Abstract and Figures

The Striated Caracara (Phalcoboenus australis) is a poorly known raptor associated with seabird colonies, restricted to the islands of southern South America, and with an estimated extant population of <2,500 mature individuals. We evaluated the number of breeding pairs, described the characteristics and spatial pattern of nest sites, and estimated breeding output of a population of Striated Caracaras in Franklin Bay, Staten Island, Argentina. We found one of the lower breeding density values reported for this species, although this population is associated with one of the biggest colonies of Rockhopper Penguins. The main material used for the construction of Striated Caracaras' nests was tussac grass, though only half of nests were placed in grassland. The spatial pattern for nest sites corresponds with global and local clustering. All successful Striated Caracaras' nests were at least 250 m from the nearest neighboring nest, were generally closer to the colony of Rockhopper Penguins than failed nests, and had more Rockhopper patches around them. Breeding success was 0.73 successful nests/active nests, productivity was 1.27 ± 1.01 young/active nest, and brood size was 1.75 ± 0.71 young/successful nest. The presence of invasive wild goats and red deer is proposed as a factor that could be restricting nest site availability in the study area.
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Status and Reproductive Outcome of the Breeding Population
of Striated Caracaras (Phalcoboenus australis) at Franklin Bay,
Staten Island, Argentina
Author(s): Ulises Balza, Nicolás A. Lois, and Andrea Raya Rey
Source: The Wilson Journal of Ornithology, 129(4):890-898.
Published By: The Wilson Ornithological Society
URL: http://www.bioone.org/doi/full/10.1676/16-189.1
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The Wilson Journal of Ornithology 129(4):890–898, 2017
Status and Reproductive Outcome of the Breeding Population of Striated Caracaras
(Phalcoboenus australis) at Franklin Bay, Staten Island, Argentina
Ulises Balza,
Nicola´s A. Lois,
and Andrea Raya Rey
ABSTRACT.—The Striated Caracara (Phalcoboenus
australis) is a poorly known raptor associated with seabird
colonies, restricted to the islands of southern South America,
and with an estimated extant population of ,2,500 mature
individuals. We evaluated the number of breeding pairs,
described the characteristics and spatial pattern of nest sites,
and estimated breeding output of a population of Striated
Caracaras in Franklin Bay, Staten Island, Argentina. We
found one of the lower breeding density values reported for
this species, although this population is associated with one
of the biggest colonies of Rockhopper Penguins. The main
material used for the construction of Striated Caracaras’
nests was tussac grass, though only half of nests were placed
in grassland. The spatial pattern for nest sites corresponds
with global and local clustering. All successful Striated
Caracaras’ nests were at least 250 m from the nearest
neighboring nest, were generally closer to the colony of
Rockhopper Penguins than failed nests, and had more
Rockhopper patches around them. Breeding success was
0.73 successful nests/active nests, productivity was 1.27 6
1.01 young/active nest, and brood size was 1.75 60.71
young/successful nest. The presence of invasive wild goats
and red deer is proposed as a factor that could be restricting
Laboratorio de Ecolog´
ıa y Conservaci ´
on la Vida
Silvestre. Centro Austral de Investigaciones Cient´
(CADIC-CONICET). Bernardo Houssay 200, Ushuaia,
Laboratorio de Ecolog´
ıa y Comportamiento Animal.
Instituto de Ecolog´
ıa, Gen ´
etica y Evoluci´
CONICET). Facultad de Ciencias Exactas y Naturales,
Universidad de Buenos Aires. Intendente G¨
uiraldes 2160,
Buenos Aires, Argentina.
Instituto de Ciencias Polares, Ambiente y Recursos
Naturales (ICPA), Universidad Nacional de Tierra del
Fuego (UNTdF). Leandro N. Alem 1036. Ushuaia,
Corresponding author; e-mail:
890 THE WILSON JOURNAL OF ORNITHOLOGY Vol. 129, No. 4, December 2017
nest site availability in the study area. Received 24 October
2016. Accepted 29 March 2017.
Key words: breeding output, invasive species, nest site
availability, spatial analysis.
The Striated Caracara (Phalcoboenus australis)
is a large subantarctic bird of prey restricted to
isolated shores and islands south of latitude 508S
in South America (Ferguson-Lees and Christie
2001). The species is classified as Near Threatened
(IUCN 2015) with an extant population of ,2,500
mature individuals. They reside mostly on Staten
Island (Isla de los Estados) in the Fuegian region
and in the Malvinas/Falkland Islands (BirdLife
International 2013), with fragmented breeding
records in southern Chile (Mar´
ın et al. 2006).
The species is largely associated with seabird
breeding colonies (i.e., Rockhopper Penguins
[Eudyptes chrysocome], Black-browed Albatross
[Thalassarche melanophris]). Its breeding season
extends from October to February during which it
lays 1–4 eggs in a single clutch. It primarily nests
on the ground in tussac grasslands of Poa
flabellata ,10 m from seabird nests and feeds its
offspring on eggs, chicks, and adults from the
nearby colony (Strange 1996, Catry et al. 2008,
Liljesthr¨om et al. 2008).
Striated Caracaras on the Malvinas/Falklands
declined as a consequence of direct persecution
during the 1900s, and their numbers have not
recovered despite the legal protection being
formalized in 1999 (Woods 2007). In this
archipelago, breeding territories have been record-
ed as close as possible to the seabird colony, and at
the same time as far as possible from the nearest
conspecific territory, following an apparently
regular pattern (Strange 1996). The published
mean distance between breeding pairs during the
breeding season ranges 47–700 m, depending on
the island studied (Strange 1996, Catry et al.
