluded
vania than in Costa Rica (35% versus 16%, from the analyses that follow any observa-
L-R x2 = 162.7, P < 0.001). tions made early and late in the day and dur-
ing rainy weather.
The logistic model indicated which condi-
TABLE 1. Results of the logistics model for the tions favored soaring flight (Table 1) and the
occurrence of flapping flight among Broad-winged ANCOVA identified which factors determined
aHnadwkinsTianlPaemnannscyal.vaCnoisat.aURSiAca..120--1298OScetpotbeemrb2e0r022.00b2y. the extent of powered flight when it occurred
temperature (°C), flock size, flight type (circle or (Table 2). Both extent and probability of flap-
straight-line soaring), and cloud cover (overcast or ping were greater during straight-line soaring
not). The log-likelihood (L-R) x2 and P-value are and gliding than during circle-soaring (Fig. 2;
shown. Sample size is 2,153. OR = 0.3). The overall flapping probability
Term df L-Rx2 p was lower when birds flew in larger flocks
than in smaller flocks or alone (OR = 2.8).
Site 1 10.24 <0.001
Temperature 1 111.56 <0.001 There was no significant difference between
Site X temperature 1 55.13 <0.001 flapping rates in Pennsylvania and Costa Rica
Flock size 1 16.76 <0.001 when birds flew in flocks of up to 50 birds
Flight type 1 77.63 <0.001 (L-R x2 = 3.75, n = 1.038, df = 1, P =
Cloud cover 1 15.24 <0.001 0.053); however, when birds were in flocks
n
Careau et al. • BROAD-WINGED HAWK MIGRATION 475
TABLE 2. Comparisons offactors influencing the numbers offlaps perobservation ofBroad-winged Hawks
in Pennsylvania, USA, 10-28 September 2002, and in Talamanca, Costa Rica, 2-19 October 2002. The AN-
COVA was restricted to birds that flapped at least once during the observation.
Pennsylvania(n = 208) CostaRica( = 156)
df /•"-ratio p /r-ratio p
Flock 1 0.10 0.76 9.17 0.003
Flight type 1 14.20 <0.001 18.63 <0.001
Temperature 1 1.42 0.24 8.34 0.005
Cloud cover 4 3.29 0.012 1.70 0.16
that ranged in size from 51 to 350 birds, flap- Costa Rica (site X temperature interaction
ping probability was significantly lower in term, OR = 0.03). Accordingly, temperature
Costa Rica than it was in Pennsylvania (L-R had an effect on the number of flaps in Costa
X2 = 10.25, n = 468, df = 1, P = 0.001). Rica but not in Pennsylvania (Table 2).
More than 94% of the Broad-winged Hawks
seen in Costa Rica were flying in flocks of DISCUSSION
>50, and flapping was far more likely in
P=en1n.s8y7;lvFaingi.a2)t.hMaonreitovwears, itnheCnoustmabeRricoaff(lOapRs dbaesnSecidencoeonfHoutbfhsefearkveeaxrtisito(en1ns8c9eo7f)osffoiartrshtienrgpmrbaoilvriddsu,epddmraaefvntiys-
decreased with flock size in Costa Rica, but
studies have shown that avian flight can be
not in Pennsylvania (Table 2).
used to gather information on meteorological
Overall, the probability of flapping was processes (Shannon et al. 2002). We present
greater during periods of complete overcast
t=ha1n.7i)t;whaoswewvheern, cclloouuddccoovveerrhwaadsa<n7e5ff%ec(tOoRn osouarrdiantgacaosndaitbiioolnosgifcoarl Bmreotahdo-dwifonrgemdeasHuarwiknsg
traveling between the temperate zone and the
the number of flaps only in Pennsylvania (Ta-
tropics during southbound migration in au-
ble 2). Although probability of flapping de- tumn, and we offer a preliminary indication
creased as temperature increased (minimum
temperature = 15° C, maximum temperature ofhow differences in soaring conditions affect
= 31° C, OR = 574.1); the relationship was the efficacy of migratory flight in the species.
In general, our observations confirm the
significantly weaker in Pennsylvania than in
flight behavior of soaring migrants document-
ed elsewhere (Kerlinger and Gauthreaux
449 1985, Spaar and Bruderer 1997, Spaar et al.
1998). For example, as temperatures and solar
radiation increase each morning, birds rely
less on flapping flight and more on soaring
and gliding flight, presumably to reduce the
energetic costs of travel by taking advantage
of the stronger mid-day thermals.
