MIGRATION PATTERNS, USE OF STOPOVER AREAS, AND AUSTRAL SUMMER MOVEMENTS OF SWAINSON’S HAWKS

Abstract

From 1995–1998, we tracked movements of adult Swainson’s Hawks (Buteo swainsoni) using satellite telemetry to characterize migration, important stopover areas, and austral summer movements. We tagged 46 hawks from July - September on their nesting grounds in seven U.S. states and two Canadian provinces. Swainson’s Hawks basically followed three routes south on a broad front, converged along the east coast of central Mexico, and followed a concentrated corridor to a communal austral summer area in central Argentina. North of 20° N, southward and northward tracks differed little for individuals from east of the Continental Divide but differed greatly (up to 1700 km) for individuals from west of the Continental Divide. Hawks left the breeding grounds mid-August to mid-October; departure dates did not differ by location, year, or sex. South migration lasted 42 to 98 days, and north migration took 51 to 82 days. On south migration, 36% of the Swainson’s Hawks departed the nesting grounds nearly 3 weeks earlier than the other radio marked hawks and made stopovers 9.0 – 26.0 days long in seven separate areas, mainly in the southern Great Plains, southern Arizona and New Mexico, and north-central Mexico. The austral period lasted 76 to 128 days. All Swainson’s Hawks used a core area in central Argentina within 23% of the 738800 km² austral summer range where they frequently moved long distances (up to 1600 km). Conservation of Swainson’s Hawks must be an international effort that considers habitats used during nesting and non-nesting seasons including migration stopovers.

INTRODUCTION

The Swainson’s Hawk (Buteo swainsoni) is one of many avian species whose long distance migration has implications for management and conservation of species (e.g., Webster et al. 2002, Greenberg and Mara 2005, Bildstein 2006). Migration is a critical period for many birds, and data from breeding and non-breeding periods are needed to manage migratory bird populations (Sillett and Holmes 2002). Documenting the timing and location of year-round movements of wide-ranging birds is essential for identifying factors that influence their survival and for developing conservation strategies (Steenhof et al. 2005, McIntyre et al. 2008).

Most Swainson’s Hawks migrate between the breeding grounds in North America and the austral summer areas in the pampas of South America (England et al. 1997). Mass mortality of Swainson’s Hawks from organophosphate pesticide poisoning in Argentina during the 1994 – 1995 austral summer (Woodbridge et al. 1995, Goldstein et al. 1999a) prompted an international conservation effort to assess threats to these hawks during migration and on the austral summer grounds. This effort included toxicological assessments (Goldstein et al. 1999b), a study of habitat relationships of Swainson’s Hawks in Argentina (Canavelli et al. 2003), and investigations of their migration and movements using satellite telemetry. In reporting the latter investigations, Schmutz et al. (1996) and Martell et al. (1998) described migration routes and timing of one Swainson’s Hawk from Saskatchewan and of five hawks from Minnesota. Fuller et al. (1998) and Bechard et al. (2006) described migration timing, routes, distances, and travel rates of 34 adult Swainson’s Hawks from seven U.S. states and two Canadian provinces in 1995 and 1996. Our paper builds on this work and includes unpublished data on hawks radio marked in 1997.

Although information has been gathered from band recoveries (Houston and Schmutz 1995) and satellite telemetry tracking (Woodbridge et al. 1995, Schmutz et al. 1996, Fuller et al. 1998, Martell et al. 1998, Bechard et al. 2006), little is known about Swainson’s Hawk ecology while on migration, use of stopover areas, and movements on the austral summer grounds (England et al. 1997). In this paper, we characterize annual long-range movement patterns and more thoroughly delineate migration routes of adult Swainson’s Hawks from separate nesting localities throughout their breeding grounds. In addition, we identify important stopover areas and describe movements and identify areas used by Swainson’s Hawks during the austral summer.

METHODS

FIELD PROCEDURES

Adult Swainson’s Hawks were radio marked on the nesting grounds in seven U.S. states and two Canadian provinces July-September 1995–1997 (Table 1). We solicited colleagues, many who provided funding, throughout the breeding range to capture and radio mark hawks. Specific trapping sites were selected by local cooperators. Trapping occurred near nests (<50 m) using dho gaza nets with a Great Horned Owl (Bubo virginianus) lure or away from nests with bal chatri traps baited with live gerbils (Gerbillus spp.; Bloom et al. 2007). We followed established guidelines (e.g., Hull and Bloom 2001) when capturing and handling hawks. All unbanded birds received an aluminum U.S.G.S. leg band. Those banded in California, Idaho, Oregon, and Utah also received an anodized colored leg band with alphanumeric symbols (Acraft Sign & NamePlate Company, Edmonton, AB). All hawks in Alberta were wearing U.S.G.S. and anodized alphanumeric leg bands when trapped. We weighed and measured 32 of the captured hawks and used weight, wing chord, and/or footpad length to sex individuals (Kochert and McKinley 2008). Four Swainson’s Hawks not weighed or measured were classed to gender based on their size relative to their mates (the larger hawk was considered the female) or behavior (the bird attending the young at the nest was considered the female).

Table 1.

Number of Swainson’s Hawks marked with satellite-received transmitters by year and location, 1995 – 1997.

		Number				Dates
Year	Location	Males	Females	Unknown	All	Deployed
1995	Southeastern Alberta	1	1		2	24 Aug
	Northern California		1		1	4 Sept
	Central Colorado			2	2	13 and 18 Sep
	Southwestern Idaho		4		4	18 to 25 Aug
	Northeastern Utah			1	1	29 Jul
	Subtotal	1	6	3	10
1996	Southeastern Alberta	1	4		5	10 to 31 Aug
	Southeastern Arizona		1	1	2	21 and 23 Sep
	Northern California	1			1	6 Sep
	Central Colorado			3	3	29 and 30 Aug
	Southwestern Idaho	1	5		6	29 Aug to 4 Sep
	Southwestern Minnesota		2		2	24 Jul
	Northeastern Oregon		6		6	6 to 27 Aug
	Southwestern Saskatchewan			2	2	27 and 28 Jul
	North-central Utah		3		3	6 to 8 Sep
	Subtotal	3	21	6	30
1997	Southwestern Minnesota		2		2	22 and 23 Jul
	Southwestern Idaho	1	3		4	6 to 21 Sep
	Subtotal	1	5		6
All years	GRAND TOTAL	5	32	9	46a	22 Jul to 23 Sep

^aNo data received from one hawk from Idaho in 1995 and one hawk in Saskatchewan in 1996.

We attached Platform Transmitter Terminals (PTTs; PTT 100, Microwave Telemetry Inc., Columbia, MD) to Swainson’s Hawks using a backpack harness (Dunstan 1972) constructed from 6-mm Teflon ribbon (Bally Ribbon, Bally, PA). The PTT and harness weighed between 32 and 36g, and we radio marked hawks only if they weighed ≥ 900 g.

DATA COLLECTION AND PROCESSING

We used the ARGOS-TIROS satellite system (Strikwerda et al. 1986, Argos 2007) to obtain location estimates of PTTs. ARGOS assigned each location estimate to a nominal Location Class (LC) based on their accuracy estimates. Standard location classes (LC = 3, 2, and 1) have an estimated 1-sigma error radius of 250, 500, and 1500 m, respectively, while the accuracy of auxiliary location classes (LC = 0, A, B, C, and Z) was undocumented at the time of our study (Argos 2007).