It is unclear whether the population on Staten
Island is threatened. Unlike populations on the
Malvinas/Falkland Islands, persecution by humans
has not been recorded on Staten Island, which has
long been a Provincial Reserve, and has recently
been declared as a National Reserve. However,
two factors could potentially affect the viability of
the population of Striated Caracaras on Staten
Island in the long term: a) the negative population
trend of their apparent main food resource during
the breeding season, the globally threatened
Rockhopper Penguins (IUCN 2015), which has
been declining in numbers in the area for the last
decades at a yearly rate of 2%(Raya Rey et al.
2014); and b) the probable, but unknown, impact
on the tussac grassland produced by two exotic
herbivores, the wild goat (Capra hircus) and the
red deer (Cervus elaphus), introduced in 1856 and
1973, respectively (Valenzuela et al. 2014).
In this context, the main objective of this work
is to increase our knowledge on the current status
and breeding biology of the Striated Caracaras’
population in Franklin Bay, Staten Island, the only
breeding population known to coexist with large
exotic herbivores, as far as we know. In this study
we, a) estimate the number of breeding pairs and
describe the characteristics of accessible nests; b)
analyze the spatial pattern of nests and productiv-
ity; c) estimate the breeding success, productivity,
and brood size and; d) evaluate the variation of
breeding success in relation to the presence of
nearest neighbors, and the distribution and density
of the main seabird reproductive patches.
We carried out this study between 14 November
and 19 December 2014, which corresponds with
late incubation period and early chick rearing of
Striated Caracaras (Strange 1996).
Study Site.—Our study took place in Franklin
Bay, Staten Island, Argentina (Fig. 1; 548530S,
648390W), where the mean annual temperature
varies from 2.7–9.0 8C, and the mean annual
precipitation is 1,500 mm. Strong winds, mainly
from the SW are predominant throughout the year
(Dudley and Crow 1983). The island is character-
ized by tussac grassland on the shores and
subantarctic forests inland, dominated by ever-
green beech (Nothofagus betuloides) and winter’s
bark (Drimys winteri). The understory is rich in
shrubs, ferns, lichens, and mosses, including
diddle dee (Empetrum rubrum) which is one of
the most abundant shrubs in the area (Niekisch and
Schiavini 1998). Several species of seabirds and
marine mammals breed on the shores of Staten
Island (Chebez and Bertonatti 1994, Schiavini
2000, Schiavini and Raya Rey 2001, Schiavini et
al. 2004). Franklin Bay holds one of the biggest
colonies of Rockhopper Penguins (127,000 breed-
ing pairs; Eudyptes chrysocome), a colony of
Imperial Shags (4,600 breeding pairs; Leucocarbo
atriceps), and a colony of Magellanic Penguins
(1,600 breeding pairs; Spheniscus magellanicus;
Raya Rey et al. 2014). Nests of Rockhopper
Penguins are arranged in 133 discrete patches
around the bay which vary largely in area (range
30–5,848 m
) and density of breeding pairs within
each patch (range 0.22–1.74 nest/m
), while
Magellanic Penguins’ nests are scattered in a
colony in the southern area of Franklin Bay.
Imperial Shags breed in 24 discrete patches which
overlap with the colony of Rockhopper Penguins,
ranging 11–1,442 breeding pairs per patch (ARR,
unpubl. data) (see Fig 1).
Breeding Population Survey.—To obtain a
census of breeding pairs of Striated Caracaras,
we surveyed 24 transects in a SW-NE direction,
perpendicular to the coastline, with variable
lengths (range: 250–1,250 m), and a 200-m
separation between each one (Fig 1). The study
area (i.e., the area bounded by the band transects)
included all the main seabird colonies on the site
and its total area was 3.69 km
. We followed
Woods (2007) to identify active territories, and
when territorial behavior was observed, we
searched exhaustively for the active nest and
aged the members of the breeding pairs following
Strange (1996). Nests were georeferenced, and
when accessible, standard variables were mea-
sured to record basic characteristics of emplace-
ment, dimensions, and construction materials
following Tapia et al. (2007). In some cases,
nests were inaccessible because of the steepness
of cliffs where they were placed. We visited each
accessible nest every 7–10 days to record its
breeding output, and depending on the time of
encounter, each nest was visited 2–4 times during
the study.
Spatial Analysis of Nest Sites.—We analyzed
the spatial pattern of nest sites in the study area
using three Ripley’s functions, which have the
advantage that all point-to-point distances are
FIG. 1. Study site showing the location of Striated Caracaras’ nests sites, the main seabird colonies, and the transects
surveyed during the breeding season of 2014–2015.
892 THE WILSON JOURNAL OF ORNITHOLOGY Vol. 129, No. 4, December 2017
evaluated to show spatial clustering at different
scales (Ripley 1977, Getis and Ord 1992, de la
Cruz Rot 2008). We used Global-K and Local-K
functions to identify global or local clustering of
events (i.e., nest sites). By contrast, Weighted-K
function measures the degree of clustering of
points by some mark, in this case by productivity
(i.e., number of young produced/nest) at the time
of the last visit of each nest. For both global
analyses, we used radial distances ranging between
50–800 m with distance categories every 50 m.
For the local analysis, we used radial distances
ranging between 20–500 m with distance catego-
ries every 20 m. In each case, we ran 99 Monte
Carlo simulations to build a confidence envelope
over the null hypothesis of complete spatial
randomness of the points. In all cases, if the
statistical L(d) (for global and local analysis) and
(d) (for global analysis of weighted points) lie
within the confidence envelope at a certain
distance, then the points (or the weighted points)
are randomly distributed. If the statistic lies in the
upper confidence interval, the events tend to be
clustered and conversely, if it lies below the
confidence envelope, the events follow a regular
pattern. We compared the distance between nesting
sites and the closest breeding patches or areas of
colonies of Rockhopper Penguins, Magellanic
Penguins, and Imperial Shags in the study area
using paired t-tests and also measured the distance
between nest sites of Striated Caracaras.
