Observation frequency and seasonality of marine birds
off the Pacific coast of Costa Rica
Bruce E. Young1 & James R. Zook2
1.
2.
NatureServe, 4600 N. Fairfax Dr., 7th Floor, Arlington, VA 22203 USA; bruce_young@natureserve.org
Apdo. 182-4200, Naranjo de Alajuela, Costa Rica; ebirdcr@gmail.com
Received 24-IV-2014.
Corrected 02-V-2015.
Accepted 07-VI-2015.
Abstract: Although the Eastern Tropical Pacific is well known for its diverse fauna, the seabirds occurring off
Costa Rica’s Pacific coast have received little scientific attention. With seabirds now the fastest declining avian
group, a better understanding of seabird diversity and abundance in this region is urgently needed. We report
on observations of Costa Rica’s Pacific seabirds made during 19 days of observations on 11 offshore trips from
2006-2010. We provide, for the first time, spatially and seasonally explicit information on the distribution of 41
species of seabirds (nine families). Species diversity is higher during the dry-wet season (36 species) and wet-dry
season transitions (36 species) than during the dry season (19 species). The fauna included three threatened species (Pterodroma phaeopygia, Procellaria parkinsoni, and Puffinus creatopus) and two near-threatened species
(Psueudobulweria rostrata and Thalasseus elegans), highlighting the importance of Costa Rican waters for the
conservation of seabirds. Rev. Biol. Trop. 64 (Suppl. 1): S235-S248. Epub 2016 February 01.
Key words: Costa Rica, diversity, Eastern Tropical Pacific, seabirds, seasonality, threatened species.
The Eastern Tropical Pacific is an area
of relatively high biological productivity that
harbors a diverse and commercially valuable vertebrate fauna. Upwelling zones and
a thermocline that lies close to the surface
help explain the diversity and abundance of
these animals (Jehl, 1974; Ballance, Pitman,
& Fiedler, 2006; Fiedler, & Talley 2006). Seabirds are no exception, with approximately 42
species in ten families recorded off the Pacific
coast of Costa Rica exclusive of the Isla del
Coco area (Sandoval, & Sánchez, 2013). Most
of these species occur as seasonal, nonbreeding visitors, although 10-12 species breed on
offshore islands (Stiles, 1984).
Recently conservationists have called
attention to worldwide declines in seabirds due
to fisheries bycatch, reduced prey bases caused
by overfishing, introduced predators on breeding sites, climate change, and other threats
(Furness, 2003; Grémillet, & Boulinier, 2009;
BirdLife International, 2013). In fact, seabirds
are now considered the most rapidly declining
group of birds (Croxall, 2012; BirdLife International, 2013). Population trends and abundances of Costa Rican seabirds are largely unknown
due to a lack of any monitoring program or
regular observations. Despite the diversity of
the Costa Rican seabird fauna, the country has
no tradition of amateur seabird observations
such as is found in many other coastal countries, particularly developed ones. Commercial
whale watching expeditions, which can provide
a platform for seabird observations, are limited
to the near-shore area around Marino Ballena
National Park in Costa Rica. Knowledge of the
distribution and abundance of seabirds in the
Pacific waters of Costa Rica is limited to sporadic expeditions that have taken place in the
1960s (King, 1974), early 1970s (Jehl, 1974),
late 1970s and early 1980s (Stiles, 1984), and
1980s (Pitman, 1986). In the late 1990s and
Rev. Biol. Trop. (Int. J. Trop. Biol. ISSN-0034-7744) Vol. 64 (Suppl. 1): S235-S248, February 2016
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early 2000s, observations from oceanographic
vessels took place in Costa Rican waters, but
the research focus was on the Eastern Tropical Pacific broadly such that the data have not
been disaggregated to describe the diversity
and abundance of seabirds in any particular
country (Ballance et al., 2006; Vilchis, Ballance, & Fiedler, 2006). Additionally, seabird
records from the vicinity of Isla del Coco were
updated in the 2000s (Montoya, 2003; Easley,
& Montoya, 2006; Montoya, 2008).
To help fill this gap in knowledge about
seabirds in Costa Rica, we present the results
of observations made in 2006-2010 from vessels traveling as far as 380km from the Pacific
coast. Our goal was to explore and describe the
relatively unknown seabird fauna and not to
calculate precise density estimates of the species present. Trips were made as opportunities
arose rather than to systematically sample seasonal abundance. Nevertheless, we were able
to perform observations during seven months,
in both rainy and dry seasons, and can report
locations of observations relative to underwater
features such as the continental slope.
