Gilmour et al.: Satellite telemetry of frigatebirds

17

SATELLITE TELEMETRY OF GREAT FRIGATEBIRDS FREGATA MINORREARING CHICKS ON TERN ISLAND, NORTH CENTRAL PACIFIC OCEAN
MORGAN E. GILMOUR1, E.A. SCHREIBER2 & DONALD C. DEARBORN1,3
1Department of Biology, Bucknell University, Moore Avenue, Lewisburg, Pennsylvania, 17837, USA (morgan.gilmour@bucknell.edu)
2122 Jump Cove Road, Weaverville, North Carolina, 28787, USA
Department of Biology, Bates College, 44 Campus Avenue, Lewiston, Maine, 04240, USA

3

Received 8 March 2011, accepted 6 September 2011
SUMMARYGILMOUR, M.E., SCHREIBER, E.A. & DEARBORN, D.C. 2012. Satellite telemetry of Great Frigatebirds Fregata minor rearing chicks
on Tern Island, North Central Pacific Ocean. Marine Ornithology 40: 17–23.
The Papahānaumokuākea Marine National Monument in the Northwestern Hawaiian Islands protects breeding habitat for many migratory
animals. We used satellite telemetry to describe the areas in which a mobile top predator, the Great Frigatebird Fregata minor, traveled on
foraging trips during the early chick-rearing period from a breeding colony on Tern Island, French Frigate Shoals. Identification of potential
foraging events, indicated by a reduction in transit rate, allowed us to assess whether wide-ranging marine species such as Great Frigatebirds
remain inside the protective boundaries of the Monument while brooding young chicks. Four of 11 foraging trips extended outside of theboundaries of the Monument. These movements may represent the shortest foraging distances that Great Frigatebirds travel from the colony
because adults need to provision young chicks frequently. We also tracked one male that abandoned its nest on a journey to the southwestof Tern Island. This bird was tracked for 16 days before the transmitter’s battery expired, and the last transmitted position was nearly 1100
km from Tern Island. These tracks, the first reports of frigatebird telemetry in the Pacific Ocean, provide information about the foraging
behaviors of a top predator during a critical life-history stage—data that will complement tracking data of other species and aid in futureconservation and management decisions concerning the Monument and surrounding waters of the Northwestern Hawaiian Islands.
Key words: Great Frigatebird, Fregata minor, Papahānaumokuākea Marine National Monument, Hawaii, satellite telemetry, habitat use

INTRODUCTION
Many animals have the capacity to move long distances. Migrationis the most common example of large-scale movements, but
animals also travel extensively to find food or mates. In the marine
environment, many animals cannot remain in a single locationfor extended periods because environmental and anthropogenic
factors drive fluctuations in resources such as prey availability.
For example, fluctuations in sea surface temperature and salinity
affect spawning, growth and development of fishes, influencing
foraging movements of juvenile and adult fishes (Rijnsdorp et al.
2009). These movements in turn affect many aspects of marine
ecology, including foraging opportunities for predators (Thompson& Ollason 2001; Schroeder et al. 2009), fish stocks and fisheries
management (Huang et al. 2007). In sum, breeding and nonbreeding movements of species in a marine environment are
determined by a suite of factors that ultimately influence survivaland breeding success.
Animal movements create difficulties for the conservation andmanagement of mobile species, in part because the animals’ ranges
may transcend the jurisdictional boundaries of single agencies
or countries (e.g. Serneels & Lambin 2001, Brindza et al. 2008,
Egevang et al. 2010). Protective efforts for migratory and wideranging species are also limited by a dearth of information on
breeding, non-breeding and foraging behaviors and habitats. One
tool for conserving and managing mobile marine species is theestablishment of reserves. Specifically, marine protected areas(MPAs) provide protection for species that reside in them during atleast part of the year. Typically, a reserve protects terrestrial breeding
habitat (for seabirds, sea turtles or pinnipeds) and might also protect
important nearshore foraging areas. In the latter scenario, shallowwater foragers such as terns (Monticelli et al. 2006) might beeffectively protected. However, because of differences in foraging
behaviors, other species that need to travel far for food, such asalbatrosses (Fernández et al. 2001), may not fully benefit from theprotection the reserve offers. Thus, assessing the effectiveness ofan MPA requires information about the movement and foraging
behavior of species that use the area (e.g. Louzao et al. 2011).
Additionally, the identification of foraging areas important to marine
animals provides useful data about animals’ relationships to fishing
operations (Karpouzi et al. 2007), environmental variables (Wilson
et al. 2002) and weather events (Schreiber & Schreiber 1984).
The establishment of the 362 000 km2 Papahānaumokuākea Marine
National Monument in the Northwestern Hawaiian Islands (NWHI)
in 2006 was a significant step toward the protection of many breeding
marine animals. More than 90% of Hawaiian Green Sea Turtles
Chelonia mydas nest on a single island within the Monument (Balazs& Chaloupka 2004), nearly all of the endangered Hawaiian Monk
Seals Monachus schauinslandi birth pups within the Monument(Antonelis et al. 2006), and millions of seabirds also breed throughout
the Monument each year. Some of these species are known to movecontinuously, foraging both within and outside the boundaries ofthe Monument (e.g. Hawaiian Monk Seals; Stewart et al. 2006),
even when tied to land during the breeding season (e.g. Laysan
Phoebastria immutabilis and Black-footed P. nigripes albatrosses;
Kappes et al. 2010). However, we lack data on habitat use of many