Breeding Output.—Finally, we estimated the
apparent breeding success as the proportion of
breeding pairs that produced at least one young by
the end of the study. We also measured apparent
productivity as the mean number of young
produced per active nest and the apparent brood
size, as the mean number of young produced per
successful nest at the same time. Breeding success,
productivity, and brood size are interpreted as
apparent because we were not able to monitor each
nest until fledging, which is the traditional way to
estimate these parameters (Newton 1979, Steenhof
and Newton 2007). The values obtained are an
overestimation of the real values, because we
assume no more breeding attempts occurred
between the end of the study and the end of the
breeding season. We also analyzed breeding
success in relation to the distance to the nearest
conspecific neighbor and variables relative to the
distribution and breeding density of seabird
We found 16 nests of Striated Caracaras in the
study area, yielding a density of 4.34 breeding
. Half of these nests were placed on cliffs
of variable height (mean 39.8 m, range 6–100 m)
at different heights from the cliff bases (mean 30.6
m, range 2–90 m). The other nests were built on
the ground, in most cases associated with one
individual tussac grass (Poa flabellata). All
breeding birds presented full-adult plumage (.5
years old, Strange 1996). In accessible nests (n¼
11), pairs were found either incubating eggs or
raising nestlings. All accessible nests were con-
structed mainly with tussac grass fibers. However,
two nests also contained branches of diddle dee
and one nest included anthropogenic elements
(pieces of rope and fishing lines). Nest area was
0.34 60.14 m
(mean, SD), ranging 0.20–0.69
. Nests were nearer patches of Rockhopper
Penguins (68 658 m, mean, SD) than patches of
Imperial Shags (376 6222 m, t¼5.74, P,
0.001) and the colony of Magellanic Penguins
(1,822 61,126 m, t¼6.18, P,0.001). Indeed,
three nests were placed at the edge or inside
Rockhopper Penguins’ breeding patches. The
distance between nest sites of Striated Caracaras
in this study was 298 6184 m (mean, SD).
The global spatial pattern analysis indicated
scale dependent clustering (Fig. 2A). However,
productivity did not correlate with spatial location,
as its pattern showed no difference with the
complete spatial randomness (Fig. 2B). In the
latter case, the statistical L
(d) takes non-null
values starting from a radial distance of 250 m.
Given that L
(d) ¼0 for null values of the mark
(i.e., a failed nest), all successful nests (i.e., 1
young produced) were separated at least 250 m
from the nearest neighbor. For the local spatial
pattern analysis, three nests located in the center of
the study area (see Fig. 1) presented significant
local clustering at radial distances of 140–240 m,
280 m, and 360–440 m respectively. All other
nests showed values of L(d) which corresponded
with randomly distributed events.
The apparent breeding success for the accessible
nests was 0.73 successful nest/active nest, apparent
productivity was 1.27 61.01 young/active nest,
and apparent brood size was 1.75 60.71 young/
successful nest (range: 1–3 young/successful nest).
When analyzing the distance to the nearest
neighbor, failed nests were consistently closer to
other nests of Striated Caracaras compared to
successful nests (Fig. 3A). Also, successful nests
were generally closer to reproductive patches of
Rockhopper Penguins and had more patches in a
200-m radius around the nest, with respect to the
failed ones (Fig. 3B, C). We did not carry out any
statistical inference in any of these data sets
because of small sample sizes.
Breeding density in birds of prey is mainly
limited by food and nest site availability (Newton
1979). In this case, food limitation seems not
likely, since this population of Striated Caracaras
is associated with one of the largest Rockhopper
Penguin colonies in the world. The breeding
density estimate in our study area is similar to that
reported by Catry et al. (2008) for a growing
population on New Island but lower than the
value reported for a stable population on Beau-
chene Island (Lewis Smith and Prince 1985,
Strange 1996), both in the Malvinas/Falkland
Islands (Table 1). The values found for apparent
breeding success, productivity, and brood size do
not seem particularly large, and we found the
lowest mean brood size reported until now for the
Nest placement in this study differs from that
previously reported. In our case, half of the nests
were built in cliffs, in contrast with the report of
FIG. 2. Spatial patterns analysis using K-Ripley’s functions. A) Global pattern analysis using Global-K function for
events (presence of a Striated Caracaras’ nest) in the study area. B) Weighted-K function for a subsample of accessible
Striated Caracaras’ nests using productivity as a mark (see text).
894 THE WILSON JOURNAL OF ORNITHOLOGY Vol. 129, No. 4, December 2017
Strange (1996), which found 95%(n¼53) of the
nests on the ground. Simultaneously, even though
the nests of Striated Caracaras were relatively
close to patches of Rockhopper Penguins, separa-
tion between the two seems to be greater than the
distance previously described. Strange (1996)
reports that 74%of his surveyed nests (n¼19)
were 10 m or less from seabird colonies, while in
TABLE 1. Reproductive parameters of Striated Caracaras published in the literature and presented in this work.
Season Location
active nests
Breeding success
(successful nests/
total nests)
active nests)
Brood size
successful nests) Source
1979/80 Beauchene Island
(Malvinas/Falkland Is.)
64 0.42
0.88 61.16
2.07 60.83
Lewis Smith and
Prince 1985
2005/06 New Island
(Malvinas/Falkland Is.)
- 18 - - 2.50 60.51 Catry et al. 2008
2006/07 New Island
(Malvinas/Falkland Is.)
4.31 36 - - 2.36 60.59 Catry et al. 2008
2014/15 Franklin Bay,
Staten Island (Argentina)
4.34 11 0.73 1.27 61.01 1.75 60.71 This study
Indicate values calculated from original data.
FIG. 3. Relationship between reproductive success and ecological variables in the study area for a subsample of
accessible nests.
our study only 19%(n¼16) were consistent with
that metric. It could be possible that the difference
in nest placement along with the low breeding
density found could be a response to the absence
of suitable nesting sites near penguin patches. The
spatial pattern analysis showed that nesting sites
tended to be clustered in our study area, while in
other sites the distribution of breeding pairs tended
to be regular (Strange 1996), which in birds of
prey is expected when availability of nesting sites
is not a limitation and the population has reached
carrying capacity (Newton 1979). Nests showing
local clustering could be influencing the global
pattern, as they represented 19%of the nest sites
we studied.
