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Overland flight by seabirds at Isla Isabela, GalÁpagos

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Oceanic seabirds are generally thought to avoid overland flight, perhaps limiting larger-scale distribution, but examples exist to the contrary. We asked whether Blue-Footed Boobies Sula nebouxii, Brown Pelicans Pelecanus occidentalis, or frigatebirds Fregata spp. crossed the Perry Isthmus (width 12.25 km, minimum height 23 m), a low point in the wasp-waist shape of Isla Isabela, Galápagos. Except for the Perry Isthmus, Isla Isabela presents an elongated, high-elevation challenge to movement between the central and western waters of Galápagos. Daytime observations over 3.5 d in June 2012 revealed ≥48 crossings by boobies and more than two crossings by frigatebirds. Overland crossing of a terrestrial barrier of this size may be facilitated by the seabirds’ ability to see water across the Perry Isthmus from an altitude of 42 m looking west and 43 m looking east, altitudes that both the boobies and frigatebirds attain during foraging.
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Anchundia et al.: Seabirds at Isla Isabela, Galápagos 139
Marine Ornithology 45: 139–141 (2017)
139
INTRODUCTION
Volant seabirds are highly mobile, and many can fly hundreds of
kilometers on most days of their lives (Prince et al. 1992). Despite
this extreme vagility, some oceanic species are thought to avoid
overland flight, except to reach their nesting sites (Friesen et al.
2007, Friesen 2015). Nonetheless, Arctic Terns Sterna paradisea
migrate across the Andes and southern Argentina (Duffy et al.
2013), and boobies and albatrosses have appeared hundreds of
kilometers inland in the absence of forcing by severe weather
(Dunn & Unitt 1977, Patten & Minnich 1997, Sullivan et al. 2009),
suggesting that some oceanic birds fly deliberately (although
maladaptively in some cases) over land. Understanding whether
pelagic seabirds cross land voluntarily or involuntarily is relevant
to seabird biology in several ways. It can help us evaluate apparent
OVERLAND FLIGHT BY SEABIRDS AT ISLA ISABELA, GALÁPAGOS
DAVID J. ANCHUNDIA1, JACOB F. ANDERSON2, & DAVID J. ANDERSON1
1Department of Biology, Wake Forest University, Winston-Salem, NC 27109, USA (da@wfu.edu)
2Department of Geosciences, Boise State University, Boise, ID 83725, USA
Received 6 February 2017, accepted 3 May 2017
ABSTRACT
ANCHUNDIA, D.J., ANDERSON, J.F. & ANDERSON, D.J. 2017. Overland flight by seabirds at Isla Isabela, Galápagos. Marine
Ornithology 45: 139–141.
Oceanic seabirds are generally thought to avoid overland flight, perhaps limiting larger-scale distribution, but examples exist to the contrary.
We asked whether Blue-Footed Boobies Sula nebouxii, Brown Pelicans Pelecanus occidentalis, or frigatebirds Fregata spp. crossed the
Perry Isthmus (width 12.25 km, minimum height 23 m), a low point in the wasp-waist shape of Isla Isabela, Galápagos. Except for the Perry
Isthmus, Isla Isabela presents an elongated, high-elevation challenge to movement between the central and western waters of Galápagos.
Daytime observations over 3.5 d in June 2012 revealed ≥48 crossings by boobies and more than two crossings by frigatebirds. Overland
crossing of a terrestrial barrier of this size may be facilitated by the seabirds’ ability to see water across the Perry Isthmus from an altitude
of 42 m looking west and 43 m looking east, altitudes that both the boobies and frigatebirds attain during foraging.
Key words: Sulidae, Fregatidae, Suliformes, seabird biogeography, seabird distribution
terrestrial barriers to gene flow (Friesen 2015), whether movements
inland of hundreds of kilometers (Sullivan et al. 2009) result
from voluntary dispersal or environmental forcing, and degree of
terrestrial isolation that impedes artificial social attraction to new
colony sites (Sawyer & Fogle 2013).
