) 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 reserves 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- bildstein@hawkmtn.org 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 observations. 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 during 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, cclloouuddccoovveerrhwaadsa132 g) should switch from LITERATURE CITED soaring to flapping flight (Hedenstrom 1993). For energy-minimizing migration, the switch Allen, P. E., L. J. Goodrich, and K. L. Bildstein. 1996. Within- and among-year effects of cold from soaring to flapping flight occurs at alow- fronts on migrating raptors at Hawk Mountain, er climbing rate. Thus, as the rate ofclimbing Pennsylvania, 1934-1991. Auk 113:329-338. decreases, time-minimizing migrants should Bildstein, K. L. 1999. The forced migration of the switch from soaring to flapping flight sooner Broad-winged Hawk. Pages 79-102 in Gathering Careau et al. • BROAD-WINGED HAWK MIGRATION All ofangels: migrating birds and theirecology (K. P. tropical landbird migrants. Studies in Avian Bi- Able, Ed.). Cornell University Press, Ithaca, New ology 20:1-133. York. Pennycuick, C.J. 1972. Animal flight. EdwardArnold Bildstein, K. L. and M. Saborio. 2000. Spring mi- Ltd., London, England. gration counts ofraptors and New World vultures Pennycuick,C.J. 1998. Fieldobservationsofthermals in Costa Rica. Ornitologia Neotropical 11:197- and thermal streets, and the theory ofcross-coun- try soaring flight. Journal of Avian Biology 29: 205. 33-43. Bildsgtreaitni,onK.alLo.nganthdeJ.MeIs.oZaamlelreisc.a2n00L1a.ndRapCotrorridmoir-. Penny1c9u79i.ckS,oaCr.inJg.,mTi.grAatlieornstoafm,theanCdomB.moLnarCsrsoann.e Pages 119-136 in Hawkwatching in the Americas Grusgrusobserved by radarand from an aircraft. (K. BildsteinandD. Klem,Eds.). HawkMigration Ornis Scandinavica 10:241-251. Association of North America, North Wales, PORRAS-PENARANDA, P., L. ROBICHAUD,ANDF. BRANCH. Pennsylvania. 2004. New full-season count sites for raptor mi- Danchin, E., L. A. Giraldeau, T. J. Valone, and R. gration in Talamanca, Costa Rica. Ornitologia H. Wagner. 2004. Public information: from nosy Neotropical 15:267-278. neighbors to cultural evolution. Science 305:487- SAS Institute, Inc. 2002. SAS JMP, ver. 5.0.1. Cary, 491. North Carolina. Fuller, M. R., W. S. Seegar, and L. S. Schueck. Shannon, H. D., G. S. Young, M. A. Yates, M. R. 1998. Routes and travel rates of migrating Pere- Fuller, and W. S. Seegar. 2002. 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SOARING AND GLIDING FLIGHT OF MIGRATING BROAD-WINGED HAWKS: BEHAVIOR IN THE NEARCTIC AND NEOTROPICS COMPARED

Similar SOARING AND GLIDING FLIGHT OF MIGRATING BROAD-WINGED HAWKS: BEHAVIOR IN THE NEARCTIC AND NEOTROPICS COMPARED