MATERIAL AND METHODS
in straight transects, occasionally slowing or
briefly altering course to allow time for accurate identification of passing birds (Fig. 1).
Birds were recorded as soon as they were
identifiable, at any distance from the observer. The method precludes interspecific comparisons of abundance due to differences in
detectability, but does allow within-species
comparisons. Whenever possible, birds that
were difficult to identify were photographed.
From 5-9 observers participated in each trip,
with JRZ joining all trips and therefore providing consistency in observations and records.
We compiled observations for each daylight
hour, recording our position with a GPS unit
at hourly increments. We additionally made
opportunistic observations at night for nocturnal species. The viewing conditions were
remarkably consistent during all trips, with
clear to partly cloudy skies, <2 m swell, and
less than 1.5 hr of rain during the entire study.
To provide a measure of seasonal occurrence,
we calculated the percentage of all one-hour
observation periods during which each species
was recorded for the periods December-March
(dry season), April-July (transition dry to wet
season), and August- November (wet season).
Although we observed other birds, we restrict
Study site: All observations took place off
the northern, central, and southern Pacific coast
of Costa Rica, within 380 km of shore (Fig. 1).
Costa Rica
Observations: We observed seabirds from
small vessels <17 m in length, except for
one trip made in a small cruise ship (Pacific
Explorer, 56 m), during 1-3 day trips (Table
1). The small size and slow speed (approximately 11km/hr) of most of the vessels allowed
for close observation of birds that crossed
paths with the boat. With the exception of the
Pacific Explorer trip which went to and from
Isla del Coco (also known as Cocos Island),
the vessels’ tracks were set to sample marine
environments both over the continental shelf
(defined as depths <200 m), continental slope
(200-800 m depth), and deep water (>800 m
depth) in northern, central, and southern territorial waters of Costa Rica. The vessels traveled
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Pacific
Ocean
Fig. 1. Study area off the Pacific coast of Costa Rica. Dark
gray lines indicate tracks where diurnal observations took
place. Stippled areas represent 200-800 m bathymetry to
indicate the location of the continental slope.
Rev. Biol. Trop. (Int. J. Trop. Biol. ISSN-0034-7744) Vol. 64 (Suppl. 1): S235-S248, February 2016
TABLE 1
Details of observations
Trip Number
1
2
3
4
5
6
7
8
9
10
11
1
Dates
19-20 Aug 2006
8 Jan 2007
4 Apr 2007
9-10 Apr 2007
11-12 Jan 2008
5 May 2008
7 March 2009
3-5 Apr 2009
26 Apr, 1 May 2009
September 18-20, 2009
November 14, 2010
Vessel
Floating Bear
Floating Bear
C Tails
Floating Bear
Floating Bear
Rosa Maria V
Reina del Sur
Floating Bear
Pacific Explorer1
Floating Bear
Floating Bear
Region of Pacific Surveyed
Central
Central
South
Central
Central
South
South
North
Central
Central
Central
Observations made en route to and from Isla del Coco.
this report to seabirds (families Procellariidae,
Hydrobatidae, Phaethontidae, Fregatidae, Sulidae, Pelecanidae, Stercorariidae, Laridae, and
the genus Phalaropus in the Scolopacidae). We
were unable to collect data on sea surface temperature or other habitat variables. Taxonomy
and sequence follow AOU (1998) and subsequent supplements through 2014 (Chesser et
al., 2014).
RESULTS
We recorded 40 species of seabirds from
nine families during 207 1-hr diurnal observation periods over 19 days during 11 trips (Table
2). One additional species, Creagrus furcatus,
was detected only opportunistically at night.