Isla Isabela, the largest of the Galápagos Islands, presents a
significant north–south barrier to seabirds unable or unwilling to
cross land along its 135 km length. The north and south lobes of the
island join at the Perry Isthmus (Fig. 1), a land bridge 12.25km wide
and 23 m high at the lowest crest. We exploited these circumstances
to ask whether this isthmus poses a barrier to movement between
western and central waters of Galápagos. Significant numbers of
Blue-Footed Boobies Sula nebouxii (>400 on the east side and
>1 300 on the west side; Anchundia et al. 2014) foraged near the
Perry Isthmus around the time of our observations, and frigatebirds
Fregata minor and/or F. magnificens and Brown Pelicans Pelecanus
occidentalis also foraged in this area (unpub. data). Movement
between these foraging sites across the Perry Isthmus would cover
much less distance than would the alternative around Isabela by
water, allowing easier connections between nesting sites (some
which are east of Isabela) and ecologically productive waters to
the west (which have supported faster nestling growth of Blue-
Footed Boobies; Ricklefs et al. 1984). We used an elevated vantage
point on the Perry Isthmus to measure the frequency of overland
crossings by seabirds, and we evaluated the shape of the isthmus as
a visual barrier to crossing.
METHODS
We observed seabird movements from Cerro Iguana (0.625178° S,
90.975286° W, elevation 68 m above sea level), a hill within the
saddle forming the Perry Isthmus, on 12–15 June 2012. From this
hill, the entire width of the isthmus was clearly visible and birds
passing through the low point of the valley at 23 m elevation above
sea level were seen easily. Atmospheric conditions were clear on
Fig 1. Isla Isabela, Galápagos, and location of the Perry Isthmus.
Topographic contour lines indicate 150 m increments in altitude
above sea level.
140 Anchundia et al.: Seabirds at Isla Isabela, Galápagos
Marine Ornithology 45: 139–141 (2017)
all days of observation. An 8×42 binocular and 60× spotting scope
were used to identify and count all seabirds observed crossing the
isthmus as they passed Cerro Iguana. We used a handheld global
positioning system (GPS) unit, compass, and direct observations
to determine the heading of each individual visually. Flight
altitudes were determined by eye with no estimate of error. Two
observers scanned for seabirds continuously and recorded counts
and identifications independently, and their data were identical
except in one case. Frigatebirds were identified to genus only due
to the difficulty of identifying males to species. In all cases, birds
that we sighted over the isthmus passed from one coast to the other
in a direct, continuous path. The altitude of passing birds was
estimated visually by comparing the vertical position of the bird
with the elevation of the observer (68 m) and with the elevation of
land under the bird.
We calculated the altitude required for a bird to see from the coast
on one side of the isthmus to the ocean’s horizon on the other side as
follows. First, we calculated the linear distance D1 from the top of
the obstacle to the horizon, accounting for the curvature of Earth, as
D1 = θ1 × r (in kilometers; see Fig. 2), Eq. 1
where
θ1 = arccos (r/(r + E)) (in radians), Eq. 2
r = 6 378 km (the radius of the Earth at the Equator), and E is the
elevation of the obstacle in kilometers. We then calculated the
altitude (A) required of the bird, flying directly over the coastline on
its own side of the obstacle, to see over the obstacle to the horizon
beyond as
A = r × (1/cos(θ1 + θ2) – 1) (in kilometers), Eq. 3
where
θ2 = D2/r (in radians) Eq. 4
and D2 is the distance in kilometers from the obstacle to the
observer. This method may overestimate A slightly because it does
not adjust A for the effect of the atmospheric refractive index on
the appearance of the horizon. We have ignored this effect because
it is minor and because its estimation requires time-dependent
information on air density and temperature along the line of sight
(French 1982). Anomalies in these variables produce the optical
illusion of mirages; their measurement is not practical in most
field situations.
RESULTS
We counted 48–50 Blue-footed Boobies and two frigatebirds
crossing the Perry Isthmus (Table 1). Most of the birds were seen
crossing during late morning and early afternoon, and many were
observed flying over land during the hottest time of the day. Brown
Pelicans were known to be present on both sides of the isthmus but
did not cross during our observations.