Populations that reach their carrying capacity
show non-breeding or non-territorial adults during
the breeding season, which reproduce only when a
given site becomes available as a consequence of
the death of one or both members of a breeding
couple (Newton 1979). Catry et al. (2008)
reported several flocks of non-breeding individu-
als (up to 65 birds) foraging near seabird colonies,
but none of these individuals presented full-adult
plumage. Moreover, the authors reported three
territorial pairs with at least one of the members of
the couple presenting immature plumage, which is
a typical sign of a growing population with
available breeding sites (Newton 1979). In our
case, not only did all breeding birds present full-
adult plumage, but we anecdotally recorded
several non-breeding adults foraging or eating
carrion in flocks with juveniles and immature
birds. Even though we could not estimate the
abundance of Striated Caracaras, it seems likely
that most adult birds in the study area are non-
reproductive, and thus we hypothesize this
population has reached its carrying capacity
limited by nest site availability.
We propose that population dynamics of the
Striated Caracaras in Franklin Bay could be
complex and include the effect of the presence of
introduced herbivore species known for their
detrimental effect on native flora communities,
the negative effect over the reproductive outcome
of seabirds and other vertebrates in oceanic
islands, and the promotion of local extinctions in
areas where similar species are absent (Cronk
1989, Veblen et al. 1989, Takatsuki 2009, Pafilis et
al. 2013). In our case, two main effects of the
exotic goats and deer are proposed.
On the one hand, these herbivores could be
affecting the abundance and/or cover of tussac
grassland. In ungrazed islands of the Malvinas/
Falkland Islands, this grass covers .60%of the
land, forming a monospecific community with
some individuals 3.5 m high and .300 years old
which provide favorable conditions for nest
placement (e.g., temperature on the ground
surrounded by tussac is between 4–20 8C higher
than in uncovered soil; Lewis Smith and Prince
1985). The reduction or the lack of this resource
could be affecting the caracaras in three ways.
First, direct reduction of potential nesting sites
could be limiting the maximum number of
breeding pairs in the study area. Second, the
relative distance to their main food resource, the
nearest reproductive patch of Rockhopper Pen-
guins, may be increased thus augmenting their
foraging energy expenditure. Third, nesting in
other emplacements, like cliffs, could be in this
context, suboptimal for egg incubation, chick
development, or both.
On the other hand, during the winter, while
seabirds are not present, both goats and deer could
potentially be an important component of the
carrion portion of Striated Caracaras’ diet, a well-
documented situation for raptors in other invaded
scenarios (Speziale and Lambertucci 2013). Stri-
ated Caracaras in the Malvinas/Falklands Islands
apparently move in winter to farmland settlements,
where they feed partly on livestock as carrion
(Rexer-Huber and Bildstein 2013). If something
similar happened on Staten Island, it would be
expected that winter survival will be higher than in
a non-invaded system. These two situations would
be resulting in a lower population recruitment rate
because of fewer breeding pairs and more
intraspecific competition.
The total eradication of the invasive species is
proposed for the restoration of island environ-
ments (Glen et al. 2013), but in this case it seems
important to have more information to assess the
consequences of this intervention, as Striated
Caracaras’ population could be suddenly losing
an important winter-time subsidy, thus abruptly
changing its dynamics with unknown conse-
quences. It would be important to know the
current status of other populations of Striated
Caracaras in other potential breeding locations of
Staten Island, A ˜
no Nuevo Island, and in Tierra del
Fuego main Island, including the seasonal
896 THE WILSON JOURNAL OF ORNITHOLOGY Vol. 129, No. 4, December 2017
movements of individuals, where seabird colonies
are not so important in numbers, but goats and
deers are absent (Schiavini 2000, Schiavini et al.
This study was funded through a research project of the
Agencia Nacional de Promoci´
on Cient´
ıfica y Tecnol´
(PICT 2012 n81832 – ANPCyT), and with funding provided
by the Inter-American Development Bank (IDB) and
Wildlife Conservation Society. This work would have not
been possible without the field assistance provided by R.
Sa´ enz Samaniego and S. Harris, the background information
on seabird populations on Staten Island collected in previous
campaigns by V. Bruno, N. Rosciano, and P. Petracci, and
the technical support of A. E. Capdevielle and G. M.
Wiemeyer from Cabure-´
ı Foundation. We are also very
grateful to L. I. Rodr´
ıguez Planes, who provided us valuable
help in the design of the analysis and drafting of this
manuscript, and to M. G. Pizzarello, M. Dom´
ınguez, and S.
Dodino, who provided important comments improving our
study. And last but not least, we thank J. C. Reboreda, R. F.
o, and J. H. Sarasola, members of the panel in the graduate
thesis presentation that led to this work, for their feedback
and precious advice provided.
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The Wilson Journal of Ornithology 129(4):898–902, 2017
An Indigo Bunting (Passerina cyanea) Transporting Snails During Spring
Theodore J. Zenzal Jr.,
Emily J. Lain,
and J. Michael Sellers Jr.
ABSTRACT.—Organisms with limited motility may use
animal transport as a mechanism for dispersal. Migratory
birds can provide a vehicle to move small organisms great
distances, which may allow them access to areas that are
otherwise inaccessible. During normal mist netting
operations at a spring migration banding station along the
northern Gulf of Mexico coast in Louisiana, USA, we
encountered an Indigo Bunting (Passerina cyanea) with
numerous snails, possibly Galba cubensis, underneath its
breast feathers. While encounters of songbirds carrying
snails appear rare, long-distance migrating songbirds
represent a possible mechanism to transport small snails
great distances to expand ranges, colonize new areas, or
maintain genetic continuity. Received 12 October 2016.
