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Distribution and at-sea behavior of Bermudan
White-tailed Tropicbirds (Phaethon lepturus catesbyi)
during the non-breeding season
Miguel A. Mejıas,
1,4
Yolanda F. Wiersma,
1
David B. Wingate,
2,5
and
Jeremy L. Madeiros
3
1
Department of Biology, Memorial University, St. John’s, NL A1B 3X9, Canada
2
P.O. Box CR 86, Crawl, Hamilton Parish CR BX, Bermuda
3
Department of Environment and Natural Resources, Ministry of the Environment, P.O. Box FL588, Flatts FL BX,
Bermuda
Received 8 February 2017; accepted 31 March 2017
ABSTRACT. The movements and behavior of many taxa of seabirds during the non-breeding season
remain poorly known. For example, although studies conducted in the Pacific and Indian oceans suggest that
White-tailed Tropicbirds (Phaethon lepturus) seldom fly more than a few thousand kilometers from nest
colonies after breeding, little is known about the post-breeding movements and behavior of a subspecies of
White-tailed Tropicbirds (P. l. catesbyi) that breeds on islands in the North Atlantic Ocean. Our objective,
therefore, was to use light-based geolocators to identify the ranges and pelagic activities of White-tailed
Tropicbirds from Bermuda during the non-breeding periods in 2014–2015 (N=25) and 2015–2016
(N=16). Locations were estimated based on changes in light intensity across time, and pelagic activities were
determined based on whether geolocators attached to leg bands were wet (i.e., birds resting on the water’s
surface) or dry (i.e., birds in flight). In 2014, birds spent late summer (July–September) near Bermuda and
the British Virgin Islands; by mid-September, most (N=17; 68%) birds took a direct easterly route to the
Sargasso Sea. In 2015, most post-breeders (N=15; 94%) flew east from Bermuda and to the Sargasso before
the end of late summer. For both years combined, fall and winter (October–February) ranges extended as far
west as North Carolina and as far east as the mid-Atlantic Ridge. In both years, all birds were located between
Bermuda and the British Virgin Islands during the spring (April–May). All birds then flew north to Bermuda
in both years, with variations in timing, during April and May. We also found extensive overlap in the ranges
of males and females during the non-breeding season in both years. During the non-breeding season, White-
tailed Tropicbirds spent 5% of night periods and 41% of day periods in flight in 2014; in 2015, birds spent
8% and 42% of night and day periods, respectively, in flight. Tropicbirds spent more time flying during the
day because they hunt by day, detecting prey on the wing by sight. Overall, our results suggest that White-
tailed Tropicbirds that breed in Bermuda are diurnal, nomadic wanderers that range over an extensive area of
the Atlantic Ocean during the non-breeding season.
RESUMEN. Distribucion no reproductiva y comportamiento en alta mar del Rabijunco comun
originario de Bermudas (Phaethon lepturus catesbyi) en el Atlantico Norte
Los movimientos y el comportamiento de muchos taxones de aves marinas durante la temporada no
reproductiva permanecen todavıa pobremente conocidos. Por ejemplo, aun cuando estudios realizados en los
oceanos Pacıfico e Indico sugieren que el Rabijunco comun (Phaethon lepturus) rara vez vuela mas de unos
pocos miles de kilometros de la colonia de nidada luego de reproducir, poco se sabe sobre los movimientos y
el comportamiento post reproductivos de una subespecie de Rabijunco comun (Paethon l. castebyi) que
reproduce en islas del Oceano Atlantico Norte. Nuestro objetivo, entonces, fue usar geolocalizadores basados
en luz para identificar los rangos de distribucion y las actividades pelagicas del Rabijunco comun de Bermuda
durante los perıodos no reproductivos en 2014–2015 (N=25) y en 2015–2016 (N=16). Se estimaron las
ubicaciones a partir de los cambios en la intensidad de luz a lo largo del tiempo, y las actividades pelagicas
fueron determinadas a partir de si los geolocalizadores adheridos a las bandas de las patas estaban mojados
(i.e., aves descansando en la superficie del agua) o secos (i.e. aves volando). En 2014, las aves pasaron el
verano tardıo (julio-septiembre) cerca de Bermuda y las Islas Vırgenes Britanicas; para mediados de
septiembre, la mayorıa (N=17, 68%) de las aves tomaron una ruta directa y hacia el este al Mar de los
Sargazos. En 2015, la mayorıa de los individuos post reproductivos (N=15; 94%) volaron hacia el este desde
Bermuda y hacia el Mar de los Sargazos antes del final del verano. Para ambos a~nos combinados, los rangos en
oto~no e invierno (octubre-febrero) se extendieron hacia el oeste tan lejos como Carolina del Norte y hacia el
este tan lejos como la Dorsal Mesoatlantica. En ambos a~nos, todas las aves fueron ubicadas entre Bermuda y
las Islas Vırgenes Britanicas durante la primavera (abril-mayo). Todas las aves luego volaron norte a Bermuda
en ambos a~nos, con variaciones en el momento, durante abril y mayo. Tambien encontramos extensa
superposicion en los rangos de los machos y las hembras durante la temporada no reproductiva en ambos
a~nos. Durante la temporada no reproductiva, los Rabijuncos comunes pasaron el 5% de los perıodos
©2017 Association of Field Ornithologists
1
J. Field Ornithol. 0(0):1–14, 2017 DOI: 10.1111/jofo.12198
nocturnos y el 41% de los perıodos diurnos en vuelo en 2014; en 2015, las aves pasaron el 8% y el 42% de
los perıodos nocturnos y diurnos, respectivamente, en vuelo. Los Rabijuncos pasaron mas tiempo volando
durante el dıa debido a que cazan de dıa, detectando las presas en vuelo por medio de la vista. En general,
nuestros resultados sugieren que los Rabijuncos comunes que crıan en Bermuda son diurnos, nomades errantes
que se distribuyen en una extensa area del Oceano Atlantico durante la temporada no reproductiva.
