The social spider Parawixia bistriata (Rengger, 1836) (Araneidae) as a potentially significant source of avian mortality in the Paraguayan Chaco

Abstract

Five cases of bird entrapment in the social webs of Parawixia bistriata (Araneidae: Araneinae: Araneini) are reported from the Paraguayan Chaco. These are the first reports of avian entrap-ment for this species, and add one new family (Thraupidae) and four new species to the global biological inventory of bird species caught in spider webs. The wing chord and masses of the largest two species, Tyrannus melancholicus (Tyrannidae) and Columbina picui (Columbidae) greatly exceed the maximum reported for other Neotropical species. It is argued that possibility of mortality from entrapment is not merely a matter of mor-phometry as has been previously suggested, but includes other factors such as spider species, silk type, and area of entangle-ment. A possible temporal relationship between the growth and life history of this spider and the potential for entrapment is postulated. It is proposed that increased deforestation in the Paraguayan Chaco could lead to an increase in avian entrapment in this webs of this species.

Full text

436 Arachnology (2020) 18 (5), 436–439
The social spider Parawixia bistriata (Rengger,
1836) (Araneidae) as a potentially signicant
source of avian mortality in the Paraguayan
Chaco
Paul Smith
FAUNA Paraguay, Encarnación,
Dpto. Itapúa, Paraguay
email: faunaparaguay@gmail.com
and
Para La Tierra, Centro IDEAL,
Mariscal Estigarribia 321 c/ Tte. Capurro,
Pilar,
Dpto. Ñeembucú, Paraguay
Arne J. Lesterhuis
Manomet, Inc.
WHSRN Executive Ofce
email: alesterhuis@manomet.org
Oscar Rodríguez
Desarrollo Turístico Paraguayo S.A.,
Asunción, Paraguay
email: oscar@dtp.com.py
Abstract
Five cases of bird entrapment in the social webs of Parawixia
bistriata (Araneidae: Araneinae: Araneini) are reported from the
Paraguayan Chaco. These are the rst reports of avian entrap-
ment for this species, and add one new family (Thraupidae) and
four new species to the global biological inventory of bird
species caught in spider webs. The wing chord and masses of the
largest two species, Tyrannus melancholicus (Tyrannidae) and
Columbina picui (Columbidae) greatly exceed the maximum
reported for other Neotropical species. It is argued that possibil-
ity of mortality from entrapment is not merely a matter of mor-
phometry as has been previously suggested, but includes other
factors such as spider species, silk type, and area of entangle-
ment. A possible temporal relationship between the growth and
life history of this spider and the potential for entrapment is pos-
tulated. It is proposed that increased deforestation in the
Paraguayan Chaco could lead to an increase in avian entrapment
in this webs of this species.
Keywords: birds • Columbina picui •Coryphospingus cucullatus •
entrapment • Saltatricula multicolor •Troglodytes aedon •Tyrannus
melancholicus
Introduction
The ñandutí spider Parawixia bistriata (Rengger, 1836)
(Araneidae: Araneinae: Araneini) is a large and conspicuous
social spider that is widespread in drier areas of Paraguay
and abundant in the Chaco, west of the Paraguay River
(Levi 1992). Though its social behaviour was reported as
early as Azara (1809), and much commented on during the
19th Century (Rengger 1836; Masterman 1869, 1891;
Darwin 1845; Goeldi 1892; Holmberg 1874a,b), it was
widely questioned by prominent arachnologists of the time
(Pickard-Cambridge 1870; McCook 1890) until the obser-
vations were conrmed by Simon (1891). The extensive
colonial orb webs built by the species are of cultural signi-
cance in Paraguay, probably inspiring the local artisanal
ñandutí lace (Fowler & Diehl 1978).
Colony structure comprises two parts: a silky, diurnal
retreat (bivouac) located 2–5 m above ground (typically in
a tree or lamp post) in which the black mass of spiders is
clearly visible, and a nocturnal capture web, composed of a
scaffold of thick support threads radiating out from the nest
for 10–30 m and upon which the individual orbs are woven
at night (Gobbi, Zucchi & Sakagami 1979). The construc-
tion of the orb of each individual spider may take as long as
an hour (Gobbi, Zucchi & Sakagami 1979). Webs are typi-
cally located at the forest edge, or along hedgerows, but not
in closed canopy forest, and a single colony may contain
over 300 individuals. The orbs are consumed each morning
before the retreat to the bivouac, but the scaffold of support
threads remains in place. The breeding system is univoltine,
with spiderlings hatching in May and the last adults disap-
pearing in March (Fowler & Diehl 1978). This synchronic-
ity in senescence (and hence body size) is essential to the
evolution of the colonial system employed.
Prey consumption is a function of body size, with larger
spiders building larger webs. Orbs may even be designed
specically to catch certain types of seasonally abundant
prey, maximising the efciency of silk usage (Sandoval
1994). Unlike most colonial orb-weaving spiders, this
species exhibits facultative, communal prey capture and
feeding. Small prey items are consumed only by the orb res-
ident, but small groups of neighbours may share prey that is
signicantly larger than themselves (Fowler & Diehl 1978;
Gobbi, Zucchi & Sakagami 1979; Sandoval 1987; Fowler &
Gobbi 1988; Fernández-Campon 2007). Prey that is too
large to be consumed is able and permitted to free itself
from the orbs, but damages it in the process (Fowler &
Gobbi 1988; Fernández-Campón 2007).
Brooks (2012) provided a review of records of birds
caught in spider webs, listing 68 cases, representing 54
species in 23 families, with additional data for Asian birds
provided by Walther (2016), meaning a total of 84 bird
species documented. Although the cause of entrapment of
birds in spider webs is not always clear, it has been hypoth-
esized to occur by three basic means: 1) foraging for insects
trapped in the web; 2) gathering material for nest building;
3) accidentally ying into an unseen web (Peloso & de
Sousa 2007). It is uncommon for spiders to actually predate
birds (Levy 1987; Peloso & de Sousa 2007), but entrapment
is nonetheless a signicant source of mortality for some bird
families (Graham 1997), and without human assistance as
many as 73% of cases of entrapment ultimately result in
death (Brooks 2012). The venom of P. bistriata is potent
enough to subdue small vertebrates, but there is no evidence
that vertebrates are consumed by this species (Araujo-Ro-
drigues et al. 2001, 2004).
In this paper we provide information on ve Paraguayan
cases of bird entrapment in the web of P. bistriata, a species
not listed by Brooks (2012) as previously associated with
bird entrapment, adding one new family (Thraupidae) and
four new species to the biological inventory of bird species.
436