The diversity of seabirds in our study
area was lower in the dry season (19 species
TABLE 2
Seasonal, breeding, and conservation status of seabirds observed
Species1
Procellariidae
Pterodroma phaeopygia (CR)
Pterodroma rostrata (NT)
Procellaria parkinsoni (VU)
Puffinus creatopus (VU)
Puffinus pacificus (LC)
Puffinus nativitatis (LC)
Puffinus subalaris (LC)
Hydrobatidae
Oceanites oceanicus (LC)
Oceanodroma leucorhoa (LC)
Oceanodroma castro (LC)
Oceanodroma tethys (LC)
Oceanodroma melania (LC)
Oceanodroma markhami (DD)
Oceanodroma microsoma (LC)
Percent of 1-hr observation periods recorded
Jan-Mar2
Apr-Jul3
Aug-Nov4
Overall
0
0
0
5
63
0
32
1
0
8
3
52
1
20
0
8
2
6
24
0
41
<1
2
4
4
45
<1
29
3
16
3
0
3
0
5
1
10
0
8
8
0
4
16
24
3
41
27
3
6
6
15
1
16
13
1
5
Rev. Biol. Trop. (Int. J. Trop. Biol. ISSN-0034-7744) Vol. 64 (Suppl. 1): S235-S248, February 2016
S237
TABLE 2 (Continued)
Percent of 1-hr observation periods recorded
Jan-Mar2
Apr-Jul3
Species1
Phaethontidae
Phaethon aethereus (LC)
Fregatidae
Fregata magnificens (LC) (b)
Sulidae
Sula dactylatra (LC) (b)
Sula granti (LC) (b)
Sula nebouxii (LC)
Sula leucogaster (LC) (b)
Sula sula (LC) (b)
Pelecanidae
Pelecanus occidentalis (LC) (b)
Scolopacidae
Phalaropus lobatus (LC)
Phalaropus fulicarius (LC)
Stercorariidae
Stercorarius pomarinus (LC)
Stercorarius parasiticus (LC)
Stercorarius longicaudus (LC)
Laridae
Creagrus furcatus (LC)
Xema sabini (LC)
Leucophaeus atricilla (LC)
Leucophaeus pipixcan (LC)
Anous stolidus (LC) (b)
Gygis alba (LC) (b)
Onychoprion fuscatus (LC)
Onychoprion anaethetus (LC) (b)
Sternula antillarum (LC)
Chlidonias niger (LC)
Sterna hirundo (LC)
Thalasseus maximus (LC)
Thalasseus sandvicensis (LC)
Thalasseus elegans (NT)
Species recorded
Aug-Nov4
Overall
0
5
11
6
34
33
40
35
0
0
0
66
0
10
13
0
71
22
2
0
2
68
10
6
7
<1
69
14
11
4
3
5
18
0
11
3
30
6
18
3
34
21
0
23
8
1
11
6
2
21
10
1
0
0
39
0
0
0
0
3
0
68
0
8
8
0
19
2
2
8
10
8
6
3
13
2
53
5
6
4
0
36
0
24
5
2
3
2
0
17
2
30
3
3
2
6
36
1
8
13
6
5
3
1
13
1
49
3
5
4
2
41
1. IUCN status (IUCN, 2013) (LC)=Least Concern, (CR)=Critically Endangered, (VU)=Vulnerable, (NT)=Near
Threatened, (DD)=Data Deficient. (b)=breeds in Costa Rica.
2. No observations were made during December. Sample size: 38 1-hr observation periods.
3. Sample size: 106 1-hr observation periods.
4. Sample size: 63 1-hr observation periods.
recorded) than in either the dry-wet transition
or wet seasons (36 species recorded in each
season). The most frequently detected species was Sula leucogaster (present in 69 %
of 1-hr observation periods), followed by
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Chlidonias niger (49 %) and Puffinus pacificus
(45 %). The rarest species were Pterodroma
phaeopygia, and Sula nebouxii, each of which
was represented by a single individual in our
observations. Three species are listed in a
Rev. Biol. Trop. (Int. J. Trop. Biol. ISSN-0034-7744) Vol. 64 (Suppl. 1): S235-S248, February 2016
Threatened Red List category and two in a
Near-Threatened one (Table 2).
SPECIES ACOUNTS
Pterodroma phaeopygia (Salvin, 1876)
(Galápagos Petrel): We observed a sole individual of this Critically Endangered species,
which breeds on the Galápagos Islands. At
05:56 on 1 May 2009, on our return from Isla
del Coco (6º39’33” N - 86º24’26” W) (Fig.
2), an individual approached the vessel on a
roughly parallel course about 100m away and
was in sight for less than a minute. Two S.
leucogaster were the only other birds visible at
the time. Flight was mostly gliding with occasional flaps at a height of 1-3 m above a calm
sea surface. The identification was confirmed
by photos (Jones, & Komar, 2009a). This species has been reported regularly over offshore
waters of Middle America (Spear, Ainley, Nur,
& Howell, 1995).