Key words: geolocators, migration, pelagic activities, tropical seabirds, sexual segregation
Tropicbirds (Phaethontidae) are medium-
sized tropical seabirds whose year-round at-sea
ranges are poorly known. White-tailed Trop-
icbirds (Phaethon lepturus catesbyi), a subspecies
of the smallest (mean mass =385 g) species,
breeds on islands in the North Atlantic Ocean,
including Bermuda which supports the largest
(~3500 nesting pairs) population in the entire
Atlantic (Lee and Walsh-McGehee 2000, Dob-
son and Madeiros 2009, J. L. Madeiros and
M. A. Mejıas, unpubl. data). In the tropics,
these tropicbirds are asynchronous breeders,
with some birds nesting year-round (Ramos
et al. 2005, Catry et al. 2009a). In contrast,
Bermudan tropicbirds, known locally as
“Longtails”, have a defined breeding season
from March to September, with birds leaving
Bermuda as late as November to unknown
non-breeding areas. At-sea surveys and GLS-
logger data collected in the Pacific and Indian
Oceans suggest that post-breeding White-tailed
Tropicbirds seldom travel more than a few
thousand kilometers from nest colonies, but
their movements and at-sea behavior in the
Atlantic Ocean during the non-breeding season
remain unexplored (Spear and Ainley 2005a,
Le Corre et al. 2012).
Our objective, therefore, was to use geolo-
cators to examine, for the first time in Atlan-
tic waters, the distribution and pelagic
activities of White-tailed Tropicbirds during
the non-breeding season. Specific objectives
were to: (i) identify the ranges of tagged trop-
icbirds during the non-breeding season, (ii)
identify late-summer, fall-winter, and spring
movements and core areas, (iii) determine if
tropicbirds exhibit sexual segregation during
the non-breeding season, and (iv) quantify
diurnal and nocturnal at-sea behaviors.
Although known to travel in loose flocks a
few kilometers offshore from Bermuda during
the breeding season, ship sightings suggest
that White-tailed Tropicbirds forage solitarily
when at sea, plunge diving for fish and squid,
and avoiding mixed-species flocks (Gross
1912, Catry et al. 2009c). Given their noma-
dic nature when far from breeding sites, we
expected no pelagic segregation between non-
breeding male and female tropicbirds. The diel
activity patterns of this species during the
non-breeding period remain uncertain. In Ber-
muda, tropicbirds spend morning and early
afternoons feeding chicks and performing
courtship flights at nest sites, with activity grad-
ually declining in late afternoon and ceasing by
nightfall (Gross 1912). If the behavior of trop-
icbirds during the breeding season matches
their behavior during the non-breeding season,
we expected greater activity by non-breeding
tropicbirds during the day than at night.
METHODS
GLS logger programming. We captured
White-tailed Tropicbirds at nine breeding
sites in Bermuda (32°310N, 64°750W) from
2014 to 2016 (Fig. 1). Prior to deployment
of geolocators (1 g; C-65 Migrate Tec Inti-
geo, Migrate Technology Ltd: Cambridge,
UK; hereafter, GLS loggers), we activated six
random GLS loggers and zip-tied them to a
low shrub with no leaves on Nonsuch Island
(G on Fig. 1) for 31 days in 2014 for open-
sky calibration (Lisovski et al. 2012). Each
GLS logger measured the light regimes at this
site, producing a single elevation angle. Fol-
lowing the calibration period, we took the
average of the sun elevation angles from the
six GLS loggers to use as the reference sun
angle for any recaptured birds in both years
(Lisovski et al. 2012). We set all GLS loggers
on “mode 6”, allowing them to estimate geo-
graphic location based on changes in light
intensity across time, and to record periods of
saltwater immersion every 30 sec. Saltwater
immersion values ranged from 0 (completely
dry) to 20 (completely saturated) and were
saved at 10-min intervals each day.
GLS logger deployment and retrieval. We
captured 30 tropicbirds in both 2014 and
2015 (N=60) and avoided re-sampling
4
Corresponding author. Email: mmejias@mun.ca
5
No affiliation.
M. A. Mejıas et al.2J. Field Ornithol.
individuals; GLS loggers were deployed from
July to August and during May in 2014 and
2015. We captured birds by removing them
from nest cavities by their bills and placing
them in a cotton weighing bag, where they
remained for the following procedures. Cap-
tured adults were at various breeding stages,
including sitting in cavities with no egg or
chick, incubating an egg, or brooding a chick.