Marine Ornithology 40: 17–23 (2012)18

Gilmour et al.: Satellite telemetry of frigatebirds

species breeding within the Monument. Papahānaumokuākea is
instrumental in protecting terrestrial breeding habitat, but morestudies are needed on the use of waters within and outside theMonument by highly mobile animals.
The Great Frigatebird Fregata minor is a long-lived, top avian
predator in the NWHI, whose at-sea worldwide distributions are
not well known (Metz & Schreiber 2002). Outside the breeding
season, these birds appear to range widely. Birds banded or tagged
in French Frigate Shoals, NWHI, have been recovered or sighted up
to 7 000 km away—on other Northwestern Hawaiian Islands and at
Johnston Atoll, Wake Island, Papua New Guinea, the Philippines,
and Japan (Dearborn et al. 2003; D.C. Dearborn & F. Juola,unpubl. data). Telemetry studies have also demonstrated wideranging movements of Great Frigatebirds during the postbreeding
period in the Indian Ocean and of Magnificent Frigatebirds Fregata
magnificens in the Atlantic Ocean. On postbreeding trips, thesebirds traveled up to 4 400 km and 1 400 km, respectively, away fromtheir breeding colonies (Weimerskirch et al. 2006). Knowledgeof foraging movements during the breeding season, however, issomewhat limited. Weimerskirch et al. (2004) tracked several GreatFrigatebirds in the Mozambique Channel and found that brooding
adults made short foraging trips within 150 km of the colony.
Incubating birds took longer foraging trips that ranged up to 600 km
from the colony. Additionally, breeding Great Frigatebirds tracked
at Aldabra Atoll in the Indian Ocean exhibited longer foraging trips
on average than those in the Mozambique Channel, presumably
because of regional differences in oceanography (Weimerskirch
et al. 2010). Wide-ranging movements of this species throughout
the annual cycle, coupled with the longest period of parental careamong birds (12–14 months; Dearborn & Anders 2006), recommend
it as an informative species to study—successful breeders engage incentral-place foraging throughout the entire year and must adjust to
seasonal changes in prey availability to feed themselves and theirchicks. Partly because we lack details of frigatebirds’ movements,
we know little about potential at-sea threats to the species. GreatFrigatebirds associate with subsurface predators such as tuna whenforaging (Au & Pitman 1986, Weimerskirch et al. 2004) but may
not interact extensively with fishing vessels (Weimerskirch et al.
2010), unlike albatrosses, whose populations have declined as aresult of bycatch (Véran et al. 2007). Great Frigatebirds also do notingest plastic (Metz & Schreiber 2002). Further study of this species
and its potential at-sea threats will aid in assessing the efficacy of
MPAs for wide-ranging foragers.