Accepted 28 February 2017.
Key words: animal transport, chenier, Gulf of Mexico,
Louisiana, migration, snails, stopover.
Migratory animals are capable of traveling
considerable distances, often traversing diverse
habitats along their journey (Rappole 1995, Dingle
1996, Newton 2008). Through these large-scale
movements, migratory animals may act as dis-
persal agents for both plants and animals (e.g.,
Raven 1963, Rees 1965, Mukherjee et al. 2014).
Migratory birds can provide a mechanism for
dispersal over great distances to habitats that some
organisms may not otherwise reach, such as
habitats located on isolated mountain summits,
islands, and across ecological barriers (e.g.,
Cockerell 1921, Rees 1965, Mukherjee et al.
2014). Additionally, many migratory birds stop to
rest and refuel en route (Rappole 1995, Newton
2008) as they encounter a number of habitats on a
single journey.
Animals like snails might seem limited in their
dispersal abilities, but birds have been shown as a
mechanism to transport them across great distanc-
es (Rees 1965, Dundee et al. 1967, Vagvolgyi
1975, van Leeuwen et al. 2012). Migratory
landbirds, shorebirds, and waterfowl have been
found to transport snails either internally or
externally (e.g., Rees 1965, Dundee et al. 1967,
Wesselingh et al. 1999, van Leeuwen et al. 2012,
Department of Biological Sciences, University of
Southern Mississippi, Hattiesburg, MS 39406, USA.
Department of Entomology and Wildlife Ecology,
University of Delaware, Newark, DE 19716, USA.
Current address: Department of Natural Resources
and Environmental Sciences, University of Illinois,
Urbana, IL 61801.
Current address: Illinois Natural History Survey,
University of Illinois, Champaign, IL 61820.
Corresponding author; e-mail: tjzenzal@gmail.com
898 THE WILSON JOURNAL OF ORNITHOLOGY Vol. 129, No. 4, December 2017
... During the breeding season, they associate with seabird colonies, breeding in their proximity and feeding on eggs, chicks, adults, and carcasses (Catry et al., 2008;Liljesthröm et al., 2008;Strange, 1996). It is expected that, to ensure breeding success, nesting attempts are preferentially associated with (i.e., restricted to) foraging habitats that include seabird nesting patches (Balza et al., 2017;Catry et al., 2008;Strange, 1996). ...
... We thus expect breeders' and chicks' isotopic niches to be a subset of the floaters' niche (e). During the nonbreeding season (b), while the rockhopper penguins overwinter at sea, we propose a range expansion for all age classes (d) with a predicted incorporation of new prey sources and a consequent isotopic niche expansion (f) some rockhopper penguin subcolonies have no associated caracara nests (Balza et al., 2017). Caracaras are the most abundant scavenger on the island (Frere et al., 1999), being over six times more abundant than the second most abundant species (i.e., southern crested caracara, Caracara plancus, UB unpublished). ...
... This technique is biased over prey that leave hard remains (e.g., hairs, feathers, exoskeletons), and, as in our case, are generally encountered in the vicinity of nest sites (Marti et al., 2007;Redpath et al., 2001). Each year, we searched for active caracara nest sites by walking systematically through the study area and observing territorial behavior of breeding caracaras (for details see Balza et al., 2017). ...
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Optimal foraging theory predicts an inverse relationship between the availability of preferred prey and niche width in animals. Moreover, when individuals within a population have identical prey preferences and preferred prey is scarce, a nested pattern of trophic niche is expected if opportunistic and selective individuals can be identified. Here, we examined intraspecific variation in the trophic niche of a resident population of striated caracara (Phalcoboenus australis) on Isla de los Estados (Staten Island), Argentina, using pellet and stable isotope analyses. While this raptor specializes on seabird prey, we assessed this population's potential to forage on terrestrial prey, especially invasive herbivores as carrion, when seabirds are less accessible. We found that the isotopic niche of this species varies with season, age, breeding status, and, to a lesser extent, year. Our results were in general consistent with classic predictions of the optimal foraging theory, but we also explore other possible explanations for the observed pattern. Isotopic niche was broader for groups identified a priori as opportunistic (i.e., nonbreeding adults during the breeding season and the whole population during the nonbreeding season) than it was for individuals identified a priori as selective. Results suggested that terrestrial input was relatively low, and invasive mammals accounted for no more than 5% of the input. The seasonal pulse of rockhopper penguins likely interacts with caracara's reproductive status by constraining the spatial scale on which individuals forage. Niche expansion in spatially flexible individuals did not reflect an increase in terrestrial prey input; rather, it may be driven by a greater variation in the types of marine prey items consumed.
... obs.). To assess the breeding density of the Striated Caracara population, we followed a procedure similar one that is used in the nearby population of Franklin Bay, Isla de los Estados, which consists of walking the island in a systematic manner and observing territorial behaviour of breeding pairs, and then looking for the nest and its content whenever possible (for details see Balza et al. 2017). Southern giant petrels' breeding pairs are conspicuous and occur in relatively low numbers in the study area . ...
... The species is known to be present on the island for a long time, as the holotype was collected in 1775 in this site (Strange 1996). Most breeding populations occur in the Malvinas/Falkland archipelago (Reeves et al. 2018), and other breeding sites in the Fuegian archipelago include Franklin Bay (Isla de los Estados), Goffré Island, Noir Island, Diego Ramírez islands and Mitre Peninsula (Tierra del Fuego Main island) (Clark 1984;Parera et al. 1997;Marín et al. 2006;Cursach et al. 2012;Balza et al. 2017). We counted 15 breeding territories and found 10 active nests of Striated Caracara, containing a total of 15 chicks. ...