We recorded mass with a 500-g Pesola spring
scale (1 g). We banded adults with a unique
identification incoloy metal band (0.5 g) on
their right leg and with a plastic Darvic band
equipped with a single GLS logger (<0.5% of
adult body mass) on the left leg. We secured
GLS loggers to the Darvic band with a small
zip tie, with excess zip tie being cut, and mod-
erate application of quick-dry two-part marine
epoxy (Amazing GOOP, Eclectic, Eugene,
OR). We kept birds in the weighing bag until
the marine epoxy was dry. GLS loggers plus
band weighed ~2g.
After processing, we returned adults to nest
cavities and placed a towel in the cavity
entrance for 5 min to prevent immediate flee-
ing and give birds time to calm down. We
briefly checked adults immediately after towel
removal. From April to June 2015 and 2016,
we revisited all nest sites weekly to remove
GLS loggers from recaptured birds. All recap-
tured birds were weighed as described above.
In addition, we collected eight to 10 flank
Fig. 1. Sites where White-tailed Tropicbirds were captured and fitted with GLS loggers across Bermuda
in 2014 and 2015. (A) Daniel’s Head, (B) Bay House, (C) Bermuda Aquarium, (D) Shelly Bay, (E)
Spittal Pond, (F) Ferry Reach, (G) Nonsuch Island, (H) Horn Rock, and (I) Cooper’s Island. Numbers
represent the number of tagged and recaptured (in parentheses) birds at each site.
Non-breeding Movements of White-tailed TropicbirdsVol. 0, No. 0 3
feathers from each recaptured adult and
placed samples in paper envelopes and refrig-
erated them until they were analyzed for
genetic sex determination (Fridolfsson and
Ellegren 1999) at the Genomics and Pro-
teomics Facility at Memorial University of
Newfoundland. Total handling time for cap-
tured and recaptured tropicbirds was 8–
10 min.
GLS logger analysis and mapping. To
view raw light data from recovered GLS log-
gers, we imported and viewed each day as
light curves using IntiProc v2.0 Geolocation
Processing Software from Migrate Technol-
ogy. We then used the “auto-mark up” com-
mand in IntiProc to estimate sunrise and
sunset events for each light curve. Deep,
abrupt dips in light curves were likely caused
by birds spending long periods in nest cavi-
ties. Therefore, approximate departure dates
(i.e., start of the non-breeding period) of
birds whose nesting fate were unknown were
determined by identifying the last date birds
exhibited nest-cavity shading in their light
curves. For birds whose nesting fates were
known, departure dates were estimated using
our nest-monitoring data.
Light curves where sunrise and sunset
events were disrupted (i.e., irregular rather
than a smooth curve) by cloud cover were
identified and removed. We used 7.3°as
our sun elevation angle based on our averaged
calibration data. We further validated this ele-
vation angle by looking at the distributions of
birds around Bermuda during the entire
breeding season using IntiProc. To account
for the natural latitudinal error associated
with GLS loggers, we smoothed validated
non-breeding data twice by taking the average
of the previous, current, and subsequent posi-
tions (Phillips et al. 2004a, Fifield et al.
2014). To avoid potential positional errors,
fixed start positions (departure date and
return date) for each bird were not smoothed
(Phillips et al. 2004a). Our tropicbird move-
ment data were erratic between 16 September
and 19 October and between 20 February
and 9 April (i.e., fall and spring equinoxes,
respectively) so these time periods were
excluded from our dataset. The earliest sight-
ings of returning White-tailed Tropicbirds in
Bermuda are from mid-February through
March so the return dates of tagged birds
coincided with the spring equinox where data
are unreliable. Although we observed much
crevice-shading among light curves in March,
corresponding to spring sightings in Ber-
muda, GLS data for one bird showed crevice-
shading post-equinox in April when it was
located close to Puerto Rico and the British
Virgin Islands south of Bermuda. This sug-
gests that some birds may visit crevices on
other islands in the Caribbean. Without reli-
able latitudinal data during the equinox, and
because Bermuda and Puerto Rico are at sim-
ilar longitudes, we could not confidently
determine which island birds were visiting
during the spring equinox.
Tropicbirds in nest cavities with neither
eggs nor chicks when recaptured were consid-
ered to be non-breeding adults and, for these
birds, the day of recapture was considered the
end of the non-breeding period. For trop-
icbirds incubating eggs when recaptured, we
estimated the approximate date of egg laying
using the hatch date in combination with evi-
dence of crevice-shading to determine the
approximate end to their non-breeding
periods.