Pterodroma rostrata (Peale, 1848) (Tahiti
Petrel): This distinctive species occurred as
scattered individuals well beyond the continental slope during our only September trip, during
which we documented its presence photographically (Jones, & Komar, 2010). One year later,
also in September, the species was reported for
first time in Panama (Jones, & Komar, 2011).
The species disperses widely from its breeding
areas, the closest of which are in the Society
Islands in the South Pacific, over 7 000 km
away (Howell, 2012).
Procellaria parkinsoni Gray, 1862 (Parkinson’s Petrel): This species has been reported
off the Pacific coast of Central America (Jehl,
1974; Stiles, & Skutch, 1989; Pitman, & Balance, 1992). We observed several birds on
4 April 2009 well offshore (Fig. 2) in the
vicinity of the Costa Rica Dome, a seasonal
oceanographic feature that occurs off of the
province of Guanacaste during the dry season
and is characterized by a shallow thermocline and significant upwelling (Fiedler, 2002;
Ballance et al., 2006). Although most Central
American records of this species are from April
and May (Stiles, & Skutch, 1989), we also
observed it in September 2009 well south of the
Nicoya Peninsula.
Puffinus creatopus (Coues, 1864) (Pinkfooted Shearwater): We observed this species
mostly as single individuals in deep water
beyond the continental slope on five separate trips. Although the species was known
previously only from the vicinity of Isla del
Coco (Stiles, & Skutch, 1989; Easley, & Montoya, 2006; Montoya, 2008), we observed it
off the north, central, and southern coasts of
Costa Rica suggesting the species is a regular,
widespread visitor. The species also occurred
nearly year round, with observations in January, March, April, September, and November,
in contrast to previous observations (Stiles, &
Skutch, 1989).
Puffinus pacificus (Gmelin, 1789)
(Wedge-tailed Shearwater): This was the most
regularly-seen tubenose species, especially for
the period January to July (Table 2). Birds were
seen in flight and on the water as individuals or
in loose flocks (the highest hourly count was
228). Occasional dark or intermediate morph
birds were seen but most were the light morph.
This species appeared to be most abundant over
the continental slope and in deeper water just
beyond (Fig. 2), where they are likely dispersing from Hawaii (Howell, 2012).
Puffinus nativitatis Streets, 1877 (Christmas Shearwater): We observed two individuals,
on 3 April 2009 in deep water 35 km from the
Nicoya Peninsula. Both birds were resting on
the water with different groups of P. pacificus.
This species is a regular non-breeding visitor
off Mexico and Central America, where it disperses from breeding areas in the south Pacific
islands including Easter Island (Carboneras,
1992; Spear, & Ainley, 1999b; Howell, 2012),
it is not surprising that this shearwater at least
occasionally wanders to Costa Rica.
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Individuals
1
2 - 10
> 10
Pterodroma phaeopygia
Pterodroma rostrata
Procellaria parkinsoni
Puffinus creatopus
Individuals
1
2 - 10
> 10 Puffinus pacificus
Puffinus nativitatis
Puffinus subalaris
Oceanites oceanicus
Oceanodroma castro
Oceanodroma tethys
Oceanodroma melania
Oceanodroma microsoma
Phaethon aethereus
Fregata magnificens
Sula granti
Sula nebouxii
Sula leucogaster
Individuals
1
2 - 10
> 10
Oceanodroma leucorhoa
Individuals
1
2 - 10
> 10
Oceanodroma markhami
Individuals
1
2 - 10
> 10
Sula dactylatra
Fig. 2. Locations and relative abundances of seabirds observed off the Pacific coast of Costa Rica, 2006-2010.
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Rev. Biol. Trop. (Int. J. Trop. Biol. ISSN-0034-7744) Vol. 64 (Suppl. 1): S235-S248, February 2016
Individuals
1
2 - 10
> 10
Sula sula
Pelecanus occidentalis
Phalaropus lobatus
Phalaropus fulicarius
Stercorarius pomarinus
Stercorarius parasiticus
Stercorarius longicaudus
Creagrus furcatus
Xema sabini
Leucophaeus atricilla
Leucophaeus pipixcan
Anous stolidus
Gygis alba
Onychoprion fuscatus
Onychoprion anaethetus
Sternula antillarum
Chlidonias niger
Sterna hirundo
Thalasseus maximus
Thalasseus sandvicensis
Individuals
Individuals
1
2 - 10
> 10
Individuals
1
2 - 10
> 10
Individuals
1
2 - 10
> 10
Individuals
1
2 - 10
> 10
Fig. 2. (Continued). Locations and relative abundances of seabirds observed off the Pacific coast of Costa Rica, 20062010.