Frigatebirds (Dearborn & Anders 2006). Tagged birds were part ofa larger, ongoing study of the reproductive ecology of the species
at Tern Island.
We first fitted two birds with dummy transmitters to ensure that the
instrumentation did not inhibit flight or other behaviors. Next, we
deployed satellite transmitters (Microwave Telemetry PTT-100),
attaching them with Tesa tape #4651 (Fernández et al. 2001) to
six Great Frigatebirds. The package weight was 23 g, and typicalbody mass in this population is 1.5 kg for females and 1.3 kg formales—thus, instruments constituted 1.5–1.8% of body mass. Birds
were captured by hand at the nest during the day. Transmitterswere attached by laying strips of tape beneath a cluster of contour
feathers in the center of the back, then wrapping the tape aroundthe feathers and the transmitter. We also attached yellow vinyl
patagial tags bearing unique alpha-numeric codes to the birds for
non-invasive identification and monitoring at the colony. During
incubation and early chick-rearing, Great Frigatebirds immediately
leave the breeding colony upon the return of their mate (Dearborn
2001; Weimerskirch et al. 2004). Thus, once instrumented, ourtagged birds left the island to undertake a foraging trip when theirmates relieved them at the nest. After one to three days, tagged birds
returned from the foraging trip to relieve their mates, and when themates returned, tagged birds left the island to undertake a second
foraging trip. Upon their return from this second trip, we removed
the transmitter.
Argos locations were recorded during the entire time a bird carrieda transmitter. We included all location classes returned by Argos
and then used movement-based criteria to filter out unrealistic
movements (e.g. Shaffer et al. 2005, Adams and Flora 2010, Kappes
et al. 2010). Specifically, we applied a speed filter (Tremblay et al.
2006) in MatLab (The MathWorks, Natick, MA), retaining only
locations in which the transit rate between two successive locations
did not exceed 65 km/h (Weimerskirch et al. 2003). The resulting
tracks retained 92.2% (n = 664) of all returned locations.
The rate at which Argos-equipped satellites pass over a given location
is not constant; therefore, animal movements are not sampled atregular intervals (Wilson et al. 2002). Interpolation resamples the data
at evenly spaced intervals (e.g. one location per hour in this study)
and allows for more fluid, realistic representations of the movements

In this study, we instrumented Great Frigatebirds with Argos
satellite transmitters during chick brooding. This stage of the
reproductive cycle places the greatest restrictions on forage-trip
duration because adults must return to the nest frequently (every
one to three days) to feed young, and these foraging trips likely
represent the shortest distances that Great Frigatebird parents travelfrom the colony. Therefore, short trips during brooding allowed us
to investigate whether the Monument provides minimal protectionof the foraging habitat of Great Frigatebirds.
STUDY AREA AND METHODS
The study was conducted in May 2005 during the early chickrearing period (mean chick age 5.5 days, range 4–12 days)on Tern Island (23°45'N, 166°17'W), French Frigate Shoals, in
the Papahānaumokuākea Marine National Monument (Fig. 1).
Tern Island supports several thousand breeding pairs of Great

Fig. 1. Study area (Tern Island, Northwestern Hawaiian Islands)
and surrounding features, including Papahānaumokuākea Marine
National Monument.