After IntiProc analysis, tracking data col-
lected during the non-breeding period were
imported into ArcGIS (ESRI, v.10.3.1: Red-
lands, CA). Data were projected using the
Transverse Mercator Complex projection
(Projected Coordinate system: WGS_1984_
Complex_UTM_Zone_21N). We generated
kernel densities representing the non-breeding
locations with Geospatial Modelling Environ-
ment (GME) (v. 0.7.4.0; Beyer 2015). In
GME, we used a raster resolution of 40 km
for all kernel densities. We are aware that
50 km is commonly used for seabird geolo-
cation studies (Phillips et al. 2005, Raine
et al. 2013, Hedd et al. 2014). However, we
used a 40-km cell size strictly for visualizing
tropicbird distribution; we did not include
any environmental layers in our GIS analy-
sis. In addition, 40 km seemed like an
appropriate compromise between considering
the spatial error of GLS loggers and captur-
ing key concentration areas. We then used
GME to calculate 30, 50, 70, and 90% con-
tours for each kernel density, with 50% con-
tours representing “core” areas. We generated
kernel densities for the following periods for
2014–2015 and 2015–2016: (i) entire non-
breeding period (July to May), (ii) late sum-
mer (July to mid-September), (iii) fall-winter
M. A. Mejıas et al.4J. Field Ornithol.
(late October to mid-February), and (iv)
spring (April and May). We chose these sea-
sonal break points to have finer-grained reso-
lution of post-breeding movements, an
approach also used in other tracking studies
of seabirds (Lorentsen and May 2012,
Reiertsen et al. 2014). Lastly, we generated
50% contours representing core ranges of
male and female tropicbirds across the non-
breeding season.
To quantify pelagic activities, we manu-
ally restored sunrise and sunset events asso-
ciated with the breeding season in IntiProc.
We then imported the immersion data for
each bird into R v2.12.1, where we catego-
rized each 24-h period into day and night
periods. We calculated the approximate
duration of day and night for each day
from the light-curve data. We then used
the plot function in R v2.12.1 (R Develop-
ment Core Team 2010) to graph the aver-
age amount of time all birds were dry
during the breeding and non-breeding peri-
ods. To avoid overestimation of dry periods
during the breeding season, we removed
sunrise and sunset events where birds were
clearly inside nest cavities.
Statistical analysis. We used paired
t-tests to compare the mean body mass of
White-tailed Tropicbirds when first captured
and when recaptured within years. We then
used independent t-tests to determine if the
proportion of time birds were wet and dry
during day and night periods differed
between the sexes across the non-breeding
period within years. All t-tests were two-
tailed, and results were considered significant
if P<0.05. All statistical tests were run in
R v2.12.1 (R Development Core Team
2010).
RESULTS
Retrieval details and body condition. In
2015, we recaptured 25 of 30 breeding birds
(83%) fitted with GLS loggers in 2014. In
2016, we recaptured 23 of 30 birds (76%)
tagged in 2015. In total, all 25 loggers recov-
ered in 2015 and 16 of 23 loggers (70%) in
2016 recorded data until birds were recap-
tured, providing movement data for 41 trop-
icbirds (N=24 males and 17 females). Birds
were lighter when GLS loggers were first
deployed than when recaptured in both
2014–2015 (t
21
=2.2, P=0.04) and
2015–2016 (t
22
=7.6, P<0.001).
Non-breeding distribution. Departure
dates of tropicbirds from breeding sites ran-
ged from 4 July to 8 September in 2014 and
from 29 June to 9 September in 2015. Dur-
ing the non-breeding period (July to May) of
2014–2015, tropicbirds were distributed
widely across the North Atlantic Ocean
(Fig. 2A). The non-breeding range of birds
with GLS loggers extended north to the
Grand Banks of Newfoundland, east to the
mid-Atlantic Ridge, south to the British Vir-
gin Islands, and west to between Bermuda
and North Carolina. Core areas (50% ker-
nels) were concentrated around Bermuda and
north of the British Virgin Islands, between
Bermuda and the mid-Atlantic Ridge (this
boundary hereafter the Sargasso Sea), and the
mid-Atlantic Ridge.
In 2015–2016, the non-breeding range of
tropicbirds was similar, extending from North
Carolina to the west and to waters between
Bermuda and the British Virgin Islands to the
south (Fig. 2B). Core areas included the Sar-
gasso Sea and waters just south of the
Georges Banks.
During the late-summer period (July to
mid-September) of 2014, core areas were near
Bermuda, the area between Bermuda and the
British Virgin Islands, and the Sargasso Sea
(Fig. 2C). Fourteen of 25 birds ranged from
Bermuda to the Sargasso Sea. Eleven other
birds wandered between Bermuda and the
British Virgin Islands, five of which returned
to Bermuda, and one stopped between Ber-
muda and the British Virgin Islands. By mid-
September, 17 (68%) birds, 16 directly from
Bermuda and one between Bermuda and the
British Virgin Islands flew east into the Sar-
gasso Sea, with two birds reaching the mid-
Atlantic Ridge. During this same period, five
(20%) birds remained in the Caribbean Sea
and three in Bermuda. In late summer 2015,
core tropicbird areas were near Bermuda and
in the Sargasso Sea (Fig. 2D). Fifteen of 16
birds (94%) ranged from Bermuda to the
Sargasso Sea, and spent most of late summer
in the Sargasso Sea. By the end of the late-
summer period, nine (56%) birds were in the
Sargasso Sea and six (38%) reached the mid-
Atlantic Ridge. The last bird spent the entire
late-summer period in waters between Ber-
muda and Turks and Caicos.