Each PTT transmitted for eight hr (hereafter a “transmission period”), every 1 to 6 days (hereafter a “duty cycle”). During transmission periods, PTTs transmitted approximately every 60 sec. We assigned data received from PTTs into three duty cycle categories based on the average elapsed time between each transmission period start date and time: Category 1 <1.5 days, Category 2 = 1.5–3.5 days; and Category 3 >3.5 days. We focused intensive study on south migration and programmed most PTTs for more frequent transmissions (Categories 1 and 2 duty cycles) on south migration with less frequent transmissions (Category 3) during the rest of the year to conserve battery life. We programmed all PTTs on a Category 1 duty cycle and half on a Category 2 duty cycle to change to Category 3 during the austral season. We also programmed a few PTTs on a Categories 2 and 3 duty cycle to not change in any season to maintain consistency among seasons.

We filtered all location estimates, except LC 3s, using the Douglas (2006) Argos-Filter that evaluated Argos locations based on two independent methods. The first filtering method required that locations have at least one other location that was consecutive in time and redundant in space, which we defined as <15 km. The second filtering method evaluated movement rates and turning angles among consecutive location estimates. We defined 90 km/hr to be a maximum rate of movement. All locations that passed the first filter were retained and considered “anchor points.” If the distance between two consecutive anchor points was >15 km, locations that passed the second filter during the intervening period were individually evaluated with respect to an additional directionality test. If passing through the candidate location did not increase the total distance traveled by >50%, the location was accepted. The Argos-Filter selected one location per duty cycle based on the best LC, or in case of LC ties based on the sequence: the highest Argos IQX value, the most messages received during the satellite overpass, and the highest IQY value (Douglas 2006). After filtering, lengths of each vector formed by two consecutive locations were computed as orthodromes (great circle routes) and the azimuth of each vector as the true departure bearing. The migratory journey (i.e., tracking path) for each individual consisted of a series of vectors connecting all location estimates, and a route was a collection of tracking paths for several birds that delineated a common course of travel. The migration corridor consisted of all paths used by all birds.

We defined four annual movement periods: south migration, austral summer, north migration, and breeding season. South migration began with the first location of a Swainson’s Hawk that was >150 km and south (<270° and >90°) of its capture location where the bird continued its southward movement. Departure azimuth for south migration was the azimuth from deployment location to the first location estimate on south migration. South migration ended when the hawk stopped consistent southward movement of >150 km and started omnidirectional movements on the austral summer grounds. North migration began with the first location of consistent northward movements (>150 km per duty cycle) away from the austral summer area and ended with the first location <150 km from its capture location. Departure azimuth for north migration was the azimuth from the last location on the austral summer area to the first location on north migration. Austral summer movements occurred between the end of the south migration and the start of the north migration. Locations <150 km of the capture site were considered to be on the breeding grounds. We defined stopovers as movements of ≤150 km for ≥24 hr during south and north migration and with movements of >150 km per duty cycle before and after the stopover. A prolonged stopover lasted > 9 days. We defined a ‘data gap’ as a period with no location estimates that spanned ≥ 1 duty cycle for Category 3 duty cycles or 12 days for the other categories. A gap at the beginning or end of migration precluded using the bird in calculations of migration duration, departure dates, arrival dates, and departure azimuths.

STATISTICAL ANALYSES

We analyzed spatial data with ARC GIS 9 Geographic Information System (GIS) software (Earth Systems Research Institute Inc., Redlands, California). We report distance moved as the sum of the length of all vectors during each season. To assess short-term tracking velocities (e.g., ground speeds; Pennycuick 2008), we examined lengths and velocities of within-duty-cycle vectors that spanned >1 h, extended ≥50 km, and had successive location estimates of LC A or better. We report long-distance travel rates based on elapsed time and cumulative tracking distance between the start of migration and the first location on the austral grounds.

To quantify the progression of migration we interpolated between successive location estimates the date (day of year) and time that vectors for each bird crossed 10° latitudinal lines between 30° N and 30° S. We calculated total passage duration as the interval (in days) between the dates and times that vectors of the first and last Swainson’s Hawk crossed each 10° line. We calculated travel rates between the 10° latitudinal lines based on the elapsed time and distance between crossings. We restricted our analyses to 25 Swainson’s Hawks that completed south migration in 1996 and 20 that completed north migration in 1997.

We modeled cumulative tracking distance as a function of variables that we believed could influence spatiotemporal progression of migrating Swainson’s Hawks. Because preliminary observations suggested that duty cycle may influence tracking distance we also included duty cycle in the model. We modeled cumulative tracking distance separately for south and north migrations. We first evaluated differences in distance due to duty cycle using analysis of variance. We addressed year marked as a random effect; but the contribution of variance by year was inconsequential, so it was evaluated as a fixed effect with the other variables. We used Akaike’s Information Criteria (AIC_c; sample size corrected, Burnham and Anderson 2002) and residual analyses to identify the simplest model that was satisfactory, based on duty cycle, longitude and latitude, and any necessary interactions. Once this simplest model was identified, additional predictors (year marked, departure date (day of year), departure azimuth, and number of stops on migration) were considered one at a time to verify whether they should be included. None of the resulting AIC_c scores were materially different from the simplest model, justifying their exclusion. Thus we considered only the simplest model for analysis of migration distance. We conducted the modeling with PROC MIXED (SAS Institute 2006) using maximum likelihood for comparable AIC_cs.

We employed Ranges 6 v1.106 software (Kenward et al. 2002) to define the austral summer range for Swainson’s Hawks and core use areas within that range. We used the best location estimate for each transmission period for all hawks in all years to develop a 100% minimum convex polygon to delineate the austral summer range. We delineated core areas by hierarchical incremental cluster analysis with a “nearest neighbor” joining rule (Kenward 1987) and used clusters that included 90% of locations to define core use area. We used clusters formed by 80% of locations to assess Swainson’s Hawk movements on the austral grounds because they retained the maximum number of locations, yet provided an adequate number of clusters to demonstrate movement within the core area. To assess seasonal hawk movements on a landscape scale, we categorized the 80% clusters into those located in the north and northeast portions of the austral range and those in the southeast portion of the range. We restricted our assessments to 22 Swainson’s Hawks that carried functioning PTTs during the entire 1996–1997 austral season. Because of the inconsistency of duty cycles among PTTs, we tallied the number of hawk occurrences in each cluster by two-week intervals starting in mid-November when most instrumented hawks had arrived in the austral range. We defined a hawk occurrence as the presence of an individual hawk in a cluster during the 2-week interval, regardless of the number of locations. We totaled the number of hawk occurrences in each northern and southern cluster and calculated a proportion based on the total number of occurrences for the 2-week period. Because some hawks used more than one cluster in a 2-week interval the total number of occurrences sometimes exceeded the number of hawks.