Marine Ornithology 40: 17–23 (2012)Gilmour et al.: Satellite telemetry of frigatebirdsof marine animals. We interpolated the filtered tracks using a Bézier
curve (µ = 0.3; Tremblay et al. 2006). We then calculated transit rates
and distances traveled using the interpolated tracks.
Although an altimeter offers a direct approach for identifying
potential foraging events, it cannot be used in conjunction withsatellite transmitters because the combined mass of the two
instruments is too great for frigatebirds to carry (Weimerskirch
et al. 2004, 2010). Therefore, we used an indirect approach. We
manually examined the telemetry tracks to identify “potential
foraging events,” which were indicated by a reduction in transit
rate (Hyrenbach et al. 2002). Specifically, we followed a modified
method for Great Frigatebirds (Weimerskirch et al. 2004) that
identifies a potential foraging event by, for instance, three or more
successive points less than 10 km apart and separated by at least
30 minutes. We further modified this method for our processeddata set, which was interpolated to one location every 60 minutes:
potential foraging events comprised three or more successive points<10 km apart and separated by 60 minutes.
We used a mixed-model ANOVA to test whether frigatebirds
exhibited different transit rates during night and day. The data points
were estimates of transit rate from each day or night period of flightfrom every bird. Time (day vs. night) was used as a fixed factor,
and the identity of the bird was incorporated as a random factor via
Restricted Maximum Likelihood (REML). Unless otherwise noted,
means + SD are reported.
RESULTSWe placed transmitters on six Great Frigatebirds caring for chicks
that were 4 to 12 days old at the beginning of the study (Table 1,
Fig. 2). There were 720 locations returned for all birds, encompassing
eleven roundtrip foraging bouts, and one incomplete trip (L80).
Parents’ trips lasted 1–3 days and covered 399 + 189 km (mean + SD)
on average (range 73–732 km, n = 11 completed trips). Four birds(not including the extended second trip of L80) traveled beyond the19boundaries of the Papahānaumokuākea Marine National Monument
before returning to feed their nestlings (Fig. 2, Table 2).
There were 19.8 + 1.9 Argos locations returned per bird per day.
The quality of locations was most commonly (64.4%) of class “0”(expected to fall within 1.5 km of true location). Visual examinationof all 12 foraging tracks (two trips by each of six birds) revealed
that four birds took their second foraging trip in a direction roughly
opposite to the first trip (Fig. 2). The incomplete data on the second
trip of male L80 (Fig. 3) were due to a dead battery or loss of the
instrument. This bird did not return to the colony in typical fashion
from its second foraging trip, but instead flew in a southeasterly
direction for 16 days, until the transmitter ceased transmitting, 1 088
km from Tern Island. The bird’s mate abandoned the nest when L80
had been gone for 10 days, and the nest failed. L80 returned to thecolony approximately one month later, but without the transmitter.
Based on 75 estimates of transit rates, birds flew more slowly atnight (20:00–06:00; 8.4 + 3.7 km/h (+ SE) at night versus 10.5 +4.1 km/h during the day; F = 5.01, P = 0.0285). Differences among
birds accounted for 14.4% of the variance in estimated transit rates.
We tracked birds during all phases of the lunar cycle (new, half, andfull moon), and birds exhibited a mix of straight and looping tracks
during each of those phases.
Based on the criterion of a short-term reduction of transit rates during
the daytime (06:00–20:00), we identified 28 potential foraging events
during the 12 trips. Fifteen of those events occurred during theextended second trip of L80 (Fig. 3). The transit rates of the outboundand return portions of the trips that contained potential foraging
events did not differ significantly (t = –0.13, P = 0.898; Table 3).
DISCUSSION
Papahānaumokuākea Marine National Monument appears toprovide only partial protection of Great Frigatebird foraging habitat
during the chick-brooding period. Our Argos tracks indicated that

TABLE 1
Transit rates, distances and durations of foraging trips from Tern Island by Great Frigatebirds during early chick-rearing
Individual

Sex

Trip

Transit rate
(km/h)

Distance (km)Farthest distance from
Tern Island (km)Time (h)Inferred
foraging events