Non-breeding Movements of White-tailed TropicbirdsVol. 0, No. 0 5
During the fall and winter period (late
October to mid-February) of 2014, core areas
were concentrated around Bermuda, the Sar-
gasso Sea, and the mid-Atlantic Ridge
(Fig. 2E). Prior to this, 19 of 25 tropicbirds
(76%) were foraging along the southern
Grand Banks of Newfoundland between late
October and mid-November, but then flew
rapidly southward, dispersing into the above
core areas. Eight (32%) birds wintered in the
Sargasso Sea and the mid-Atlantic Ridge, five
(20%) ranged from the Sargasso Sea to Ber-
muda, and two (8%) birds spent most of
winter near Bermuda. Eight (32%) other
tropicbirds largely used the Sargasso Sea, and
one spent the period from mid-October to
AB
CD
Fig. 2. Pelagic distribution of White-tailed Tropicbirds (2014–2015, N=25; 2015–2016, N=16)
from Bermuda (indicated by stars) during the non-breeding period (i.e., from departure to approximate
return; A and B), late-summer period (C and D), fall-winter period (E and F), and spring period (G
and H) and by sex for the entire non-breeding period (I and J). Bermudan waters (i.e., Bermuda’s exclu-
sive economic zone, or EEZ) are indicated on maps by dashed circles with a radius of 370.4 km. Colors
indicate the relative concentration of tropicbirds during specific periods, with areas of greatest use shown
in red, expanding outward to areas used least shown in green. Tropicbird distributions in 2014–2015
and 2015–2016 are shown in the left and right columns, respectively. Sea-floor layer downloaded from
www.NaturalEarthData.com.
M. A. Mejıas et al.6J. Field Ornithol.
IJ
EF
GH
Fig. 2. Continued
Non-breeding Movements of White-tailed TropicbirdsVol. 0, No. 0 7
late December just south of the Nova Scotian
shelf before flying south to the Sargasso Sea
where it spent the rest of the winter period.
The last bird spent parts of December and
January along the coast of North Carolina,
then flew to the southern George’s Banks in
early February, before ending its winter per-
iod near Bermuda.
During the fall and winter of 2015, the
core area included the Sargasso Sea and
waters south of the Georges Banks (Fig. 2F).
Between late October and mid-November, 10
of 16 birds (63%) were foraging in the south-
ern Grand Banks of Newfoundland before
flying south to core wintering areas by
December. The Sargasso Sea was used by 13
of 16 wintering birds (81%). Six (38%) trop-
icbirds wintered in the Sargasso Sea and south
of the Georges Banks, and two (13%) pri-
marily in the Sargasso Sea. Four (25%) birds
wintered in the Sargasso Sea and the mid-
Atlantic Ridge, and two wintered mostly
along the coasts of North Carolina before fly-
ing eastward to Bermuda, and to the Sargasso
Sea by the end of winter. The remaining two
birds spent half of their fall and winter period
in the core area (i.e., one in the Sargasso Sea,
and the other south of the George’s Banks)
and the other half in Bermudan waters.
During the spring periods, we were unable
to determine the movements of six birds in
both years because our nest-monitoring data
suggested that they began breeding during the
equinox period. During the spring period
(April–May) of 2015, all tropicbirds were
located in a core area between Bermuda and
the British Virgin Islands (Fig. 2G). Sixteen of
19 birds (84%) spent most of the spring period
there. During early and mid-April, 11 trop-
icbirds were in the Caribbean Sea, near His-
paniola, Puerto Rico, and the British Virgin
Islands. The onset of a movement north to
Bermuda began by mid-April, with 12 individ-
uals arriving in Bermuda before the end of
April. The seven remaining birds stayed in the
spring core area, flying north to Bermuda in
May. During spring 2016, all tropicbirds were
in the area between Bermuda and the British
Virgin Islands (Fig. 2H). Eight (80%) of 10
birds spent most of the spring in this core area.
Five tropicbirds were located near Puerto Rico,
Hispaniola, and the British Virgin Islands
through early and mid-April. Nine birds flew
north, back to nest sites by the end of April;
the last individual flew north from the core
area to Bermuda in late May.
Distribution of males and females. We
recaptured 14 males and 11 females in 2015
and 10 males and six females in 2016. In
2014–2015, core areas during the non-breed-
ing period for both sexes were concentrated
in Bermuda, the Sargasso Sea, and the mid-
Atlantic Ridge; ranges of males and females
overlapped extensively (Fig. 2I). In 2015–
2016, core areas of males and females were
concentrated largely in the same areas of the
Sargasso Sea and south of the Georges Banks
(Fig. 2J).
At-sea activity patterns. During the
2014 and 2015 breeding seasons, birds spent
a greater percentage of their time dry during
both day and night periods (Fig. 3A, B). In
both years, the start of the non-breeding sea-
son was marked by a sharp decline in the per-
centage of time birds were dry, particularly at
night (Fig. 3A, B). This trend persisted
throughout the non-breeding periods, with
the breeding seasons of both 2015 and 2016
beginning with an abrupt increase in time
birds spent dry during the day and night
(Fig. 3A, B).