Rev. Biol. Trop. (Int. J. Trop. Biol. ISSN-0034-7744) Vol. 64 (Suppl. 1): S235-S248, February 2016
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Oceanodroma leucorhoa (Vieillot, 1818)
(Leach’s Storm-Petrel): This species was regularly seen over the continental slope and in
deeper water (Fig. 2). Although the species has
been reported as a seasonal visitor to Costa
Rica (Stiles, & Skutch, 1989), we observed it
year round, on nine of the 11 trips. We observed
birds that exhibited a full range of rump patterns from white to intermediate to dark, as has
been noted in California (Howell, McGrath,
Hunefeld, & Feenstra, 2010), but the data presented here are for all morphs combined.
Individuals
1
2 - 10
> 10
Thalasseus elegans
Fig. 2. (Continued). Locations and relative abundances
of seabirds observed off the Pacific coast of Costa Rica,
2006-2010.
Puffinus subalaris Ridgway, 1897 (Galapagos Shearwater): We observed this species
regularly, year round. Observations appeared
to be most frequent over the continental slope,
with fewer individuals appearing over deeper
or shallower water (Fig. 2). Most often seen
as pairs or individuals, this was one of the few
species that would customarily follow the boat,
often for periods of up to one hour. We did
not distinguish between “white-winged” and
“black-winged” types, which may represent
separate species (Howell, 2012).
Oceanites oceanicus (Kuhl, 1820) (Wilson’s Storm-Petrel): We recorded this panoceanic storm-petrel most frequently during
August and September trips. Although the
species usually occurred as single individuals, we documented one aggregation of 81
individuals on 20 August 2006 in the mouth
of the Gulf of Nicoya. We do not know
whether birds off Costa Rica are from Antarctic (nominate oceanicus) or South American
(chilensis) populations.
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Oceanodroma castro (Harcourt, 1851)
(Band-rumped Storm-Petrel): We observed
only three individuals, one in January 2007
and two in September 2009, all in deep water.
Other Costa Rican reports are from the months
of May and June (Stiles, & Skutch, 1989), and
one Panama report is from September (Jones,
& Komar, 2011). These observations suggest
that there may be little seasonality to when the
species visits Costa Rican waters from its Galapagos breeding areas (Howell, 2012). Unfortunately, we were unable to obtain photographic
evidence of this confusing species, which still
needs unequivocal evidence of its occurrence
in Costa Rica.
Oceanodroma tethys (Bonaparte, 1852)
(Wedge-rumped Storm-Petrel): This species
is likely a seasonal visitor to Costa Rica. We
detected it most regularly in August and September, and less frequently in April and May
(Table 2). Although we did not distinguish
between the subspecies, based on observations
in Mexico and the Gulf of Panama, the South
American kelsalli is the most likely in Costa
Rican waters within 500km of the coast (Spear,
& Ainley, 2007; Howell, 2012).
Oceanodroma melania (Bonaparte, 1854)
(Black Storm-Petrel): Although we observed
this large storm-petrel nearly year-round, it
was most regular in April, when concentrations
of up to 72 individuals were encountered. The
species appears to prefer inshore waters along
the continental slope (Spear, & Ainley, 2007).
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Oceanodroma markhami (Salvin, 1883)
(Markham’s Storm-Petrel): On 19 September
2009 we observed two individuals, the first
documented records for Costa Rica. As has
been reported for Central America (Spear, &
Ainley, 2007; Howell, 2012), the storm-petrels
were well offshore, approximately 240km from
the nearest land (Fig. 2). We were able to distinguish O markhami from O. melania only
by examining photographs taken of birds that
closely approached our observation vessels.
Several other birds were suspected of being
this species but we were unable to photograph
them. Due to the identification challenge, the
species may be more common in Costa Rica
than the paucity of records indicates (Stiles, &
Skutch, 1989).
Oceanodroma microsoma (Coues, 1864)
(Least Storm-Petrel): We observed this species infrequently, during April, August, and
September either over the continental slope or
farther offshore (Fig. 2).
Phaethon aethereus Linnaeus, 1758
(Red-billed Tropicbird): We regularly observed
this species well offshore, always as single
birds, during both the dry-wet and wet-dry
season transitions (Fig. 2, Table 2). Of the 12
individuals we saw, only 2 were adults.