L77

Male

Trip 1

10.4

227.9

83.3

22

1

Trip 2

11.4

732.2

249.9

64

3

Trip 1

3.3

73.5

22.7

22

0

Trip 2

9.5

262.2

54.7

29

2

Trip 1

11.2

392.5

131.0

37

2

Trip 2

11.1

431.5

153.8

39

0

Trip 1

6.9

436.7

132.5

63

1

Trip 28.4

3070.81087.9367

15

Trip 1

12.7

519.7

190.8

41

1

Trip 2

10.4

575.8

209.0

56

2

Trip 1

13.5

514.7

181.5

38

1

Trip 2

10.1

222.3

49.9

22

0

L78
L79
L80
L82
L83

FemaleFemaleMale
FemaleMale

Marine Ornithology 40: 17–23 (2012)20

Gilmour et al.: Satellite telemetry of frigatebirds

frigatebirds foraged both inside and outside of the boundaries ofthe Monument during this part of the breeding season (Figs. 1 &2; Table 2). This brooding period puts the greatest constraint on thebirds’ foraging time, and frigatebirds are expected to forage nearer
the colony than they would during incubation and post-brooding.
However, our tracking data demonstrate that even the shortest trips(22 h) ranged up to 228 km. Four of the six birds traveled outside
the boundaries of the Monument at least once while foraging(Table 2). As chicks age, Great Frigatebird parents spend longer
periods away from the nest, and may travel farther to forage. The
Monument effectively protects nesting habitat and some portion of
the foraging area of Great Frigatebirds, but it does not protect all of
the foraging areas used by breeding birds.
We used a reduction in transit rate as an indicator of potential foraging
events, an indirect approach that could err in either direction—either
missing true foraging events or inferring events that did not occur.

Fig. 2. Foraging tracks of six Great Frigatebirds (two foraging trips
per bird, except one foraging trip for L80) during early chick-rearing
from Tern Island, French Frigate Shoals. Black circles within bird
tracks indicate slower flight periods thought to correspond toforaging events. Dotted lines delineate bathymetry.

The horizontal transit rates of frigatebirds are reduced during some
portions of flight because frigatebirds continuously change altitude
while climbing thermals and descending; more time may be spent
gaining or descending altitudes of several hundred meters thanis spent moving horizontally (Weimerskirch et al. 2003). Slower
horizontal movement causes satellite tracks to appear stationary. Slowtransit rates and the resulting static tracks therefore present difficulties
in interpretation of Great Frigatebird movements that are based onchanges in transit rate. It is possible that the “foraging events” weinferred simply reflected birds riding in a thermal or resting on thewing (Weimerskirch et al. 2010). In our data, potential foraging
events were infrequent (not all birds exhibited the slow transit rates
of inferred foraging activity on all days they were tracked; Table 1).
Inferred foraging occurred throughout the day, but some birds flew
for up to 27 h before a potential foraging event occurred (L82), andthree birds (L78, L79, L83) did not exhibit any potential foraging
activity during one of their foraging trips (Table 1). There were also
several Argos locations that indicated that the birds were moving very
slowly within French Frigate Shoals. Great Frigatebirds immediately
vacate the nest when they are relieved by their mate (Dearborn 2001;
Weimerskirch et al. 2004), and we did not observe tagged birds inthe colony except when they were on their nests. Furthermore, adultGreat Frigatebirds from this colony have not been observed to forage
within the atoll. Therefore, the slow, local flight we observed likely
involved birds flying in thermals above the island, which is common(M.E. Gilmour, pers. obs.). Conversely, although a reduction oftransit rate may be a useful indicator of potential foraging activitiesof pelagic seabirds traveling great distances, this method may fail todetect foraging events. Great Frigatebirds at times approach the sea
surface, catch a fish, and continue flying (E.A. Schreiber, pers. obs.).
Without altimeter data, telemetry studies would not be able to capturesuch behavior.
From previous work on diet or at-sea observations of frigatebirds,
three environmental factors seem important in determining foraging
locations. First, Great Frigatebirds may use physical cues to locate
areas of available prey. Potential prey are known to congregate at
upwellings and around eddies (Tew Kai et al. 2009). Tagged birds

TABLE 2
Residence times, inferred foraging events and percentage
of total foraging events within the boundaries ofPapahānaumokuākea Marine National Monument by six
Great Frigatebirds rearing young chicks on Tern Island
Hours
Individual
tracked% inferred% time inside Inferred
foraging eventsmonument foraging
inside monument
boundaries
eventsboundaries

L77

88

52

4

25

L78

51

100

2

100

L79

76

84

2

50

L80

430

21

16

6

L82

97

79

3

0

L83

60

100

1

100

Fig. 3. Foraging tracks of L80 from Tern Island. The second trip
was transmitted for 16 days until the transmitter battery expired.
Black circles within track indicate slower flight periods thought tocorrespond to foraging events. Dotted lines delineate bathymetry.