In 2014, non-breeding males and females
did not differ in time spent wet during either
the day (t
18
=1.4, P=0.18) or night
(t
22
=1.5, P=0.15). Similarly, non-breeding
males and females did not differ in time spent
wet during the day (t
13
=0.02, P=0.97) or
night (t
8
=1.0, P=0.33) in 2015. During
the non-breeding periods of both 2014 and
2015, tropicbirds (males and females com-
bined) flew more during the day than night
(Fig. 3A, B). However, during the non-breed-
ing period in 2014, tropicbirds spent more
time on the water (59% of time; mean =7h)
than in flight (41%; 5 h) during the day. Dur-
ing the non-breeding period in 2015, trop-
icbirds also spent more time on the water
(58%; 7 h) than flying (42%; 5 h) during the
day. All non-breeding birds spent most of the
night periods on the water in both 2014 (95%;
11 h) and 2015 (92%; 10.5 h) (Fig. 3A, B).
DISCUSSION
We recaptured most White-tailed Trop-
icbirds with GLS loggers attached during the
previous breeding season in both 2015 (83%)
and 2016 (76%). The high nest-site fidelity
M. A. Mejıas et al.8J. Field Ornithol.
of Bermudan White-tailed Tropicbirds, whose
nest cavities limited escape possibilities, con-
tributed to our high recapture rates. Birds we
failed to recapture (N=5 in 2015; N=7in
2016) were not observed and may have died
during the non-breeding period. In support
of this possibility, the survival rate of White-
tailed Tropicbirds on Aride Island was 0.81
(Catry et al. 2009a), a percentage similar to
our recovery rates. Alternatively, birds that
were not recaptured may have used different
nest cavities. For example, in 2015, we recap-
tured one adult in a nest cavity located a few
meters from the one it used in 2014.
We found that the mass of White-tailed
Tropicbirds when recaptured was greater than
that when tagged in both years. Other investi-
gators have also reported that tracking devices
did not cause seabirds to lose body mass
(Adams et al. 2009, Nisbet et al. 2011).
Although seabirds carrying tracking devices
may experience reduced flight efficiency (Pas-
sos et al. 2010) and have lower colony atten-
dance (S€ohle et al. 2000), the high return
rates of tagged tropicbirds in our study, plus
the increase in mass, suggest that the small
GLS loggers had minimal negative effects.
During the late-summer period (July to
mid-September), distribution patterns of non-
breeding tropicbirds in our study differed
slightly in 2014 and 2015. In both years,
birds ranged from Bermuda to the mid-
Atlantic Ridge. Some post-breeding birds
wandered between Bermuda and the British
Virgin Islands, supporting the hypothesis that
Bermudan tropicbirds migrate directly to the
Caribbean after breeding (Amos 1991). In
2014, 11 post-breeding tropicbirds flew to
the Caribbean, five of which returned to Ber-
muda before the end of the late-summer per-
iod. However, the most common flyway, used
mainly by post-breeding birds near Bermuda,
in both years, was a strong eastern departure
from the island to the Sargasso Sea, which we
interpret as the typical departure route of Ber-
mudan tropicbirds. The one exception was a
bird in 2014 that took an eastward route to
the Sargasso Sea from waters between Ber-
muda and the British Virgin Islands. This
eastward movement of birds from Bermuda
by mid-September may indicate a decline in
prey availability in Bermudan waters, such as
the Caribbean reef squid (Sepioteuthis sepi-
oidea), Atlantic flying fish (Exocoetidae spp.),
and pufferfish (Tetraodontidae) (M. A.
Mejıas, unpubl. data).
During the fall and winter (late October to
mid-February) of 2014, White-tailed Trop-
icbirds in our study exhibited considerable
variation in their distributions, with some
found near Bermuda, the Sargasso Sea, and
the mid-Atlantic Ridge. Bermuda Petrels
(Pterodroma cahow) have a similar distribution
during their non-breeding period (Madeiros
Fig. 3. Average proportion of time White-tailed Tropicbirds were dry during day (hollow circles) and
night (dark circles) periods during the breeding and non-breeding periods of (A) 2014 (N=25) and (B)
2015 (N=16). The approximate start and end of the non-breeding period are denoted with solid and
dashed lines, respectively. During both years, the average percent time that birds were dry during day
and night periods declined during the non-breeding period, and steadily increased during the following
breeding period.
Non-breeding Movements of White-tailed TropicbirdsVol. 0, No. 0 9
et al. 2013). In the case of Bermudan trop-
icbirds, the full extent of their non-breeding
ranges was similar in both years. Despite
White-tailed Tropicbirds being viewed largely
as a tropical species, many individuals from
our subtropical Bermuda population were
located in the southern Grand Banks of New-
foundland from late October to mid-Novem-
ber in both 2014 and 2015. The Grand
Banks is a nutrient-rich zone, with significant
nutrient upwelling that supports large num-
bers of fish and squid (Anderson and Gardner
1986, Montevecchi and Myers 1995). These
fish and squid densities, in turn, support an
estimated 40 million seabirds annually (Mon-
tevecchi and Tuck 1987, Lock et al. 1994,
Hedd et al. 2011).