We used SAS version 9.1.3 (SAS Institute 2006) for modeling and SYSTAT 12 (SYSTAT 2007) to conduct two sample t-tests, analysis of variance, Pearson correlations, and nonparametric tests for assessing factors related to departure dates, travel rates, distances moved, velocity rates, distance between north and south vectors, and use of clusters. We used Oriana software for circular statistics (http://www.kovcomp.co.uk/index.html) to calculate the circular mean departure azimuth for south and for north migration. Except where noted, values reported in the RESULTS section are means ± SD, and we used an α-level of 0.05 for all tests.

RESULTS

THE MARKED SAMPLE

We radio marked 46 adult Swainson’s Hawks on the breeding grounds in seven U.S. states and two Canadian provinces (Table 1). Idaho, the only site studied in all three years, had the most radio marked hawks of any locality and the only radio marked mated pair (Table 1).

We received 6813 location records from 44 PTTs between July 1995 and September 1998 and obtained no data from two PTTs. Filtering retained 4619 location estimates (68% of total locations); 7% were classed in the highest Location Classes (LC) categories LC 2 or 3, 16% were LC 1, 40% were LC 0, 17% were LC A, and 20% were LC B or Z. Tracking duration ranged from 1.05 to 13.05 months. We obtained data for part of a second south migration from one bird from Colorado.

At the start of south migration, 16 (36%), 21 (48%), and seven (16%) PTTs transmitted on Category 1, 2, and 3 duty cycles, respectively (see METHODS for definitions), and 34 functioning PTTs completed the trip. During the austral season, eight (33%) and 16 (67%) PTTs transmitted on Category 2 and 3 duty cycles, respectively. Of 20 PTTs on hawks that completed north migration, 19 and one transmitted on a Category 3 and 2 duty cycles, respectively.

MIGRATION ROUTES AND PATTERNS

South migration

All Swainson’s Hawks nesting east of the Continental Divide (n = 17) migrated on a route east of the Continental Divide and along the east side of the Sierra Madre Oriental in eastern Mexico (Fig. 1). Most Swainson’s Hawks nesting west of the Continental Divide (n = 25) followed one of two routes to eastern Mexico (Fig. 1). Fifteen (60%) crossed the Continental Divide between northwestern Colorado and west-central New Mexico, and went to the southern Great Plains or northern Chihuahuan Desert before flying along the east side of the Sierra Madre Oriental. Eight hawks (32%) crossed the Continental Divide between west-central New Mexico and central Mexico, traversed the southern Chihuahuan Desert, and crossed the Sierra Madre Oriental to the east coastal plains of Mexico. Only two hawks deviated from these two general routes (Fig. 1)

Figure 1.

Migration paths north of 20° N used by 17 and 25 adult Swainson’s Hawks respectively from east and west of the Continental Divide on south migration and by 8 and 15 hawks respectively from east and west of the Continental Divide on north migration, 1995 −1998. Blue lines depict paths of hawks from east of the Continental Divide. Red and orange lines depict paths of hawks that cross the Continental Divide on south migration at > and <35° N, respectively. Green lines depict paths of two California hawks. Dotted lines show paths of hawks that deviated from the routes of the group.

Swainson’s Hawk locations converged in east-central Mexico, with locations concentrating on the east side of the Sierra Madre Oriental around 20° N (Fig. 2). The width of the south migration corridor (the distance between the outermost east and west orthodrome tracking vectors of < 500 km) measured 3220 km at 40° N, 1310 km at 30° N, 180 km at 20° N, and <100 km at Veracruz, Mexico (19° N). We excluded vectors of ≥500 km because they artificially widened the corridor. The corridor remained relatively narrow as it crossed the Andes Mountains near Medellin, Colombia, rounded the “elbow” of the Andes near Santa Cruz, Bolivia, and extended to the austral summer grounds in Argentina (Fig. 2); measuring about 380 km at 0° and 20° S and remaining < 400 km wide to the austral grounds.

Figure 2.

South and north migration based on all filtered location estimates of 43 radio marked adult Swainson’s Hawks between nine localities throughout the breeding grounds and the austral summer grounds, 1995 – 1998.

North migration

Although the general corridors of northward and southward migration were similar (Fig. 2), paths of many individuals varied. Of 23 Swainson’s Hawks tracked completely on both migrations, only 4 (17%) had northward and southward vectors <200 km apart at all 10° latitudinal lines. Maximum separation of northward and southward migration vectors at 10° latitudinal lines for individual hawks ranged from 189 to 485 km between 30° S and 10° N and from 198 km to 1036 km north of 10° N°. Northward vectors for all individuals were predominately east of their southward vectors south of 10° N and west of southward vectors north of 10° N.

All Swainson’s Hawks from east of the Continental Divide (eastern hawks; n = 8) followed the same route on northward and southward migration north of 20° N (Fig. 1), and <200 km separated north and south migration vectors for individuals at 30° N and 40° N. In contrast, northbound and southbound paths north of 20° N of most hawks from west of the Continental Divide (western hawks; n = 15) varied greatly. Northward migration paths of most western hawks were south and west of the paths they took in the autumn (Fig 1). Northward and southward tracking vectors for 11 (73%) western Swainson’s Hawks were separated by >200 km, with >600 km separating northward and southward vectors for eight of these hawks (Table 2). Nine (60%) of the western hawks crossed the Sierra Madre Oriental, migrated through central and northwestern Mexico, and crossed the Continental Divide between southwestern New Mexico and southern Mexico (Fig. 1). The remaining 6 hawks flew northward along the east side of the Sierra Madre Oriental, after which five went to the southern Great Plains or northern Chihuahuan Desert and crossed the Continental Divide north of west-central New Mexico. The remaining hawk traversed the central Chihuahuan Desert and crossed the Continental Divide near the Mexico-Arizona border at 31° N. Of the hawks with northward and southward vectors separated by >600 km, four diverged from their south paths in east central Mexico, and four continued along the east side of the Sierra Madre Oriental and departed west from their southward paths between 27° and 34° N latitude (Table 2).

Table 2.

Northward paths in relation to departure from southward paths north of 20 N° for eight Swainson’s Hawks from west of the Continental Divide where north and south vectors were separated by >600 km.

Nesting grounds	Year	Number of hawks	Location of departure from southward path	Azimuth of path after departure	Path to nesting grounds
Idaho, Oregon, Utah	1997	4	Mexico 18° to 22° N latitude	310° to 320°	Central and northern Mexico to nesting grounds via central and southern California and central Arizona
Oregon	1997	1	Texas-Mexico boarder 27° N	296°	Northern Mexico to Oregon via central California
Oregon	1997	1	Texas Panhandle 34° N	310°	Continental Divide 450 km south of the south path and to Oregon via northern Nevada
Idaho	1997, 1998	2	Texas Panhandle 33° to 34° N	275° and 282°	West 775 to 1000 km to western and central Arizona and to Idaho

PROGRESSION OF MIGRATION

Effect of duty cycle on distance

Swainson’s Hawks carrying PTTs with Category 1 duty cycles on south migration had significantly larger cumulative tracking distances (F_2,28 = 4.18, P = 0.03) compared to hawks carrying PTTs with Categories 2 and 3 duty cycles. The simplest model indicated that cumulative tracking distance for the south migration was significantly related to duty cycle (F_2,24 = 6.11; P= 0.007 deployment latitude (F_1,24 = 21.66; P = 0.001), duty cycle by latitude (F_2,24 = 7.79; P= 0.003), and deployment longitude (F_{1,2 4}= 6.25; P=0.02). The interaction between latitude and duty cycle was the key element of the model, and coefficients of the best fitting model represented changes in cumulative distance per unit change in latitude, for each duty cycle category (Table 3). Cumulative tracking distance increased 287 km per degree increase in latitude of origin for PTTs with a category 1 duty cycle, which was higher than the change per degree of latitude for the other two duty cycle categories. Also for each degree increase in longitude (westward offset), south migration distance increased by 53 km (Table 3).