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Continental islands are often sites of low diversity and endemism, as well as important areas for the protection of bird populations, especially seabirds. On Isla Observatorio and the Año Nuevo Islands, in the Southwestern Atlantic, the latest assessment of avifauna dates from more than 20 years ago. In this study, we use a combination of methods to update the status of the main seabird colonies and the most abundant avian terrestrial predator at Observatorio and Goffré Islands during the breeding season. In only 4.5 km², the islands would harbour ~ 90,000 breeding seabirds. Seabird colonies occupied different areas of the islands and varied in their population status, with Imperial Shags (Leucocarbo atriceps) showing an increase and Southern Giant Petrels (Macronectes giganteus) a decrease according to the last surveys. Magellanic Penguin (Spheniscus magellanicus) population estimations also suggest a decrease but the last survey was based on total, and not on occupied nest sites. We recorded and assessed one new breeding species: The globally near-threatened Striated Caracara (Phalcoboenus australis), which has an important breeding population of around 15 territorial pairs at Observatorio Island. These islands appear to be an important regional bird site and future studies would determine their trends and threats, especially those related with invasive species.
... As top predators and facultative scavengers, caracaras may be highly exposed to mercury accumulation. In particular, the resident population in Franklin Bay (Isla de los Estados) build their nests in close association with Southern Rockhopper penguins (Eudyptes chrysocome, hereafter rockhopper) and are the main predator of their eggs and chicks (Liljesthröm et al. 2008;Balza et al. 2017). Moreover, caracaras of all ages and breeding status depend on marine resources and particularly on penguin subsidies during breeding season (Balza et al. 2020). ...
... Fieldwork was conducted in Franklin Bay, a ~ 4 km 2 bay on Isla de los Estados (Tierra del Fuego, Argentina, 54°85′30 S, 64°83′90 W). During December 2016 and May 2017, moulted wing (n = 26) and tail (n = 10) feathers were collected during systematic occupancy surveys of caracara territories (for details see Balza et al. 2017). These samples included wing feathers from three birds found dead during the surveys. ...
Mercury is a widely available pollutant associated with negative effects on wildlife, especially top predators. Here, we characterized the mercury concentrations in feathers of Striated Caracara Phalcoboenus australis on Isla de los Estados (Argentina). With feather mercury levels averaging 26.3 mg/kg, this population has the highest mean feather mercury ever reported for a bird population in South America. We propose that the high mercury concentrations are related to the feeding habits of the species: during feather moult, they are strongly associated with a Southern Rockhopper Penguin (Eudyptes chrysocome) colony known to be highly exposed to mercury contamination. Our results suggest that this Striated Caracara population should be monitored for acute effects and potential impacts of mercury toxicity.
... Striated Caracaras (Phalcoboenus australis; hereafter "caracaras") are near threatened falconids who inhabit the extreme southern coasts of South America and the Falkland Islands (Malvinas; Balza et al. 2017;Reeves et al. 2018;BirdLife International 2020). During summer, caracaras feed on a seasonal resource pulse (sensu Yang 2010) associated with colonial seabird populations (Strange 1996;Balza et al. 2017;Harrington et al. 2018). ...
... Striated Caracaras (Phalcoboenus australis; hereafter "caracaras") are near threatened falconids who inhabit the extreme southern coasts of South America and the Falkland Islands (Malvinas; Balza et al. 2017;Reeves et al. 2018;BirdLife International 2020). During summer, caracaras feed on a seasonal resource pulse (sensu Yang 2010) associated with colonial seabird populations (Strange 1996;Balza et al. 2017;Harrington et al. 2018). In winter, when most seabirds migrate offshore, caracaras' diets include native Upland Geese (Chloephaga picta), the feces and carrion of resident southern fur seals (Arctocephalus australis), southern sea lions (Mirounga leonina), and Gentoo Penguins (Pygoscelis papua), beetles (Coleoptera), and subsidies available at farms (Strange 1996;Rexer-Huber and Bildstein 2013;Harrington et al. 2018). ...
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Biological invasions can drive ecosystem change and alter predator ecology by providing trophic subsidies that mitigate the effects of seasonally pulsed resources. Striated Caracaras (Phalcoboenus australis) are near threatened, non-migratory falconids who inhabit the southern coasts of South America and the Falkland Islands (Malvinas) and feed on seasonally migratory colonial seabirds. Here we report the first multi-day observation of caracaras foraging extensively for earthworms (Lumbricidae) and beetle larvae (Coleoptera) in introduced Yorkshire fog (Holcus lanatus) on New Island, Falkland Islands. Our results suggest that invertebrates may be a more important winter resource than previously thought, and that caracaras benefit indirectly from introduced grasses in the Falklands, a relationship that merits special consideration when identifying ecological restoration plans.
... Striated Caracara (Phalcoboenus australis; hereafter ''caracaras'') are Near Threatened scavenging falconids that breed in high densities on the extreme southern coasts of South America and the Falkland Islands (Balza et al. 2017, BirdLife International 2020. During the austral summer, they forage in colonies of breeding seabirds; in winter, when most seabirds migrate offshore, caracaras consume invertebrates, penguin and seal excreta, and bonanzas of carrion (e.g., carcasses of Gentoo Penguin [Pygoscelis papua], cast sheep [Ovis aries], and pinnipeds; Strange 1996, Rexer-Huber andBildstein 2013). ...