Other investigators have also reported
sightings of tropicbirds near Newfoundland.
For example, a Red-billed Tropicbird (Phae-
thon aethereus) was observed on the New-
foundland Banks in 1876 (Mactavish 2005).
Similarly, in 2006, a carcass of a White-tailed
Tropicbird was found in St. John’s, New-
foundland, in mid-September 2006 after a
tropical storm (Mactavish 2007). The pres-
ence of tropicbirds off the coast of New-
foundland is likely explained by the warm
subtropical waters of the Gulf Steam that run
from the southern tip of Florida to eastern
Newfoundland. Our observation of many
Bermudan tropicbirds on the Grand Banks
during both years of our study suggests that
this is an important foraging area for this
species in the fall. Although their prey in
these waters remain unknown, possibilities
include Atlantic saury (Scomberesox saurus)
and northern shortfin squid (Illex illecebrosus)
(Hurley 1980, Dudnik et al. 1981, Perez
1994). Both of these species can be as small
as 20 cm and are found near the surface,
traits favoring plunge diving by foraging trop-
icbirds (Squires 1957, Dudnik et al. 1981,
Wigley 1982). Time spent on the Grand
Banks by tropicbirds appears to be con-
strained by temperature. In both years of our
study, all birds flew south from the Grand
Banks by mid-November, coinciding with the
cooling of the area by the Labrador Current
(Han et al. 2010).
Bermuda is seemingly void of tropicbirds
during the winter months, but our results indi-
cate that a few spend much of the non-breed-
ing season within Bermuda’s exclusive
economic zone, albeit far offshore. In 2014,
two tropicbirds spent most of the fall and win-
ter period in Bermudan waters, coinciding
with rare onshore sightings of this species in
December and January (J. L. Madeiros,
unpubl. data). This also matches the distribu-
tion of White-tailed Tropicbirds in the Indian
Ocean, where geolocators revealed that some
post-breeders remained among their breeding
colony in the Seychelles (Le Corre et al.
2012). A likely reason why most tropicbirds
did not winter near Bermuda is because
weather conditions typically deteriorate during
the winter. Being subtropical, Bermudan win-
ters are dominated by strong gales and heavy
rains with few calm days, conditions that con-
trast with those during the breeding season
(Diamond 1975, Phillips 1987, Amos 1991).
The Sargasso Sea, an area bordered by cur-
rents and so-named because of the presence
of floating mats of Sargassum seaweed (Trott
et al. 2010), supported the greatest number
of wintering tropicbirds during both years of
our study. In addition to being crucial habitat
for juvenile turtles and a diverse assemblage
of invertebrates, Sargassum seaweed serves as
spawning substrate for Atlantic flyingfish an
important food source for tropicbirds (Adams
1960, Dooley 1972, Sterrer 1992, Mansfield
et al. 2014). The influx of tropicbirds to the
Sargasso Sea was apparent by mid-November,
after many birds flew there from the Grand
Banks in both years of our study. Similarly,
Haney (1986) reported more than 50% of
White-tailed Tropicbird sightings off the east-
ern coast of Florida occurred over Sargassum
patches where they were foraging.
Our fall and winter kernel analysis did not
capture the extreme movements displayed by
some birds. Across both years, nine trop-
icbirds were located in the Labrador Sea in
late October, including three birds just south
of Greenland, before all birds left the Labra-
dor Sea and flew south by November.
Although latitudinal error of the geolocators
could explain these extreme northern distribu-
tions, the removal of equinox periods fol-
lowed by smoothing reduced the likelihood
of such errors. Alternatively, strong weather
systems could have forced these birds further
north. However, tropicbirds were still in the
Grand Banks several weeks after Hurricane
Gonzalo, which first passed over Bermuda on
18 October 2014 and passed the Avalon
M. A. Mejıas et al.10 J. Field Ornithol.
Peninsula of Newfoundland several days later.
Therefore, our data suggests that the presence
of tropicbirds in Newfoundland waters was
not entirely storm dependent. The greatest
longitudinal distance moved by a White-tailed
Tropicbird in our study was by one that flew
east in late November 2014, stopping west of
the Azores and ~2500 km from Bermuda.
Previously, Monticelli and Aalto (2012)
reported a rare sighting of a White-tailed
Tropicbird in the Azores in late October.
During the spring (April–May) of 2015
and 2016, all non-breeding White-tailed
Tropicbirds were located between Bermuda
and the British Virgin Islands, demonstrating
a more localized core area that contrasts with
their more extensive movements during the
winter. The aggregation of tropicbirds in this
core area could coincide with when they
become more social as the breeding season
approaches. Alternatively, this area may be a
productive, pre-breeding foraging area (Catry
et al. 2009b). Our data also confirm that the
ranges of Bermudan tropicbirds and Carib-
bean tropicbirds overlap. For both years com-
bined, 16 White-tailed Tropicbirds in our
study spent the early to mid-spring period
near the British Virgin Islands and Puerto
Rico, the latter island supporting 200–300
nesting pairs of White-tailed Tropicbirds (Lee
and Walsh-McGehee 2000). We also found
abrupt changes in the light data for one bird,
indicating cavity shading, during the time it
was located close to Puerto Rico and the Bri-
tish Virgin Islands in early April. This sug-
gests Bermudan tropicbirds may enter cavities
or crevices while in the Caribbean, possibly
following resident tropicbirds to their nest
sites, a behavior also observed at breeding
sites in Bermuda, where prospecting adults
enter cavities belonging to established pairs.