Fregata magnificens Matthews, 1914
(Magnificent Frigatebird): This species is
widespread, occurring along the length of the
coast and continuously well offshore (Fig. 2).
It was most abundant near Isla Cabo Blanco,
located off the southern tip of the Nicoya Peninsula, where the species breeds.
Sula dactylatra Lesson, 1831 (Masked
Booby): We regularly observed this species
well offshore during both the dry-wet and wetdry season transitions. The species was seen as
individuals or adult/immature pairs.
Sula granti Rothschild, 1902 (Nazca
Booby): Our observations of this species were
restricted to areas well beyond the continental
slope during April and May (Fig. 2, Table 2).
This species, as well as the previous one, would
characteristically approach our vessel and circle us once or twice before continuing on.
Sula nebouxii Milne-Edwards, 1882
(Blue-footed Booby): This species is primarily a coastal species that is occasionally found
further offshore. We observed a single individual of this species in the mouth of the Gulf
of Nicoya on 20 September 2009 together with
hundreds of S. leucogaster, F. magnificens,
Pelecanus occidentalis, and other seabirds
that were attracted to a fishing boat dumping
bycatch (Fig. 2). This is the only Sula species
we observed that does not breed in Costa Rica,
although it does breed as close as Panama Bay
(Ridgley, & Gwynne, 1992) and possibly in the
Gulf of Fonseca (Howell, & Webb, 1995). The
scarcity of records in Costa Rica suggests that
individuals from these breeding populations do
not disperse widely.
Sula leucogaster (Boddaert, 1783)
(Brown Booby): Together with F. magnificens,
S. leucogaster was one of the most frequently
detected species, occurring year round and at
all distances from shore (Fig. 2, Table 2). We
observed hundreds of individuals apparently
nesting on Isla Cabo Blanco, and frequently
saw groups of birds flying to and from the
island. Occasional individuals of the race S. l.
brewsteri were observed.
Sula sula (Linnaeus, 1766) (Red-footed
Booby): We regularly detected this species well
offshore in all seasons except the dry season
(Fig. 2, Table 2). In early May 2009, the species was particularly common with up to 131
individuals counted during one hour of observations. This was at our closest approach to
Isla del Coco where the species breeds (AOU,
1998). All birds were of the typical all-brown
Pacific morph except for one white bird and
two brown birds with white tails, all of which
were observed on the trip en route to and from
Isla del Coco.
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Pelecanus occidentalis Linnaeus, 1766
(Brown Pelican): Pelicans favor the coasts,
where they occur year round (Table 2). Only
once, in May 2009 off the Osa Peninsula where
deep water is close to shore, did we observe this
species beyond the continental slope (Fig. 2).
Phalaropus lobatus (Linnaeus, 1758)
(Red-necked Phalarope): We observed small
groups of this species on all trips with small
vessels where proximity to the water aided
detection of this small pelagic shorebird (Table
2). Although our results suggest the species
inhabits Costa Rican waters year-round, we
note that we made no observations in July
when the species nests in northern North
America (AOU, 1998). We observed the species over the continental shelf as well as further
offshore (Fig. 2).
Phalaropus fulicarius (Linnaeus, 1758)
(Red Phalarope): This species was detected
in similar areas but less frequently than its
congener (Fig. 2). Its absence during the dry
season (Table 2) suggests that the species
may only occur regularly in Costa Rica as a
passage migrant.
Stercorarius pomarinus (Temminck,
1815) (Pomarine Jaeger): This was the most
frequently observed jaeger, occurring yearround over the continental shelf and deeper
water (Fig. 2, Table 2).
Stercorarius parasiticus (Linnaeus,
1758) (Parasitic Jaeger): Although less commonly observed than S. pomarinus, this species
occurred in the same areas and was equally
aseasonal (Fig. 2, Table 2).
Stercorarius longicaudus Vieillot, 1819
(Long-tailed Jaeger): We recorded this species twice, on 4 April 2009 and 14 November
2010. Both individuals (immatures), which we
identified from photographs, occurred over the
continental slope (Fig. 2).
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Creagrus furcatus (Néboux, 1842) (Swallow-tailed Gull): We observed two individuals
of this species, both attracted to our boat at
night and well offshore (Fig. 1). As we have
documented elsewhere, some individuals of
this species regularly disperse to Costa Rican
waters from breeding colonies in the Galapagos
Islands from late March to early May (Young
et al., 2010).