Table 3.

Coefficients for terms in the best fitting model for south migration of 31 adult Swainson’s Hawks, 1995 – 1997.

Effect	Coefficient	SE	95 % CI
Latitude, Duty cycle 1a	286.9	54.7	(147.1 ; 399.7)
Latitude, Duty cycle 2b	35.1	35.8	(−38.8 ; 109.0)
Latitude, Duty cycle 3b	67.3	50.5	(−36.9 ; 171.5)
Longitudec	−52.7	21.1	(−96.2 ; −9.2)

^aPer unit change for Category 1 duty cycle was significantly higher than for Categories 2 (P = 0.005) and 3 (P = 0.01) duty cycles.

^bDuty cycle Categories 2 and 3 are not statistically different from 0.

South migration

Swainson’s Hawks left their breeding areas between 12 August and 9 October for all years and breeding localities (Table 4). Departure dates did not differ among localities (F_7,15 = 1.72, P = 0.18) or between sexes (Mann-Whitney U = 28.5, P = 0.75, n = 20), considering just hawks with PTTs on Category 1 and 2 duty cycles in 1996 when all localities were represented. Both members of a pair in Idaho began migration on 12 September 1996. Mean departure day did not differ among years (18, 20, and 21 September for 1995, 1996, and 1997 respectively; F_2,32 = 0.64, P = 0.55) for all localities combined, nor did it differ among years in Idaho alone for all other variables held constant (F_2,8 = 1.88, P = 0.21). Departure differed by 8 days in two consecutive years for a hawk from Colorado. Departure azimuth tended to be southeast (x̄ = 133 ± 21° SD; range = 102 – 203; n = 42). Only two hawks (both from Minnesota) departed west of south. The male and female of the Idaho pair departed at 165° and 121°, respectively. Departure azimuth differed little (155° vs 159°) in two consecutive years for a hawk from Colorado.

Table 4.

Start and end dates of south migration for radio marked Swainson’s Hawks from nine localities arranged in decreasing latitude, 1995 – 1997.

Year/State/province	Number of hawks	Mean start date	SD	Range	Number of hawks	Mean end date	SD	Range
1995
Southeast Alberta	2	23 Sep	0.7	22 – 23 Sept	1	7 Nov
Southwest Idaho	3	12 Sep	13.6	27 Aug – 20 Sep	3	29 Nov	5.1	23 Nov – 3 Dec
Northern California	1	20 Sep			1	23 Nov
Central Colorado	2	26 Sep	16.3	14 Sep – 7 Oct	2	25 Nov	9.9	18 Nov – 2 Dec
Northeast Utah	1	6 Oct
Subtotal	9	21 Sep	12.1	27 Aug – 7 Oct	7	24 Nov	9.2	7 Nov – 3 Dec
1996
Southeast Alberta	5	28 Sep	3.5	24 Sep – 5 Oct	4	27 Nov	17.7	15 Nov – 23 Dec
Southwest Saskatchewan	1	21 Sep			1	30 Nov
Northeast Oregon	4	8 Sep	21.9	12 Aug – 30 Sep	5	30 Nov	11.8	13 Nov – 10 Dec
Southwest Minnesota	2	27 Sep	3.5	24 – 29 Sep	1	18 Nov
Southwest Idaho	6	19 Sep	7.8	6 – 30 Sep	6	26 Nov	6.5	15 Nov – 4 Dec
Northern California	1	8 Sep			1	4 Dec
Central Colorado	3	23 Sep	12.0	11 Sep – 5 Oct	3	1 Dec	25.2	14 Nov – 30 Dec
North-central Utah	3	25 Sep	0.6	25 – 26 Sep	3	17 Nov	7.6	12 – 26 Nov
Southeast Arizona	2	9 Oct		9 Oct	1	25 Nov
Subtotal	27	22 Sep	12.3	12 Aug – 9 Oct	25	27 Nov	12.5	13 Nov – 30 Dec
1997
Southwest Minnesota	2	28 Sep	2.8	26 – 30 Sep
Southwest Idaho	3	7 Sep	15.5	21 Aug – 19 Sep	1	7 Dec
Central Coloradoa	1	28 Sep
Subtotal	6	16 Sep	15.7	21 Aug – 30 Sep	1
GRAND TOTAL	42b			12 Aug – 9 Oct	33			7 Nov – 30 Dec

^aSecond south departure for a bird instrumented in 1996

^bFive of the 46 radio marked hawks were excluded due to PTT failure and data gaps at onset of migration

The interval between departure of the first and last radio marked Swainson’s Hawk from the breeding grounds in 1996 spanned 58 days. Passage duration for these hawks ranged from 21 to 25 days between 30° N and 10° N, increased sharply to 40 days at 0°, and changed little thereafter (Fig. 3A). The increase in passage duration between 20° N and 0° resulted from four hawks that traveled significantly slower than the rest of the group between 10° N and 0° (t₂₆ = 3.44, P = 0.002) and lagged behind for the remainder of their migration. Travel rates differed among 10° latitudinal zones (F_5,115 = 4.53, P = 0.001), with rates highest between 30° and 20° N and lowest between 20° and 10° N (Fig. 3B). Rates increased significantly between 10° N and 30° S (Fig. 3B). Travel rates for the entire south migration averaged 176.7 + 36.0 km day⁻¹ (range = 136–263) for 16 hawks with Category 1 PTTs. Velocities for within-duty-cycle vectors ranged from 8.9 to 86.4 km h⁻¹ (x̄ = 38.6 ± 17.4) for intervals lasting 1.0 to 7.2 h and vectors 50 to 274 km long (n=133). We pooled tracking velocities for all migration periods because velocities on south and north migration and on the austral and breeding grounds did not differ (F_3,129 = 1.12, P = 0.34). The higher velocity range (e.g., ≥70 km h^–1, n = 8 vectors) represents our best estimate of short-term sustained flight speed.

Figure 3.

A. Passage duration (number of days between the dates and times that vectors of the first and last radio marked Swainson’s Hawk crossed each 10° latitudinal line) for south (dashed line) and north (solid line) migration. B. Travel rates (km/day) between 10° latitudinal lines from 30° N to 30° S for Swainson’s Hawks on south migration. Means are depicted by horizontal lines and 95% confidence intervals by vertical lines. Based on 24 and 20 radio marked Swainson’s Hawks respectively completing migration south in 1996 and north in 1997.

Swainson’s Hawks ended their south migration in November and December (Table 4), 42 to 98 days after start of migration (Table 5). We observed no correlation between latitude at the end of south migration and latitude of origin (r = 0.30, P = 0.80, n = 36). Cumulative tracking distance for south migration for all years ranged from 8449 – 13209 km (x̄ = 11052 ± 1123; Table 6).