(M. A. Mejıas, unpubl. data).
As expected, we found no evidence of pela-
gic segregation by non-breeding male and
female White-tailed Tropicbirds. Both males
and females were located in Bermudan waters,
the area between Bermuda and the British
Virgin Islands, the Sargasso Sea, and the mid-
Atlantic Ridge in 2014, and in the Sargasso
Sea and waters south of the Georges Banks in
2015. Although nothing is known about the
possibility of sexual segregation among either
tropicbirds or ecologically similar seabirds,
such segregation has been reported among
species in the order Procellariiformes. For
example, sexual segregation during the
non-breeding period has been reported for
Wandering Albatrosses (Diomedea exulans),
Grey-headed Albatrosses (Thalassarche chrysos-
toma), Black-browed Albatrosses (Thalassarche
melanophris), and Northern Giant Petrels
(Xavier et al. 2004, Phillips et al. 2004b,
Gonzalez-Solıs et al. 2007). These species
exhibit moderate-to-extreme sexual size
dimorphism, allowing one sex to exploit prey
in specific areas more effectively than the
other sex. In contrast, male and female
White-tailed Tropicbirds are similar in size,
minimizing the likelihood of behavioral dom-
inance by one sex, and hunt solitarily, mini-
mizing the likelihood of competition for
resources by male and female tropicbirds
(Catry et al. 2009c, J. L. Madeiros, unpubl.
data).
The pelagic activities of White-tailed Trop-
icbirds differed between the breeding and
non-breeding periods in 2014 and 2015. In
both years, breeding birds spent a greater per-
centage of time dry during both the day and
night. This was expected because nesting
birds regularly visit nest sites. In contrast,
non-breeding seasons began with an abrupt
and consistent trend of birds spending less
time dry during both day and night. In addi-
tion, White-tailed Tropicbirds in our study
flew more during the day than night during
the non-breeding period. The limited time
spent flying during the night by White-tailed
Tropicbirds in our study was not surprising
because tropicbirds hunt by day, detecting
prey on the wing by sight rather than by use
of olfactory cues like nocturnal seabirds
(Nevitt 1999). In both years, non-breeding
tropicbirds spent an average of 5 h in daily
flight. We interpret these long dry periods as
time spent foraging in areas where prey are
patchily distributed in nutrient-poor tropical
and subtropical waters, especially when travel-
ing between rafts of Sargassum seaweed (Rus-
sel-Hunter 1970, Flint 1991). Although non-
breeding tropicbirds in our study spent more
time flying during the day than at night, they
still spent a greater proportion of day periods
resting on the water. Similarly, Spear and
Ainley (2005b) found that White-tailed Trop-
icbirds in the Pacific spent most of the day
resting on the water. In both years, all trop-
icbirds spent most night periods on the water,
Non-breeding Movements of White-tailed TropicbirdsVol. 0, No. 0 11
confirming for the first time that tropicbirds
roost on the water at night.
Our results show that the fall and winter
(October–February) ranges of White-tailed
Tropicbirds extended as far west as North Car-
olina and as far east as the mid-Atlantic Ridge.
The range of post-breeding birds from Ber-
muda also overlapped that of Caribbean trop-
icbirds during the late summer and spring.
Despite variation in movements among indi-
viduals, males and females largely shared the
same non-breeding areas. Overall, our results
suggest that White-tailed Tropicbirds that
breed in Bermuda range over an extensive area
of the Atlantic Ocean during the non-breeding
season.
ACKNOWLEDGMENTS
This is Contribution #255, Bermuda Biodiversity
Project (BBP), Bermuda Aquarium, Natural History
Museum and Zoo, Department of Environment and
Natural Resources. We thank the Bermuda Depart-
ment of Environment and Natural Resources for their
support of this research, specifically D. Pettit, for
granting permission to conduct tropicbird surveys
throughout Bermuda’s nature reserves and M. Outer-
bridge for issuing the research permit. Special thanks
to D. Butterfield who allowed us to conduct fieldwork
on his private property. We also extend our thanks to
Albert and Patricia Mejıas who assisted with the cap-
ture and recapture of tropicbirds. Our field work was
made possible by generous funding provided by the
Bermuda Zoological Society and the NSERC grant of
I. Jones, the latter individual who originally proposed
studying the at-sea movements of tropicbirds with
geolocators and created the tropicbird silhouettes in
Figure 3. We thank C. Schacter, K. Robbins, and D.
Mercer for the immense help with geolocator analysis,
and T. Miller, G. Robertson, W. Montevecchi, and A.
Bond for helpful comments on an earlier version of
this manuscript. We are indebted to B. Pilgrim for
doing genetic work on feathers for sex determination.
Lastly, we would like to thank G. Ritchison and the
two anonymous reviewers for feedback that greatly
improved our paper.
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