Marine Ornithology 40: 17–23 (2012)Gilmour et al.: Satellite telemetry of frigatebirds
traveled along the edges of shallow areas to the west (L82 and L83;
Fig. 2) and to the southwest (L80; Fig. 3) of Tern Island, and one
bird (L77) visited an area containing small seamounts to the northof Tern Island (4 500 m approximate depth; Fig. 2). Those areas
may exhibit some upwelling activity. In the ocean surrounding
the NWHI, northeasterly trade winds, in combination with the
Hawaiian Lee Current and the North Hawaiian Ridge Current,
influence water movement and nutrient exchange (Calil et al. 2008),
creating potentially important areas of local prey aggregation.
Notably, however, strong currents occur in an area 500 km southeastof Tern Island, and to a lesser extent 300 km to the northeast (Calilet al. 2008; Yoshida et al. 2010)—areas to which our tagged GreatFrigatebirds did not travel (Fig. 2).
Second, frigatebirds are reported to associate with predators such as
dolphins (Au & Pitman 1986) and tuna (Weimerskirch et al. 2004)
that drive schools of fish to the surface. These subsurface predators
are found in pelagic waters throughout the Pacific Ocean (Scott &Cattanach 1998; Bertrand et al. 2002), and Great Frigatebirds may
travel to deeper waters on both sides of the Monument’s boundaries
in search of feeding opportunities associated with them. Indeed,most of the potential foraging events we identified occurred overdeeper waters (4 500–5 100 m) where chlorophyll levels were low(0.35 mg/m2; NOAA CoastWatch Program, NASA Goddard Space
Flight Center & GeoEye, accessed 23 January 2011; http://las.pfeg.
noaa.gov/oceanWatch/oceanwatch.php#). In those situations, there
was an absence of upwelling or other obvious physical features that
could aid frigatebirds in finding food.
Third, some of the frigatebirds’ preferred prey exhibit high temporaland spatial variability. Important prey include flying fish and squid,
which are consumed throughout the year (Harrison et al. 1983;
Metz & Schreiber 2002; M.E. Gilmour and D.C. Dearborn, pers.
obs.), but other prey such as scad (Carangidae) and halfbeaks(Hemiramphidae) appear to be locally and seasonally important(Harrison et al. 1983). Changes in sea temperatures and productivity
affect the movements of adult and juvenile fish and squid (Pierce etal. 2008), potentially shifting foraging areas of Great Frigatebirds
throughout the breeding season.