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Background Animals need adaptive strategies to cope with seasonal changes in prey availability to survive and reproduce, which can include migrating, prey-switching, or reducing metabolic needs. Human settlements can disrupt spatiotemporal patterning in resource availability, which can affect animals’ foraging success, particularly for juveniles who are behaviorally developing and learning efficient foraging skills. Here, we investigate behavioral responses of juvenile striated caracaras, a near-threatened scavenging falconid, to seasonally migratory seabird prey and a farm settlement on Saunders Island, Falklands. We deployed accelerometer–GPS dataloggers (n = 27) to measure seasonal differences in daily and hourly vectorial dynamic body acceleration (VeDBA; an activity index measured in gravitational g) and space use, and investigated seasonal variation in body mass of the tagged subset and an additional 65 caracaras. Results Juvenile caracaras were overall similarly active in winter and summer. However, during winter, caracaras made the most of limited daylight by increasing average daytime activity (winter males: 0.16 ± 0.03 g, summer males: 0.09 ± 0.01 g, winter females: 0.12 ± 0.02 g, summer females: 0.08 ± 0.01 g). During winter, both sexes increased the percentage of daylight spent in high activity (winter males: 35 ± 5%, summer males: 21 ± 3%, winter females: 25 ± 6%, summer females: 16 ± 3%, p < 0.001) and ranged nearly 4 times farther (95% kernel density estimate winter: 2.36 ± 0.96 km², summer: 0.61 ± 0.20 km²; p < 0.001). Furthermore, on a daily scale, males were 21% more active than females year-round (24-h average VeDBA: males 0.07 ± 0.01 g, females 0.06 ± 0.01 g; p < 0.01). We did not observe a significant seasonal difference in mass. Conclusion That caracaras’ daily activity and body mass did not vary between seasons suggests that wintering birds on Saunders are meeting resource requirements despite the absence of seasonally migratory prey. We hypothesize that human subsidies may mitigate the effect of seasonal food limitations. Further research should include studies on seasonal energetics to improve our understanding of baseline body condition, and comparative studies on other islands and including adults to understand the importance of human subsidies.
Existing networks of nature reserves contain a biased sample of biodiversity. In Patagonia Argentina, most nature reserves focus their protection objectives on a particular ecosystem, geoform or scenic value, and usually are located in inaccessible areas. However, unique species or assemblages could inhabit less protected ecosystems, areas or habitats, which could be threatened depending of management. In this study, we assessed the conservation value of different ecosystem types and areas (fjords) in Isla de los Estados Provincial Reserve (RPIE, Argentina), using birds as study case. We chose three fjords (east, central and west) and five ecosystems types (forests at low and high elevation, open-lands at low and high elevation, and sea coasts). Bird's assemblage richness, density, biomass, trophic level, migratory status, and use of strata per ecosystems and fjords were characterized in 75 points (3 fjords × 5 ecosystems × 5 replicates) and evaluated using ANOVA and multivariate methods. Also, Shannon (H') and Pielou (J) indices were estimated for fjords and ecosystems. Passerine was the most abundant group, being mainly residents, omnivorous and carnivorous-scavenger, and they were observed mainly flying or in the canopy. Assemblage structure and function varied with ecosystem types, with higher richness and biomass in coasts and open-lands than in other ecosystems, but with greater density in forests. Multivariate analyses showed conspicuous groups for forests and coast sampling units, with significant differences among all ecosystem types except between low and high forests. Also, east fjord significantly differed in density and biomass from the others, but west fjord also differed in structure, function and bird assemblage. We conclude that greater conservation value must be assigned to ecosystem types or areas inhabited by threatened species (as open-lands at high elevation) and highest richness and variety of use of strata (as sea coasts). However, bird assemblage patterns have particularities in less valuable ecosystems and areas, which also justify their importance for conservation, or at least, prescriptions of low impact uses and activities in the management planning. Nature reserves are opportunities to preserve endemic species, habitats or areas of special interest, as low latitude or unique isolated landscape communities, and ecosystems underrepresented in the network of local, regional or world protected areas.
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Understanding the dynamics and causes of population trends are essential for seabird conservation. Long-term studies of seabirds at high-latitude (Antarctic, sub-Antarctic and Arctic) regions have shown contrasting species-specific trends in population size in response to climate change and anthropogenic pressures. We have studied for the last 20 years (1992–2012) the population trends of seven seabird species that breed in the Beagle Channel, south-eastern Tierra del Fuego and at Staten Island, a sub-Antarctic region in southern Argentina. The numbers of Magellanic and Gentoo Penguins increased significantly since 1992 (by >15 % year−1). In comparison, the populations of Imperial Cormorants, Dolphin Gulls and Kelp Gulls increased at slower rates (<5 % year−1), while the Rock Cormorant population even decreased by 1.3 % year−1. At Staten Island, the numbers of Rockhopper Penguins decreased by 24 % between the censuses of 1998 and 2010, whereas the population of Magellanic Penguins increased by 227 % during the same period. Over the study period, air and sea-surface temperatures remained stable in our study area, suggesting that the detected population changes are not driven by the climate. This finding contrasts with the detected links between increasing temperature trends and seabird population changes reported from Antarctic and Arctic regions. The level of tourism and size of the permanent human population has increased in the Beagle Channel area during the last 20 years and could be responsible for the increase of gull populations. The seabird species that received the highest number of visitors (Imperial Cormorants and penguin species) seem to be adapted or at least indifferent to pressures exerted by tourism, as their populations increased during the study period. In addition, increasing numbers of seabirds in the area may generally be leading to higher abundances of scavenging species (e.g. gulls).
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Invasive species are the greatest threat to island ecosystems, which harbour nearly half the world’s endangered biodiversity. However, eradication is more feasible on islands than on continents. We present a global analysis of 1,224 successful eradications of invasive plants and animals on 808 islands. Most involve single vertebrate species on uninhabited islands, but plant and invertebrate eradications occur more often on inhabited islands. Inhabited islands are often highly modified and support numerous introduced species. Consequently, targeting a single invasive species can be ineffective or counterproductive. The impacts of other pests will continue and, in some cases, be exacerbated. The presence of people also creates regulatory, logistical and socio-political constraints. Real or perceived health risks to inhabitants, pets and livestock may restrict the use of some eradication tools, and communities or individuals sometimes oppose eradication. Despite such challenges, managing invasive species is vital to conserve and restore the unique biodiversity of many inhabited islands, and to maintain or improve the welfare and livelihoods of island residents. We present a brief case study of the Juan Fernández Archipelago, Chile, and discuss the feasibility of eradicating large suites of invasive plants and animals from inhabited islands while managing other invaders for which eradication is not feasible or desirable. Eradications must be planned to account for species interactions. Monitoring and contingency plans must detect and address any ‘surprise effects’. Above all, it is important that the local community derives social, cultural and/or economic benefits, and that people support and are engaged in the restoration effort.