Table 5.

Duration (days) of south and north migrations as determined by satellite telemetry for adult Swainson’s Hawks from nine nesting localities arranged by latitude in the U. S. and Canada, 1995 – 1997.

	South migration				North migration
Year/Locality	Number of hawks	Mean	SD	Range	Number of hawks	Mean	SD	Range
1995
Southeast Alberta	1	45.3
Southwest Idaho	3	78.5	9.0	71.6 – 88.7
Northern California	1	64.1
Central Colorado	2	60.4	26.0	42.0 – 78.8
Subtotal	7	66.5	17.3	42.0 – 88.7
1996
Southeast Alberta	4	59.6	14.0	47.8 – 79.6	2	58.0	9.9	51.0 – 65.0
Southwest Saskatchewan	1	62.6			1	56.3
Northeast Oregon	4	83.8	22.1	65.5 – 97.8	4	68.7	10.7	54.8 – 80.2
Southwest Minnesota	1	49.3			1	70.4
Southwest Idaho	6	68.2	8.0	46.3 – 77.8	5	64.1	9.4	51.6 – 74.8
Northern California	1	87.1			1	53.7
Central Colorado	3	69.5	14.7	58.1 – 86.1	2	60.0	4.0	57.2– 62.7
North-central Utah	3	53.1	6.9	48.2 – 61.0	2	65.8	2.4	64.1 – 67.5
Southeast Arizona	1	46.9
Subtotal	24	66.5	15.5	46.3 – 97.8	18	63.5	8.5	51.6 – 80.2
1997
Southwest Idaho	1	79.0			2	71.8	2.4	60.2 – 81.9
GRAND TOTAL	32a	66.9	15.5	42.0 – 97.8	20a	61.6	9.1	51.0 – 81.9

^aWe excluded 14 of 44 hawks that started southward and 13 of 33 that started northward migration because of PTT failure, bird mortality, and data gaps.

Table 6.

Cumulative tracking distance (km) as determined by satellite telemetry for adult Swainson’s Hawks from nine nesting localities arranged by latitude in the U. S. and Canada during south and north migration, 1995 – 1997.

	South migration					North migration
Year/Locality	Number of hawks	Mean	SD	Range	Duty cyclea	Number of hawks	Mean	SD	Range	Duty cyclea
1995
Alberta	1	10016
Idaho	3	11127	547	10518 – 11577	2
California	1	11659			2
Colorado	2	10436	41	10407 – 10465	2, 3
Subtotal	7	10847	648	10016 – 11659	2
1996
Alberta	4	11437	249	11227 – 11778	2, 3	2	11095	343	10852 – 11337	2, 3
Saskatchewan	1	10327			3	1	10049			3
Oregon	5	11294	1,310	10051 – 12831	1, 2	4	10833	616	10090 – 11585	3
Minnesota	1	11368			1	1	9047			3
Idaho	6	12094	539	11455 – 12809	1	5	10594	511	10081 – 11231	3
California	1	13209			1	1	10214			3
Colorado	3	9946	413	9653 – 10465	1, 2, 3	2	9575	487	9230 – 9919	3
Utah	3	10160	1,182	9263 – 11499	2, 3	2	10185	294	9977 – 10393	3
Arizona	1	8449			1
Subtotal	25	11156	1,224	8449 – 13209		18	10417	667
1997
Idaho						2	10279			3
GRAND TOTAL	32b	11058	1126	8449 – 13209		20b	10394

^aDuty cycle category. See text for definitions

^bExcluded 14 of 44 hawks that started south and 13 of 33 that started north because of PTT failure, bird mortality, and data gaps.

North migration

Swainson’s Hawks (n = 23) began migrating north mid-February through March (Table 7), 76 to 128 days after start of the austral period (x̄ = 92.7±12.1). Departure azimuth from the last location on the austral grounds to the first location >150 km away on the north migration averaged 1.5 ± 20.3° (range = 299 to 48 °). Swainson’s Hawks ended their north migration from mid-April through May (Table 7), 51 to 82 days after start of migration. Duration of north and south migration did not differ for all years (t₅₀ = 0.70, P > 0.49) or 1996, the year of our best sample (t₄₁ = 1.03, P = 0.30, Table 5). We observed no correlation between breeding locality [latitude (r = −0.17) or longitude (r = 0.04)] and duration of north migration. Cumulative tracking distance on north migration ranged from 9740 to 11585 km (x̄ = 10394 ± 649; Table 6). Cumulative tracking distance for 20 hawks was positively related with latitude (F_1,16 = 11.11; P = 0.004) and longitude (F_1,16 = 19.39; P > 0.001).

Table 7.

Start and end dates of north migration for radio marked Swainson’s Hawks from nine localities arranged in decreasing latitude of nesting area, 1995 – 1997.

Year/Locality	Number of hawks	Mean start date	SD	Range	Number of hawks	Mean end date	SD	Range
1995
Central Colorado	1	11 Mar
1996
Southeast Alberta	4	15 Mar	11.8	2 – 26 Mar	2	11 May	7.1	6 – 16 May
Southwest Saskatchewan	1	3 Mar			1	21 Apr
Northeast Oregon	3	5 Mar	4.8	1 – 10 Mar	3	5 May	15.1	25 Apr – 16 May
Southwest Minnesota	1	25 Feb			1	6 May
Southwest Idaho	5	24 Feb	7.8	14 Feb – 5 Mar	5	29 Apr	11.4	21 Apr – 19 May
Northern California	1	26 Feb			1	20 Apr
Central Colorado	2	8 Mar	11.6	21 Feb – 16 Mar	2	6 May	15.6	25 Apr – 17 May
North-central Utah	2	28 Feb	9.0	22 Feb –6 Mar	2	4 May	11.4	27 Apr – 13 May
Southeast Arizona	1	13 Feb
Subtotal	20	3 Mar		13 Feb – 26 Mar	17			20 Apr – 19 May
1997
Southwest Minnesota	1	16 Mar
Southwest Idaho	2	6 Mar	7.6	1 – 11 Mar	2	16 May		30 Apr – 1 Jun
Subtotal	3	9 Mar			2	16 May
GRAND TOTAL	23			13 Feb – 26 Mar	21			20 Apr – 1 Jun

The interval between departure dates of the first and last radio marked Swainson’s Hawk from the austral grounds spanned 41 days in 1997. Since northward migration began north of 30° S for 70% of the instrumented hawks, we assessed passage duration starting at 20° S. Passage duration steadily decreased from 32 days at 20° S to 21 days at 0°, after which, passage duration tended to increase (Fig. 3A). This decrease in passage duration resulted from two late departing hawks (6 and 16 days later than the rest of the instrumented group) that caught up with the group at the Equator. Travel rates for these two hawks averaged 272 km day⁻¹ between 10° S and 0° compared to 123 km day⁻¹ for the remaining radio marked hawks.