The negative correlation between flapping
rates and flock size suggests that Broad-
winged Hawks use information available in
flocks to increase their flight efficiency (Ker-
linger 1989). That said, although smaller flock
Circlesoaring Straight-linesoaringandgliding sizes and higher flapping rates in Pennsylva-
Flighttype
nia were probably due at least in part to this
FIG. 2. Mean number of wing flaps per 30-sec ob- effect, smaller and weaker thermals in Penn-
servation period (± SE) made by Broad-winged Hawks sylvania also may have contributed to a great-
incircle soaring orstraight-line soaringandglidingflight
in Pennsylvania, USA, 10-28 September 2002, and in er likelihood of flapping at the site.
Talamanca, Costa Rica, 2-19 October 2002. Numbers We suggest that migrating Broad-winged
above the errorbars represent sample sizes. Hawks do not pursue a pure soaring and glid-
476 THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 118, No. 4, December2006
ing strategy throughout their migration be- than energy-minimizing migrants (Heden-
cause they are constrained from doing so in strom 1993). These temporal and energetic as-
two ways. First, they cannot soar when ver- pects may explain why Broad-winged Hawks
tical airspeeds within thermals fail to reach a are more likely to resort to flapping in Penn-
critical threshold value, and second, they can- sylvania than in Costa Rica.
not glide efficiently when inter-thermal dis- Our observations indicate that Broad-
tances exceed their maximum gliding range winged Hawks shift from a mixed strategy of
(Kerlinger 1989). Our data indicate that soaring and gliding supplemented by powered
Broad-winged Hawks respond to these con- flight to a nearly pure strategy of soaring and
straints by using powered flight preferentially gliding as they proceed during their south-
during straight-line soaring and gliding flight bound migrations, suggesting that the instan-
and secondarily when circle-soaring. This tac- taneous metabolic cost of migration declines
tic also has been observed in migrating Com- from north to south. By relying more on pow-
mon Cranes (Grus grus\ Pennycuick et al. ered flight in the north, where conditions are
1979), as well as in other raptor species. By less favorable for soaring, Broad-winged
stretching inter-thermal glides with flapping Hawks may trade off energy against time, a
flight, birds increase the distance realized, phenomenon also observed in Levant Spar-
thereby extending their ability to reach and rowhawks (Accipiter brevipes\ Spaar et al.
use the next thermal (Pennycuick 1998). Sec- 1998). This would allow them to move along
ond, under certain circumstances, soaring and the corridor at a faster rate at the expense of
gliding can slow travel compared with flap- depleting fat reserves.
ping flight, particularly when the birds are Finally, we highlight the fact that we did
soaring in small thermals. Indeed, migrants not discriminate adult from juvenile Broad-
are likely to pursue a pure soaring strategy winged Hawks, and that we observed mi-
only when they have time to wait forthe prop- grants at only two sites. Additional observa-
er conditions and are able to move slowly tions in which the flight behaviorofadults and
along the migration corridor. For Broad- juveniles are compared and in which other
winged Hawks, time limitations may be more species are observed at other temperate and
important in Pennsylvania than in Costa Rica, tropical sites are likely to provide important
because solar intensity and photoperiod de- insights into the extent to which age and lat-
crease rapidly during September in Pennsyl- itudinal geography affects the flight behavior
vania, thereby forcing birds to move south in of migrating birds of prey.
a brief window of time (Bildstein 1999). On
ACKNOWLEDGMENTS
the otherhand, solar intensity and photoperiod
remain relatively high and constant year-round This study was conducted as part of VC and JFT’s
in Costa Rica, resulting in a more prolonged undergraduate research project at the Universite de
window oftime for hawk movements (Porras- Sherbrooke and when VC and JFT wereConservation
Penaranda et al. 2004). Science Interns at Hawk Mountain Sanctuary (sup-
morAes laikreelsyultt.o uBsreoaad-twiimneg-meidniHmaiwzaktsiomnasytrabte- tpisoutrmetrneed2db0ey021’fEoidrnuetciegranntsisoatntudHdieuaswQskucehMboeolcaur)ns.thaiWipnsefStrahnoacmntkuthaterhyeMiaannu-d-
egy in temperate than in tropical zones, re- the interns and Bribri people at Kekoldi Indigenous
sulting in a higher flapping rate in Pennsyl- Reservefortheirassistanceandhospitality. Finally,we
vania. Assuming an energy consumption of are grateful to R. Spaar, P. Kerlinger, C. J. Farmer, and
approximately 4 X BMR in soaring flight and three anonymous referees for helping us improve our
a climbing rate of 1 m/sec, flight theory pre- bmuatniuosncrtioptC.onTsheirsviastiHoanwSkciMeonucne,taniunmSbaenrct1u3a5r.yContri-
dicts that during time-minimizing migration,
heavy birds (>132 g) should switch from LITERATURE CITED
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