Xema sabini (Sabine, 1819) (Sabine’s
Gull): We observed this species most frequently from August to November, although we also
observed single individuals in late April and
early May (Table 2). The species occurred over
the continental slope or somewhat further out,
but rarely far offshore (Fig. 2).
Leucophaeus atricilla (Linnaeus, 1758)
(Laughing Gull): This species was abundant
year-round in coastal waters out to the continental slope.
Leucophaeus pipixcan (Wagler, 1831)
(Franklin’s Gull): This species passes through
Costa Rica during southward and northward
migrations from its North American breeding
areas (Stiles, & Skutch 1989). Although the
species is known to migrate up the Golfo de
Nicoya (Stiles, & Skutch 1989), we observed
it more frequently well out at sea (Fig. 2), suggesting the species follows multiple routes during its passage through Costa Rica. This species
is known to migrate offshore between South
and Central America (Howell, & Dunn, 2007).
Anous stolidus (Linnaeus, 1758) (Brown
Noddy): We observed this species infrequently
at almost any distance from shore (Fig. 2). It
was absent during the dry season (Table 2).
Gygis alba (Sparrman, 1786) (White
Tern): This species breeds on Isla del Coco,
approaching the mainland only rarely (Stiles, &
Skutch 1989). Our observations of the species
were all made well offshore (Fig. 2) with all but
one of our sightings made during the voyages
to and from Isla del Coco.
Rev. Biol. Trop. (Int. J. Trop. Biol. ISSN-0034-7744) Vol. 64 (Suppl. 1): S235-S248, February 2016
Onychoprion fuscatus (Linnaeus, 1766)
(Sooty Tern): We observed three individuals of
this species, all well offshore during April and
May (Fig. 2, Table 2).
Onychoprion anaethetus (Scopoli, 1786)
(Bridled Tern): We observed scattered individuals of this species year-round and generally
fairly close to shore (Fig. 2, Table 2).
Sternula antillarum Lesson, 1847 (Least
Tern): This species is typically observed close
to shore (Stiles, & Skutch 1989), but we
observed three individuals over the continental
slope on three separate trips that took place in
April and August.
Chlidonias niger (Linnaeus, 1758) (Black
Tern): Although this tern does not breed closer
to the study area than North America (AOU,
1998), the species occurs in Costa Rica yearround. It was one of the most frequently
detected species in our observations (Table
2), usually occurring in flocks with up to 132
individuals counted in an hour. The abundance
of this species declined well offshore (Fig. 2).
Sterna hirundo Linnaeus, 1758 (Common Tern): We observed a few individuals of
this species during April, May, and November,
when it can appear almost any distance from
shore (Fig. 2).
Thalasseus maximus (Boddaert, 1783)
(Royal Tern): This species is abundant yearround close to shore (Stiles, & Skutch, 1989),
and was occasionally observed in deeper water
(Fig. 2).
Thalasseus sandvicensis (Latham, 1787)
(Sandwich Tern): Similar to T. maximus, this
species is primarily a coastal species that is
occasionally found further offshore (Fig. 2),
possibly during migrations. We observed the
species during March, April, and September.
Thalasseus elegans (Gambel, 1849) (Elegant Tern): As with the other Thalasseaus
terns, this is primarily a coastal species that we
observed at sea in September and November
(Table 2).
DISCUSSION
Our observations emphasize the diversity
of seabirds that occur off the Pacific coast of
Costa Rica. Of the 42 species reported from
the area prior to the initiation of our study
(Stiles, & Skutch, 1989; Sandoval, & Sánchez,
2013), we recorded 38. The lower diversity
we recorded during dry season months relative to other seasons in Costa Rica matches
that reported previously for Gulf of Panama
(Loftin, 1991). Our observations added two
species (P. phaeopygia, P. nativitatis) to the
list of birds known from Costa Rica (Jones,
& Komar, 2009a, 2009b), and provided the
first photographic evidence for occurrence in
Costa Rica of an additional two species (O.
markhami, and P. creatopus; Jones, & Komar,
2009b, 2011). Due to the paucity of previous
observations in our study area, we suggest that
these species have been overlooked previously
(but see Pitman, 1986).