STOPOVERS

We recorded at least one stopover for 33 of 41 Swainson’s Hawks on south migration. We detected stops for all 16 hawks carrying PTTs on Category 1 duty cycles, 16 (84%) of 19 on Category 2 duty cycles, and only 1 (16%) of 6 wearing PTTs on Category 3. Considering just PTTs with Category 1 duty cycles, hawks stopped 2 to 7 times, and 14 of 16 hawks stopped 3 to 6 times. The first stopover (x̄ = 12.1±8.1days) was longer than all other stopovers combined (2.3±1.7 days; t₆₈ = 8.38; P > 0.001). Mean stopover duration tended to decrease with the number of stopovers made (r = 0.72, P = 0.07, n = 7). Stopovers occurred throughout the south migration with the longer stops occurring in the more northern latitudes (Fig. 4). Most hawks with PTTs on Category 1 or 2 duty cycles used at least one stopover north of 25° N, comprising 11 (85%) of 13 hawks from east of the Continental Divide, 13 (87%) of 15 hawks that crossed the Continental Divide to the Southern Great Plains, and 9 (100%) of 9 hawks that crossed the Continental Divide and the Sierra Madre Oriental. Duration of south migration correlated with total number of days birds stopped on south migration (r = 0.58, P = 0.004, n = 23).

Figure 4.

Distribution of stopover locations by duration class for north and south migrations of adult Swainson’s Hawks, 1995 – 1998. Based on 33 hawks stopping 98 times on south migration and 10 hawks stopping 16 times on north migration.

Of 41 Swainson’s Hawks, 15 (36%) made prolonged stopovers (range = 9.3 – 26.0 days) on south migration in seven widely separated areas north of 25° N latitude (Fig. 5). Only 2 (12%) of 17 hawks from east of the Continental Divide made prolonged stopovers compared to 6 (46%) of 13 hawks that crossed the Continental Divide to the southern Great Plains, and 6 (67%) of 9 hawks that crossed the Continental Divide and the Sierra Madre Oriental. All but one of the prolonged stopovers occurred on the first stop and north of the U.S. - Mexico border. The remaining hawk made a prolonged stop in northern Mexico on its second of six stops. Of the 15 prolonged stopovers, 12 (80%) occurred in the southern Great Plains and in the deserts of north-central Mexico and southern Arizona and New Mexico (Fig. 5). Swainson’s Hawks that made prolonged stopovers departed the nesting grounds earlier than hawks that made no prolonged stops (x̄ = 9 September ±12.4 days vs. 27 September ±8.1 days; t ₃₃= −4.911, P > 0.001; Category 1 and 2 PTTs only). Swainson’s Hawks from four of nine breeding localities made prolonged stopovers (Fig. 5), and most (67%) came from Idaho. Ten of 12 (83%) Swainson’s Hawks from Idaho used 5 of the 7 areas where prolonged stopovers occurred (Fig. 5). The Idaho hawks began prolonged stopovers between 2 and 27 September each year 1995 – 1997, with no difference in starting date among years (F_2,7 = 7.57, P = 0.50). Four of the prolonged stopover areas were used in multiple years and two areas were used by birds from >1 breeding locality (Fig. 5).

Figure 5.

Location of prolonged stopover areas for 15 adult Swainson’s Hawks on south migration, 1995 – 1997. The first digit depicts the number of hawks in the stopover area and the U.S. Postal Service abbreviation identifies the state from where they originated. The last two digits depict the year of the stop.

Ten (43%) of 23 Swainson’s Hawks that migrated north made mostly one to two stopovers (x̄ = 6.1 ± 1.1 days, range = 4.0 – 8.4). One hawk made two additional stopovers, stopping 12.8 and 13.0 days before the PTT quit moving, and evidence suggests it did not return to its nesting area. Because all but one of these hawks wore Category 3 PTTs, we considered only stops ≥ 3.5 days. Although the proportion of hawks stopping ≥ 3.5 days did not differ between north and south migration (χ² = 1.82, df = 1, P > 0.10), fewer hawks made prolonged stopovers on north migration (χ² = 8.17, df = 1, P = 0.003). All but two stopovers occurred north of 25° N latitude (Fig. 4). Of 14 stopovers north of 25° N, 13 occurred in the southern Great Plains and one in the Chihuahuan Desert. Eight of 13 hawks migrating north through the southern Great Plains stopped at least once, which included six of eight hawks and two of six from east and west of the Continental Divide, respectively. Hawks that migrated up the eastern Sierra Madre Oriental and through the Great Plains or Chihuahuan Desert stopped over more than those who crossed the Sierra Madre Oriental and migrated through central Mexico (χ² = 4.87, df = 1, P = 0.03).

AUSTRAL SUMMER MOVEMENTS

The austral summer area for 36 Swainson’s Hawks from 1995 to 1998 encompassed 738800 km² in northern Argentina and western Uruguay (Fig. 7). The core area cluster of 90% of the locations encompassed 172700 km² (23% of the 100% minimum convex polygon) in northeastern La Pampa, northwestern Buenos Aires, south and eastern Cordoba, and southwestern Santa Fe provinces of Argentina (Fig. 6).

Figure 7.

Percent of occurrences of radio marked adult Swainson’s Hawks in the 13 northern 80% clusters of 26 clusters (see Fig. 6) on the austral range in Argentina by 2-week intervals during the 1996 – 1997 austral summer.

Figure 6.

Austral summer range (100% convex polygon) and core areas (90% and 80% use areas, cluster analysis) of 36 radio marked adult Swainson’s Hawks based on all filtered location estimates, 1995 – 1998. Dotted line separates the northern and southern 80% clusters.

Swainson’s Hawks frequently moved among 26 clusters formed by 80% of the locations from all instrumented hawks (Fig. 6). We recorded more clusters used (Mann-Whitney U = 99.5, P = 0.03), more moves among clusters (Mann-Whitney U = 104.5, P = 0.01), and larger cumulative tracking distances (Mann-Whitney U = 98.0, P = 0.04) by eight birds carrying PTTs on Category 2 duty cycles than by 16 with PTTs on Category 3. Maximum vector length did not differ between the two groups (Mann-Whitney U = 57.0, P = 0.67). Therefore, we examined number of clusters used and moves among clusters for Swainson’s Hawks carrying PTTs on Category 2 as well as all birds combined. Of 24 hawks that carried functioning PTTs for the entire austral season, 23 (96%) used ≥3 clusters, and 15 (62%) used 5 to 10 clusters. All clusters used by male Swainson’s Hawks were also used by females. Of 23 hawks that used >1 cluster, 17 (74%) reused 1 to 6 clusters (x̄ = 1.5±1.3) and revisited the same cluster up to three times. Birds moved from 0 to 16 times (x̄ = 6.0±4.0) among clusters during the austral season (Table 8). Of the eight Swainson’s Hawks with PTTs on a Category 2 duty cycle, 7 (88%) used 5 to 10 clusters and moved an average of 9.1±5.1 times between clusters. Cumulative tracking for distances during the austral summer season ranged from 791 to 3553 km for all 24 Swainson’s Hawks (1,370 and 3,184 for eight with Category 2 PPTs). Maximum single vector distance for each bird ranged from 100 to 893 km (Table 8).

Table 8.

Use of 80% clusters and movements of 24 radio marked adult Swainson’s Hawks during the austral summer season, 1995 – 1998.