P. Smith,A. J. Lesterhuis & O. Rodríguez 437
Picui Ground Dove Columbina picui (Columbidae)
An individual in a state of decomposition (Fig. 1) was
found tangled in a web at Laguna Capitán, Presidente Hayes
department, -22.540278°S -59.676389°W, by AJL and Rob
Clay on 05 July 2018. Given the poor state of preservation,
it was assumed to have been trapped for a considerable time.
Mean wing chord of four specimens of Columbina picui
from the Paraguayan Humid Chaco were 87.71 mm
(+/-1.69) and mean mass 44.75 g (+/-1.52) (Smith, Betuel &
Batjes 2008).
Tropical Kingbird Tyrannus melancholichus (Tyran-
nidae)
An individual of this species was found dead at Puerto
Barbero, Presidente Hayes department, -24.914173°S
-57.304178°W, by OR on 05 January 2009 suspended by
one wing and one leg from the spider silk.
Mean wing chord of four specimens of Tyrannus melan-
cholichus from the Paraguayan Humid Chaco was 112.00
mm (+/-1.30) and mean mass 46.00 g (+/-2.00) (Peris
1990).
Many-coloured Chaco Finch Saltatricula multicolor
(Thraupidae)
A freshly deceased adult was found by OR suspended by
the left wing (Fig. 2) on 01 February 2018 at approximately
15 km SE of Teniente Manuel Irala Fernández, on the Ruta
Trans Chaco at Estancia Millón, Boquerón department,
-22.904094°S -59.480793°W.
Mean wing chord of 22 specimens of Saltatricula multi-
color from the Paraguayan Dry Chaco was 68.18 mm
(+/-1.78) and mean mass 22 g (+/-1.53) (Smith, Betuel &
Batjes 2008).
Red-crested Finch Coryphospingus cucullatus fargoi
(Thraupidae)
On 14 February 2018 an adult male caught by the left
wing in a support line of a P. bistriata web was found by
AJL, Hugo Cabral and Mateo Lesterhuis close to Loma
Plata, Boquerón department, approximately -22.38°S
-59.79°W. The bird (Fig. 3) was hanging just under an elec-
tric cable at a height of approximately 15–20 m. The indi-
vidual, at rst believed to be dead, was suspended by its left
wing and tried unsuccessfully to escape on several occa-
sions. Just above the tip of the left wing a thick noose could
be observed, indicating the bird had been struggling for
some time. It was considered that the bird would be unlikely
to survive.
Mean wing chord of 19 male specimens of Coryphospin-
gus cucullatus fargoi from the Paraguayan Dry Chaco was
62.61 mm (+/-1.66) and mean mass 13.75 g (+/-0.80)
(Smith, Betuel & Batjes 2008).
Fig. 1: Dead Picui Ground Dove Columbina picui tangled in a web at
Laguna Capitán, Presidente Hayes department. © A. J. Lesterhuis.
Fig. 3: A living adult male Red-crested Finch Coryphospingus cucullatus
caught in a support line of a P. bistriata web close to Loma Plata,
Boquerón department. © A. J. Lesterhuis.
Fig. 2: A freshly deceased Many-coloured Chaco Finch Saltatricula multi-
color found at approximately 15 km SE of Teniente Manuel Irala
Fernández, on the Ruta Trans Chaco. © O. Rodríguez.