21

In our study, all tagged birds were feeding chicks of similar age(4–12 days old) but foraged in very different areas surrounding
French Frigate Shoals. Birds traveled 74–732 km on foraging trips
lasting one to three days (Table 1). None of the six birds returnedto the same area on the second of two foraging trips, and four birds
made consecutive trips that were essentially in opposite directions(Fig. 2). Additionally, although transit rates from Tern Island toeach bird’s first potential foraging event (8.9–16.6 km/h) weresimilar to the return transit rates (8.9–17.4 km/h; Table 3), there
was variation among birds in both transit rates and linearity offlight paths on both the outbound and return portions of foraging
trips. For example, Great Frigatebirds exhibited both looping
and straight courses (Fig. 2) while flying at various transit rates(Table 1). A combination of slower transit rates and looping
courses suggests a bird continuously searching for prey, whereas
faster transit rates coupled with straight tracks may indicate abird quickly flying to known feeding areas. Experience in both
breeding and local foraging likely varied among our tracked GreatFrigatebirds. Foraging strategies incorporating experience havebeen demonstrated in northern seabirds such as Black-legged
Kittiwakes Rissa tridactyla (Irons 1998) and Thick-billed Murres
Uria lomvia (Woo et al. 2008).
Weimerskirch (2007) concluded that looping foraging courses
are more common in tropical seabirds than in seabirds foraging
in temperate or polar regions, and our tracked Great Frigatebird
movements contrast with tracking studies that found seabirds flying
directly to foraging grounds (Freeman et al. 2010) and/or making
straight-line return trips (Pettex et al. 2010). Tropical seabirds may
use varying foraging strategies because productive foraging areas insubtropical, pelagic waters are patchy (Longhurst & Pauly 1987). In
the absence of any one especially productive foraging area, GreatFrigatebird parents appear to explore a variety of areas around Tern
Island and the Monument.
We identified several potential foraging events at night (not depicted
in figures). While Weimerskirch et al. (2010) also observed alarger number of periods of reduced flight at night than during
the daytime, Weimerskirch et al. (2004) used altimeter data to

TABLE 3Time spent foraging and characteristics of outbound and return flights
of six Great Frigatebirds during early chick-rearing at Tern Islanda
Individual
L77

Outbound distance to first
Time
Transit rate Time spent Return distance from last
Time
Transit rate
foraging event (km)outbound (h) (km/h)
foraging (h) foraging event (km)inbound (h)(km/h)
149.6

14

10.7

3

59.5

4

14.9

213.4

18

11.9

2

313.6

18

17.4

L78

116.0

13

8.9

3

32.3

3

10.8

L79

53.2

4

13.3

3

240.4

23

10.5

L80

18.4

2

9.2

6

374.9

42

8.9

L82

315.3

19

16.6

3

185.1

18

10.3

319.9

27

11.9

3

191.6

18

10.7

243.0

18

13.5

2

251.2

18

14.0

L83
a

Twenty-eight foraging events were inferred among all tagged birds, but only two birds (L77 and L82) exhibited likely foraging events
during both tracked foraging trips. Because L80 did not return to Tern Island during the second foraging trip, only the first foraging trip
is included in the table.
Marine Ornithology 40: 17–23 (2012)22

Gilmour et al.: Satellite telemetry of frigatebirds

show that frigatebirds do not approach the surface after dark andconcluded that frigatebirds do not feed at night. It is possible thatfrigatebirds do not feed at night for lack of visibility, although we
tracked birds during all parts of the lunar cycle (new, half, andfull moon) and observed looping tracks and reduced transit rates(suggesting feeding) during each lunar phase, as well as during theday. Additionally, birds flew within the colony on moonlit nights,
suggesting that there is enough light for them to see. It is possible
that, during nights with sufficient moonlight, frigatebirds forage on
prey such as nocturnal, vertically migrating squid. Diet alone cannot
resolve the question—Black-footed and Laysan albatrosses from
Tern Island are thought to scavenge dead squid from the sea surface
during the day (Fernández & Anderson 2000), and it is possible that
Great Frigatebirds do the same.
CONCLUSION
Papahānaumokuākea is a protected area for many endemic andmigratory terrestrial and marine species. The protection andpreservation of the NWHI provides habitat for many species that use
the land and waters within the Monument for breeding. However,
because species such as Great Frigatebirds and other pelagic foragers(e.g. Black-footed and Laysan albatrosses, Kappes et al. 2010; Redfooted Boobies Sula sula, Young et al. 2010) travel long distances
to feed, the Monument only partially protects their food resources.
Knowledge of large-scale movements, feeding areas and wintering
areas are essential for developing management plans and protectedareas that safeguard the resources needed by these species.
ACKNOWLEDGEMENTS
We thank Beth Flint and the US Fish and Wildlife Service forlogistical support and access to Tern Island, and Angela Anders andFrans Juola for assistance in the field. We thank Scott Shaffer forproviding assistance with data analysis. Funding sources included
Bucknell University and Seabird Research Inc.
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