	Number of clusters used	Number of moves among clusters	Number of clusters reused	Cumulative tracking distance (km)	Maximum vector distance (km)
Mean	5.3	6.0	1.5	1,795	307
SD	2.4	4.0	1.3	785	169
Range	1 to 10	0 to 16	0 to 6	791 to 3,553	100 to 893

We observed seasonal shifts in hawk use of the north and south 80% clusters during the 1996 −1997 austral season. During the first 2 weeks of the austral season (last 2 weeks of November) 60% of the Swainson’s Hawks occurrences were in the northern clusters (Fig. 7). By the first week of December this occurrence decreased to 24%, remained low to mid-January, and then steadily increased to 73% by 8 March (Fig. 7). With exception of the one cluster in the extreme north of the austral range, the distance between the most northeast and most southwest cluster was 700 km (Fig. 7).

We did not see evidence of exclusive areas being used by hawks from specific breeding localities. Of 26 clusters, 22 contained hawks from 2 to 4 breeding localities, and 4 clusters contained hawks from 5 to 8 localities. The two largest clusters (one in the south and one in the north; Fig. 6) each contained birds from eight of the nine breeding localities. Collectively these two clusters contained hawks from all the nine breeding localities.

DISCUSSION

MIGRATION

Our findings support the assertion that most of the Swainson’s Hawk population migrates between the breeding grounds in North America and the austral summer grounds in the pampas of South America (England et al. 1997, Sarasola et al. 2008). No radio marked hawks went to Florida or central California where wintering Swainson’s Hawks occur regularly in small numbers (Clark 1990), and none wintered in Mexico as did Swainson’s Hawks radio marked in the Central valley of California (M. Bradberry unpublished data, Wheeler 2003).

Swainson’s Hawks from across the breeding range followed various routes south on a broad front in a funnel pattern (Berthold 2001) to converge along the east coast of central Mexico into a common corridor to a communal austral summer area in central Argentina. The pattern constricted between the Sierra Madre Oriental and the Gulf of Mexico at about 20° N and remained concentrated to the austral grounds. North and south migration patterns were similar.

Swainson’s Hawks differed from other migratory raptors studied thus far in that timing of migration did not differ by sexes and was relatively synchronous across geographic locations. In contrast, Osprey (Pandion haliaetus) departure dates from the breeding grounds in North America and Sweden differed significantly between sexes (females departed significantly earlier) and among geographic regions (Hake et al. 2001, Martell et al. 2001). Female Honey Buzzards (Pernis apivorus) left the breeding grounds in Sweden somewhat earlier than males (Hake et al. 2003). Although our failure to find a difference between sexes may be partially due to the small sample size of males, we have no evidence to suspect a difference occurred. Values for males were well within the range of females and departure date was the same for both members of a pair. Duration of north migration was shorter than that of south migration for adult Osprey and Peregrine Falcons (Falco peregrinus; Alerstam et al. 2006, McGrady et al. 2002) but not for Swainson’s Hawks.

The distribution of migrating Swainson’s Hawks varied in space and time for both south and north migration. Although hawks departed from the nesting grounds over a 2-month period, their migration south across 20° N became concentrated into a 3-week period. This latitude is just north of Veracruz, Mexico where the migration corridor was most constricted and appeared to be a temporal goal. Migrating hawks slowed and passage duration increased through southern Mexico and Central America, which is an area where migrating hawks may feed (Kirkley 1991). The Equator may have been an intermediate temporal goal for Swainson’s Hawks migrating north with most hawks slowing as they approached the Equator and trailing birds traveling faster. In contrast to south migration, travel rates increased between 0° and 20° N, suggesting that 20° N may have been a temporal goal as it was on south migration

Many factors affected our estimate of total migration distance. Swainson’s Hawks from northern latitudes and western longitudes had greater cumulative distances. Hawks from the western areas moved more longitudinally than birds from easterly longitudes to reach the east coast of Mexico where all Swainson’s Hawks converged. A critical factor in estimating migration distance is the role of transmission frequency (duty cycle). Shorter duty cycles produce more transmission periods and more location estimates, yielding more vectors which result in greater cumulative distance. Any studies that depend on multiple duty cycle transmissions must account for this to prevent faulty comparisons, either qualitatively or quantitatively. Any statistical analysis must incorporate correction for multiple duty cycles if more than one is used in the study.

USE OF STOPOVER AREAS

Birds use stopovers on migration to refuel and replenish fat reserves (Berthold 2001, Skagen 2006) and/or to complete molt (Leu and Thompson 2002, Newton 2008. Although data suggest that we missed short-duration stopovers of hawks carrying Category 2 and 3 PTTs, results from hawks carrying Category 1 PTTs suggest that all Swainson’s Hawks tended to stop on south migration. About half of the hawks departed the breeding grounds early and stopped at intermediate destinations for prolonged periods possibly to fuel (Bechard et al. 2006). The other half stayed on the breeding grounds nearly three weeks longer where they could add fuel and complete molt before migrating south. Ospreys that departed early (mainly females) engaged in prolonged stopovers presumably to amass fuel, whereas late-departing Ospreys that accumulated energy for migration and completed their molt on the breeding grounds had fewer stopover days (Alerstam et al. 2006, Hake et al. 2001, Kjellen et al. 2001). Most of the prolonged stopover areas were used by different birds from the same breeding locality (e.g., Idaho) in the same and different years, and some of the areas were used by birds from different localities in the same year. This area fidelity suggests that these stopover areas were important destinations (Newton 2008). Most Idaho hawks made prolonged stops in widely separated areas 1200 to 1500 km (straight-line distance) from their nesting area in about the same time period in all 3 years. These birds may have departed the breeding grounds early to exploit seasonal food resources such as insects (Littlefield 1973) in these prolonged stopover areas before continuing their migration.

Smith et al. (1986) proposed that Swainson’s Hawks must acquire all the fat they need at some region in the Northern Hemisphere to complete the migration south while fasting. Alternatively, Kirkley (1991) suggested that Swainson’s Hawks likely feed on migration, and mass measurements of hawks before and after migration indicate that this may be the case (Goldstein et al. 1999c, Bechard et al. 2006). Swainson’s Hawks that made prolonged stops in the southern Great Plains and northern Chihuahuan Desert probably were accumulating fuel for the rest of migration, as proposed by Smith et al. (1986). However, most of these hawks made subsequent stops along the route south. Stopovers by Swainson’s Hawks elsewhere along the route present an opportunity to accumulate fuel before embarking on the next segment of the migration (Kirkley 1991, Bechard et al. 2006). Swainson’s Hawks tended not to stop for prolonged periods on north migration. These hawks may have obtained sufficient food resources on the austral grounds to reduce the need to refuel at intermediate destinations as discussed by Alerstam et al. (2006)

Stopover behavior can influence migration patterns of birds (Berthold 2001) and might have influenced differences in migration patterns north of 20° N between Swainson’s Hawks from east and those from west of the Continental Divide. Hawks from east of the Continental Divide tended to stopover in the southern Great Plains on both south and north migration, and individuals followed similar paths on both migrations. In contrast, hawks from west of the Continental Divide tended not to stop on north migration and use different routes on south and north migration. On south migration many hawks from west of the Continental Divide deviated from a direct trajectory to east-central Mexico to stopover in the southern Great Plains. On north migration most of these hawks crossed the Sierra Madre Oriental, a logical strategy for taking a direct path to the western nesting grounds. Although some of these hawks took fairly direct paths to their nesting areas, we tracked others westerly resulting in a wide separation between southward and northward paths for these individuals. Alerstam et al. (2006) attributed the wide separation of outbound and return tracks of migrating Ospreys to wind drift. Wind drift allows birds to complete their journey in less time and at a lower cost rather than correcting for crosswinds en route (Bildstein 2006). Swainson’s Hawks whose northbound paths diverged northwest from their southbound paths between 20° and 30° N latitude migrated at northwesterly azimuths perpendicular to the prevailing winds, which are from the northeast (http://www.ace.mmu.ac.uk/eae/climate/older/Prevailing Winds.html) in this area. These Swainson’s Hawks tended not to stop over and may not have had a need to do so. Wind drift, however, does not explain the wide separation of tracks for the three hawks whose northbound and southbound tracks widely diverged above 30° N.