Spatial point processes may be analysed at two levels. Quadrat and distance methods were designed for the sampling of a population in the field. In this paper we consider those situations in which a map of a spatial pattern has been produced at some cost and we wish to extract the maximum possible information. We review the stochastic models which have been proposed for spatial point patterns and discuss methods by which the fit of such a model can be tested. Certain models are shown to be the equilibrium distributions of spatial–temporal stochastic processes. The theory is illustrated by several case studies.
Introduced in this paper is a family of statistics, G, that can be used as a measure of spatial association in a number of circumstances. The basic statistic is derived, its properties are identified, and its advantages explained. Several of the G statistics make it possible to evaluate the spatial association of a variable within a specified distance of a single point. A comparison is made between a general G statistic andMoran’s I for similar hypothetical and empirical conditions. The empiricalwork includes studies of sudden infant death syndrome by county in North Carolina and dwelling unit prices in metropolitan San Diego by zip-code districts. Results indicate that G statistics should be used in conjunction with I in order to identify characteristics of patterns not revealed by the I statistic alone and, specifically, the Gi and G∗ i statistics enable us to detect local “pockets” of dependence that may not show up when using global statistics.
This paper provides new data on the location and size of Southern Rockhopper Penguin (Eudyptes chrysocome chrysocome) populations at Staten Island, Tierra del Fuego, Argentina, in view of population sizes previously reported for all the subspecies. Surveys were performed mainly during November-December 1998 and 1999. Mean nests were calculated in circular plots of 100 m2. Nest density was then extrapolated to the area occupied by nests, estimated from aerial pictures and through ground controls. Nests are distributed mainly on areas of tussock grass (Poa flabellata). Most of the nests were located in tightly-knit subcolonies. Nest density (±SD) was estimated at 102.5 (±29.7) nests/100m2. Two localities, Cabo San Juan and Bahia Franklin, together held 173,793 nests, 166,762 located in Bahia Franklin, extending over a radius of 3.2 km. Based on the reported population data for this subspecies, the global population for Southern Rockhopper Penguins is likely to be close to 636,000 pairs, with Staten Island holding 27.3% of the world population and Bahia Franklin alone holding 26.2% of the world population. The Southern Rockhopper Penguins of Staten Island were almost undisturbed and unknown during the 20th century. The population importance of Bahia Franklin for the species was not previously recognized, but it should now occupy a central place of concern for the conservation of this species. Received 6 December 1999, accepted 26 January 2000.
The original vegetation of St Helena has been almost entirely destroyed and little is known about it. Using relict occurrences and historical records, hypothetical distributions have been constructed for the indigenous plants. Multivatiate methods are used to reconstruct seven putative former vegetation zones which correspond well with historical records. The replacement of past vegetation zones by the present ones has been complicated by the interaction of history with site factors through browsing, grazing, erosion, cutting, clearance and plant introduction.
Grazing of goats on Mediterranean islets is a common practice. Its consequences on plant communities are well documented, although not on vertebrates. We aim to shed light on the effect of livestock farming on lizards by investigating five populations of the insular lizard, Podarcis gaigeae, differing in the duration and intensity of grazing. Data on grazing regime, invertebrate abundance, tick prevalence, infestation levels, gull nests and lizard densities were collected during a period of 6 consecutive years. Grazing had a negative impact on insect populations, thus decreasing food availability for lizards. Tick prevalence and infestation levels were higher in places of continuous grazing. Goat activity disturbed gulls, which avoid nesting, so depriving the islets of marine subsidies. As a consequence of all these factors, lizard densities were higher in ungrazed and lower in grazed biotopes. Grazing effects were more severe on islets communities than on the main island populations. Our data imply that management action should be taken to conserve the highly diverse islet populations.
Understanding processes and impacts of biological invasions is fundamental for ecology and management. Recent reviews summarized the mechanisms by which invasive species alter entire ecosystems,but quantitative assessments of these mechanisms are lacking for actual assemblages to determine their relative importance, frequency and patterns. We updated information on introduced vertebrates in theTierra del Fuego Archipelago (TDF) via an exhaustive literature review and new data to evaluate eco-system impact mechanisms and provide management recommendations. To date, 24 exotic vertebrateshave naturalized in TDF, outnumbering natives nearly 2:1, with the North American beaver (Castor canadensis) and muskrat (Ondatra zibethica) being the most widely distributed species and alsoimpacting the ecosystem through the greatest number of mechanisms. Introduced vertebrates occupiedmost parts of the archipelago with human-inhabited islands having greater taxa richness. All exoticspotentially altered ecosystems by one or more mechanisms: 100% food webs, 92% invasional meltdown,42% habitat modification, 38% disease or parasite transmission, 21% soil property and disturbance regimechanges. Impact to habitat structure was the main clustering criterion for this assemblage. Within thespecies that physically alter habitats, we found two sub-groups: 1) large herbivores and 2) “others” including beavers and muskrats. Species that did not alter habitat were divided further into those withpredatory trophic effects (carnivorous mammals and trout, sub-group 4) and the rest with assortedimpacts (sub-group 3). By establishing high quality information on archipelago-wide assemblage, dis-tribution, impacts and mechanisms for exotic vertebrates, we recommend, based on ecological criteria,prioritizing the management of sub-group 2. A secondary priority might be given to the carnivores insub-group 4, while species in sub-groups 1 and 3 are less urgent. As the first systematic survey of introduced fauna on an archipelago-scale, we identified knowledge gaps, such as population abundanceand dynamics for specific species, which are needed to orient future work, but the notable progress madeto date is highlighted.