TABLE 1
Birds observed crossing the Isthmus Perry, Isla Isabela, Galápagos Islands, in June 2012a
Date, time Time of
sighting Species Count Direction of flight Altitude above
groundb
12 June, 12h15–16h45 14h52 Frigatebird 1 NE to SW ~90 m
15h39 Blue-footed Booby 17 E to W ~25 m
13 June, 06h37–16h30 no sightings
14 June, 06h35–16h45 08h58 Blue-footed Booby 11 E to W ~20 m
09h06 Blue-footed Booby 5E to W ~35 m
09h38 Frigatebird 1 W to SE ~80 m
15 June, 06h38–16h45 14h24 Blue-footed Booby 6E to W ~35 m
15h22 Blue-footed Booby 10–12cW to E ~40 m
a Flight paths of birds crossing were to the north of the hill (Cerro Iguana) from which our observations were made.
b The surface of the ground was approximately 23 m above sea level.
c The two observers had different counts. Civil twilight on 12–15 June 2012 in the Galápagos Islands was at 05h34 and 18h23.
Fig. 2. Trigonometric values described in Methods (see Eq. 1–4),
superimposed on a diagram of the curved surface of the Earth
with water (gray) and a terrestrial mountain and adjoining lowland
(black). The straight solid line from the bird to ocean is the line
of sight from the minimum bird altitude to the ocean beyond the
mountain, assuming that the bird remains over the ocean on its own
side of the land.
D1
D2
Ѳ2
Ѳ1
r
A
E
Anchundia et al.: Seabirds at Isla Isabela, Galápagos 141
Marine Ornithology 45: 139–141 (2017)
Blue-footed Boobies flying over the sea alternate level flapping
flight with gliding (Nelson 1978), but over the Perry Isthmus they
flapped their wings constantly, and we judged them to be moving
faster than when flying over water. In contrast, frigatebirds never
flapped their wings during crossing. All crossing Blue-footed
Boobies passed in groups below the observers’ elevation (flying
~20–40 m above the ground), while frigatebirds were seen alone
and flying above the observers’ elevation (flying ~80–90 m above
the ground). Two additional frigatebirds were observed flying
behind our location, >100 m above land, moving from north to
south, but they were not counted because they did not cross Isabela
on the east–west axis.
To establish a line of sight from above the coastline over the crest of
the Perry Isthmus to the ocean on the other side requires an altitude
of at least 42 m from the east coast looking west, and at least 43m
from the west coast looking east.
DISCUSSION
Seabirds crossed the Perry Isthmus on three of the four days of
our observations, indicating that this landform with a width of
12.25km, but a minimum elevation of only 23 m, poses little barrier
to movement of these species. Geometric considerations indicate
that seabirds have a line of sight from the coast on one side across
the Perry Isthmus to water on the other side from a height of 42 m or
more above sea level under clear weather conditions; birds farther
out to sea would require a higher altitude (Fig. 2). Delano reported
(1817, in Murphy 1936, p. 838) that dives of Galápagos Blue-footed
Boobies began at an altitude of 60–100 m, exceeding this altitude
requirement. Our own observations are roughly consistent with
Delano’s, although we consider an altitude of 100 m to be unlikely
for this species, and dives from lower altitudes are common (pers.
obs.). Frigatebirds exceed the minimum altitude regularly, and
probably on most days that they are at sea (Rattenborg et al. 2016,
Weimerskirch et al. 2016). Brown Pelicans may not reach the
required altitude during normal flight at sea, perhaps preventing
them from having any reason to fly inland at the Perry Isthmus;
however, our brief observations certainly do not rule out crossings
by brown pelicans.
Attraction to water on the other side of the Perry Isthmus may
lead to movement across the isthmus in species that reach the
altitude required to see the water. Of course, after an individual’s
first crossing of a barrier like the Perry Isthmus, it may add this
information to its spatial understanding of its foraging range and
be more likely to cross again in the future. Even without personal
knowledge of the geography, a naive individual may imitate
informed individuals as they voluntarily cross terrestrial barriers
like the Perry Isthmus.
ACKNOWLEDGEMENTS
We thank Maria Escobar for assistance in the fieldwork; F. Estela,
J. Grace, E. Tompkins, M. Silman, and T. Anderson for comments
on the manuscript; D. Duffy, S. Cruz, D. Ainley, and an anonymous
reviewer for constructive reviews; Galápagos Conservancy, Swiss
Friends of Galápagos, and Galápagos Conservation Trust for
funding; Galápagos National Park Service for permission to work
in the Park; and the Charles Darwin Research Station and TAME
Airline for logistic support. Field work conformed to Animal Care
and Use Protocol A11-095 issued by Wake Forest University.
This material is based upon work supported by National Science
Foundation Grant DEB 0842199 to D.J. Anderson.
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