AUSTRAL SUMMER MOVEMENTS

All radio marked Swainson’s Hawks from nine widely separated localities across the breeding range used common austral summer grounds. Stable isotope analyses showed that flocks of Swainson’s Hawks on the austral range consisted of a mixture of individuals from across the breeding range (Sarasola et al. 2008a). Although the austral summer range for radio marked Swainson’s Hawks encompassed a large expanse in north-central Argentina and western Uruguay, these hawks concentrated in a core area in central Argentina comprising about 20% of the austral range. The large total range resulted from 80% of the hawks making infrequent forays across a broad geographic area outside the core area. In contrast, Ospreys from different parts of the breeding range, as well as individuals from the same areas on the breeding grounds, wintered in widely separated regions (Kjellén et al. 1997, Martell et al. 2001). Unlike Ospreys from North America and Europe, where males and females from the same breeding area wintered in separate areas (Kjellén et al. 2001, Martell et al. 2001), male and female adult Swainson’s Hawks used the same regions on the austral grounds, which agrees with Sarasola et al. (2008a). Swainson’s Hawks on the austral summer grounds foraged in large flocks feeding on swarms of insects which favors communal behavior (Canavelli et al. 2003, Sarasola and Negro 2005) as opposed to osprey where factors, including competition for food, favor separation (Martell et al. 2001).

Adult Swainson’s Hawks frequently moved long distances (up to 1600 km) on their austral summer range. Goldstein et al. (2000) located four radio marked adult Swainson’s Hawks in late January 1997 500 km north of where they were trapped six weeks earlier. These hawks moved from a southern to a northern cluster on the austral grounds, supporting our observations. In contrast, adult Peregrine Falcons and Ospreys and juvenile Golden Eagles (Aquila chrysaetos) remained in relatively small areas, moving short distances (< 75 km) on their wintering areas (Kjellén et al. 1997, Hake et al. 2001, McGrady et al. 2002, Ganusevich et al. 2004, McIntyre et al. 2008). The smaller wintering areas of Peregrine Falcons were attributed to concentrations of relatively stable prey in localized areas (McGrady et al. 2002, Ganusevich et al. 2004). The frequent and long-distance movements of Swainson’s Hawks on the austral summer grounds reflected behavior of a bird preying on temporally abundant, easily captured, and often spatially unpredictable insect prey (Alerstam 1990, Sherry and Holmes 1995). Swainson’s Hawks associated with agricultural habitats on the austral summer range and fed mainly on grasshoppers (order Orthoptera) and other insects, which can be sporadic, locally abundant, and often unpredictable (Goldstein et al. 2000, Canavelli et al. 2003, Sarasola and Negro 2005). Swainson’s Hawks track migrating swarms of insects, and migrant raptors that depend on swarming insects on the wintering grounds travel long distances to find food resources (Jaramillo 1993, Newton 1998). Swainson’s Hawk movements among clusters and the shift of use from the south to the north regions of the core austral summer area as the season progressed might have reflected responses to agricultural activities (tilling, mowing, harvesting, burning, etc.) and the availability of abundant but transient food sources, such as insect outbreaks (Canavelli et al. 2003, Sarasola and Negro 2005).

CONSERVATION IMPLICATIONS

Conservation strategies for migratory birds often rely on information that identifies areas used during migration, molting, staging, and resting as well as information on potential limitations or threats in these areas (Senner and Fuller 1989, Berthold 2001, Bildstein 2006). Migratory species face particular risks, and long-distance migrations are some of the most difficult and dangerous activities birds undertake (Bildstein 2006). Results of our radio tracking study in Argentina were used to define the area for banning the pesticide monocrotophos to provide immediate protection to the species on the austral summer grounds (Goldstein et al. 1999b). Our study also provided insights into connectivity among the areas used by Swainson’s Hawks. Survival and condition of hawks at any of these use areas can have carry-over effects for the birds’ performance at other locations, such as the breeding grounds (Norris and Mara 2007, Newton 2008). Within the Swainson’s Hawk migration corridor, paths of many individuals overlay in a pattern that suggests there are commonly used routes and thus resources (e.g., perches and roosts) and conditions (e.g., thermals that allow soaring flight) required by the hawks. The concentrated migration of Swainson’s Hawks from across the breeding range, particularity from east-central Mexico through the Isthmus of Panama, has broad implications. Conservation actions or catastrophic events in these concentration areas could impact most of the breeding population. Conservation efforts also should consider potential risks to migrants, such as construction of wind energy farms across the Isthmus of Tehuantepec, where Swainson’s Hawk migration is particularity concentrated. Stopover areas are likely used for feeding and resting and are candidates for conservation efforts such as those implemented for waterfowl and waterbirds (Berthold 2001) and needed for other groups of avian migrants (Moore 2000). The intermingling of sexes and hawks from numerous breeding localities on the austral range has implications on localized mass mortality events (e.g., pesticide poisoning, hailstorms; Goldstein et al. 1999a, Sarasola et al. 2005) because effects would be diluted across the Swainson’s Hawk’s breeding range instead of impacting local nesting populations (Sarasola et al. 2008a).

Our study also provided new information on the movements of migratory adult Swainson’s Hawks from across their breeding range in North America, but much remains to be learned. How Swainson’s Hawks respond to environmental and land use changes on the austral grounds is largely unknown. Record high precipitation during the summers of 1997 and 2000 through 2002 resulted in a distinct change in the distribution of Swainson’s Hawks on the austral summer grounds (Canavelli 2000, Sarasola et al, 2008b, J. Sarasola pers. com., M. Bechard pers. observation). The areas occupied with large numbers of Swainson’s Hawks in northern La Pampa Province during our study were virtually devoid of Swainson’s Hawks. Also much of the pasturelands, which were mainly used by the Swainson’s Hawks during our study (Canavelli et al. 2003), have been converted to row crops (Sarasola et al. 2008b). Transformation of Argentinean agriculture from a system of range-based livestock production to one of intensive agricultural cultivation could affect Swainson’s Hawks negatively (Woodbridge et al. 1995). Our research identified important stopover areas for migrating adult Swainson’s Hawks; however, why certain birds engage in prolonged stopovers and others do not and what these birds do on the stopover areas remain to be answered.