Identifying Native and Exotic Predators of Ground-Nesting Songbirds in Subantarctic Forests in Southern Chile Page: 53
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PREDATORS IN SUBANTARCTIC FORESTS
impact of either native or exotic predators on the
forest avifauna of the CHBR.
Here, we explored the consequences of mink
invasion on ground-nesting Passeriformes. To better
understand the ecological role played by the invasive
American mink in sub-Antarctic forests, we carried
out an artificial nest study in the Omora Ethnobo-
tanical Park (OEP: 55 S) to measure the relative
risk of depredation by mink and other potential
predators in the three dominant habitat types of
the north coast of Navarino Island: anthropic shru-
bland, beaver meadows, and forested habitat. We
hypothesized that introduced mink depredation on
artificial nests would be most important in habitats
with greater disturbance, which have been shown
to decrease safe nesting sites (Gates & Gysel 1978,
Willson et al. 2001).
We characterized the community of potential
predators and also bird species that occupy the sub-
Antarctic forest, using the OEP's database of over
6,000 captures of principally forest Passeriformes
during a long-term mist netting program (period:
2000-2007) (Anderson et al. 2002). Utilizing this
database, we determined: a) the total assemblage of
forest avifauna and b) the relative frequency of the
principal forest songbird species in the CHBR that
represented >1% of captures. Opportunistic mist
netting surveys conducted at high elevation sites (i.e.
above tree line) were excluded from this analysis. In
addition, we used data from previous surveys of the
archipelago-wide vertebrate fauna (Anderson et al.
2006) and a literature review to determine potential
nest predators. Nesting location and type of nest
were determined from personal observation and a
decade of field notes and experience, accumulated
by the Omora Sub-Antarctic Bird Observatory (2000
The study took place in the OEP on the north
shore of Navarino Island in the CHBR, Chile (55S).
The OEP is one of three long-term socio-ecological
research (LTSER) sites coordinated by the Institute of
Ecology and Biodiversity (www.ieb-chile.cl/ltser). The
eco-region, known as the Magellanic Sub-Antarctic
Forest Biome, is considered one of the world's most
pristine remaining wilderness areas (Mittermeier
et al. 2003). However, while the archipelago has a
low degree of habitat fragmentation and a sparse
human population, it is paradoxically replete with
exotic species, including the American mink, North
American beavers (Castor canadensis Kuhl 1820),
muskrats (Ondatra zibethicus Linnaeus 1766), feral
dogs, cats, and livestock (Anderson et al. 2006).
Artificial nest experiment
To determine nest predator identity and the
relationship of nest failure with habitat type, we used
an artificial nest experiment with plasticine eggs.
Two sites per habitat type were used to distribute
artificial nests (n = 60), which were made of 100%
dehydrated and sterilized coconut fiber (approx.120
mm diameter). Eggs were made using a mix of white
and blue colored plasticine clay (approx. 30 mm
long). Both size and color mimicked natural Aus-
tral Thrush (Turdus falcklandii Quoy & Gaimard
1824) nests and eggs. To standardize clutch size,
three eggs were placed in each nest, and nests were
placed at locations that simulated natural nesting
sites for ground-nesting songbirds found at the
OEP, usually under downed logs or at the base of
shrubs. Nests were checked every three days from 3
to 30 October 2005. Nitrile gloves were worn when
handling nests and eggs, and they were considered
depredated if they had scratch marks, were missing,
had puncture holes, or were torn apart. If a nest
failed between subsequent checks, we marked the
failure date as the midpoint between these checks.
The identity of nest predators was determined using
a reference collection of potential predator teeth
marks on eggs. A preserved specimen of mink was
obtained from the Martin Gusinde Anthropological
Museum. Various bird specimens, including wrens
and rayaditos, were obtained during the course of
the Omora Park's monthly bird banding project. A
simultaneous study of native mice that were trapped
in the OEP with Sherman traps allowed us to obtain
local native mice species for creating their teeth
prints on reference eggs.
We calculated the probability of daily nest
mortality due to mink using the Mayfield logistic
regression (Hazler 2004). For nests that were
depredated by other predators, we only included
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Maley, Brett M.; Anderson, Christopher B.; Stodola, Kirk & Rosemond, Amy D. Identifying Native and Exotic Predators of Ground-Nesting Songbirds in Subantarctic Forests in Southern Chile, article, April 28, 2011; Punta Arenas, Chile. (https://digital.library.unt.edu/ark:/67531/metadc1181154/m1/3/: accessed July 20, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT College of Arts and Sciences.