Four species, Puffinus tenuirostris, Puffinus griseus, Puffinus opisthomelas, and Stercorarius maccormicki, have been reported
previously in Pacific waters of Costa Rica
outside of the Isla del Coco area but not during
our study. Puffinus tenuirostris and P. opisthomelas appear to be accidental to Costa Rica,
known from few sightings (Stiles, & Skutch,
1989) and therefore not surprisingly missed
during our study. We did have one sighting,
on 18 September 2009, of a small, black and
white shearwater that could have been P.
puffinus but we were unable to photograph it,
and therefore could not confirm identification.
Stiles and Skutch (1989) reported several sightings of S. maccormicki along the Pacific coast
of Costa Rica.
Another previously reported species that
we did not observe was Puffinus griseus.
Although reported as common to abundant from
May to October (Stiles, & Smith, 1977; Stiles,
& Skutch, 1989), we failed to observe this
Rev. Biol. Trop. (Int. J. Trop. Biol. ISSN-0034-7744) Vol. 64 (Suppl. 1): S235-S248, February 2016
S245
species during any of the seven days of observations across three years of the study in this
time period. Costa Rican birds likely originate
from New Zealand breeding colonies (Spear,
& Ainley, 1999a), which undertake spectacular
migrations of over 60 000km covering a wide
area of the Pacific Ocean (Shaffer et al., 2006).
Tracks of data-logged birds in 2005 suggest
that the species may not regularly migrate near
Costa Rica (Shaffer et al., 2006). Also, the
species has declined widely in the Pacific (Scofield, & Christie, 2002; Scott,Scofield, Hunter,
& Fletcher, 2008). Other authors note that this
species tends to avoid warm, windless waters
and generally considered it scarce off tropical
Mexico and Central America (Pitman, 1986;
Howell, 2012).
With more observations, additional species will surely be recorded from the Pacific
waters of Costa Rica. For example, Pterodroma
externa is known to be widespread in the
Eastern Tropical Pacific (Ballance et al. 2006)
and could be expected to wander into Costa
Rican waters from time to time. Subsequent
to our study, the albatross Phoebastria irrorata was photographed south of the Nicoya
Peninsula and the tern Larosterna inca was
photographed in the Gulf of Nicoya (Sandoval,
& Sánchez, 2013).
We recorded the presence of three threatened procellariids and two near-threatened
species (a procellariid and a tern) in the Pacific
waters of Costa Rica (Table 2). None of the
threatened species breed in Costa Rica and,
with the exception of P. creatopus, these species were absent from January to March. With
these species occurring in territorial waters,
Costa Rica has partial responsibility for their
conservation. During our observation periods,
the only potential localized threats to these
species that we could discern were occasional
artisanal long-line fishing operations. Monitoring of seabird populations, a conservation
measure first proposed by Stiles (1984), has
still not been implemented. Our results, covering the extent of the Costa Rican Pacific coast
and providing quantitative, spatially explicit
relative abundance data, can serve as valuable
S246
background information for the establishment
of such a program.
ACKNOWLEDGMENTS
We thank Paul Murgatroyd for inviting us
to perform observations aboard the Floating
Bear, the many observers and photographers
that accompanied us on the cruises, the ship
captains for returning us safely to port, and four
anonymous reviewers for comments that significantly improved the quality of the manuscript.
RESUMEN
El Pacifico Tropical Oriental es bien conocido por
su diversa fauna; sin embargo, las aves marinas que se
encuentran frente a la costa pacífica de Costa Rica no han
recibido mucha atención científica. Reconociendo que las
aves marinas representan el grupo de aves que más rápidamente está disminuyendo, se requiere urgentemente un
mejor conocimiento de la diversidad y abundancia de las
aves marinas en esta región. Reportamos observaciones de
aves marinas del Pacifico de Costa Rica hechas durante 19
días de observación en 11 giras mar afuera entre el 2006 y
el 2010. Proveemos, por primera vez, información espacialmente y estacionalmente sobre la distribución de 41
especies de aves marinas (nueve familias). La diversidad
de especies es mayor durante las transiciones entre las
estaciones seca-húmeda (36 especies) y húmeda-seca (36
especies) que durante la estación seca (19 especies). La
avifauna incluye tres especies amenazadas (Pterodroma
phaeopygia, Procellaria parkinsoni y Puffinus creatopus)
y dos especies casi amenazadas (Psueudobulweria rostrata y Thalasseus elegans), lo que resalta la importancia
de las aguas costarricenses para la conservación de las
aves marinas.
Palabras clave: Costa Rica, diversidas, Pacific Tropical
Este, aves marinas, estacionalidad, especies amenazadas.
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