438 Bird mortality in Parawixia webs
Southern House Wren Troglodytes aedon musculus
(Troglodytidae)
On 30 January 2018 a deceased adult Southern House
Wren Troglodytes aedon musculus (Fig. 4) was found by PS
entangled in a spider web at Fortín Toledo, -22.333333°S
-60.350000°W, Boquerón department, Paraguayan Chaco.
The web was partly entwined within a Stetsonia cactus and
at least partially continuous with the support lines of a social
web of the spider P. bistriata. The bird was somewhat
decomposed, suggesting it had been trapped for some time,
and was bound tightly with spider silk around the feet, tail
and more loosely around one wing. There were no signs of
spider predation.
Mean wing chord of 13 specimens of Troglodytes aedon
musculus from Paraguay was 50.23 mm (range = 47–56; P.
Smith unpublished data) and the mean mass of the species
from across its range varies from 10.4–11.6 g (Dunning
2008). There have been previous reports of House Wren
entrapment in spider webs (Gosling 1984; Hallbeck 2007;
Brooks 2012).
Discussion
Brooks (2012) stated that the mean mass of birds that
died in spider webs was 9 g and mean wing chord was 53
mm, whilst those that were able to free themselves had a
mean mass of 11 g and mean wing chord of 66 mm. Though
these differences were not statistically signicant, the
values for some of the deceased birds published here are
much greater than the mean masses and wing chord values
proposed as potential limits. In the cases of Tyrannus melan-
cholichus and Columbina picui (the two largest species in
our sample), it is over four times the proposed mass and, in
the case of T. melancholichus, almost double the wing chord
length, the greatest yet documented for any Neotropical
species. Walther (2016) provided data for several Asian
species that surpassed even these numbers. It would seem
reasonable to assume therefore that the factors affecting a
bird’s ability to free itself would be more complex than
simple morphometrics, and might additionally include fac-
tors such as the species of spider (and type and tensile
strength of its silk), the area of the body that is ensnared, and
the condition of the bird. A bird ensnared by an extremity
may be expected to tire quickly, something that would
rapidly be lethal in the Chaco where day time summer tem-
peratures regularly surpass 40°C (Fariña Sanchez 1973).
Given the complex, strengthened structure of the colonial
webs of P. bistriata, we hypothesize that even heavier birds
than those recorded here might struggle to free themselves
if bound by the thickened support lines. We performed a
simple test of the tensile strength of these lines by hanging
a weighted bag from a collected piece of support line. It was
found that a single strand could comfortably hold 300 g
without breaking. Whilst a dead weight exerts less force
than a moving weight, it should be noted that a bound bird
is often entangled by more than a single strand, and,
depending on how heavily and by what part of the body it is
bound, it may have to exert an even greater force than that
measured here in order to free itself. This suggests that
potentially far bigger birds than those recorded here could
feasibly become entangled in the webs of P. bistriata.
The construction of orbs is a time-consuming and ener-
getically costly undertaking (Gobbi, Zucchi & Sakagami
1979; Sandoval 1994; Blackledge & Wenzel 1999), and
some spiders build structures (stabilimenta) into their webs
to increase visibility. These have been proposed as a protec-
tive device against web destruction by birds, thereby saving
time and energy for the spiders (Eisner & Nowicki 1983;
Blackledge & Wenzel 1999). Despite the costs of rebuilding
broken webs, the orbs of P. bistriata do not gure stabili-
menta (Sandoval 1994).
With the individual orbs being consumed each morning,
the only part of the structure remaining by day is the much
thicker scaffold of support lines. These are present in high
density, and their location in prime hunting areas for insects
might be considered to put them at risk of frequent collision
by birds. Support lines are the product of several individual
spiders spinning their silk together to make thicker threads
(Masterman 1869) and, given their permanence, it may be
assumed that the overall cost to each individual spider in the
colony is lower for a broken support line than it is for a
broken orb web. This fact, coupled with the extreme tensile
strength of these lines, suggests that natural breakage by
birds, as opposed to entrapment or rebounding, is likely to
be a relatively rare event, and that such events may be toler-
Fig. 4: Adeceased adult Southern House Wren Troglodytes aedon found in
a spider web at Fortín Toledo, Boquerón department. © P. Smith.

P. Smith,A. J. Lesterhuis & O. Rodríguez 439
ated. Indeed the presence of the decomposing Picui Ground
Dove and House Wren still hanging from the web indicates
that the spiders take no measures to remove entrapped birds,
and that the function of the support line is unaffected by the
extra weight.
P. bistriata is an exceptionally abundant spider in the
Paraguayan Chaco (Fowler & Diehl 1978), and with
increasing deforestation levels (Yanosky 2013) creating
ever more of the forest edge habitats that the species
favours, it would seem that its populations are likely to
increase. However, this reduction in forest cover, coupled
with increased edge effect and possibly greater numbers of
spiders, also threatens to bring birds and spiders into
increased contact, potentially making these colonial webs
an signicant source of mortality for bird species in the
region.
Bearing in mind that four of the ve cases of entrapment
occurred during the months of January and February (when
adult spiders are at their largest and eldworkers are rarely
in the eld because of the extreme heat), it is perhaps
worthy of further study whether the risk of avian entrapment
increases temporally, proportional to the growth of the spi-
ders and hence the possible increased strength of their webs.
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