The Wilson Journal of Ornithology 121(4):803?807, 2009
Breeding Success and Social Mating System of the Bay-capped
Wren-Spinetail (Spartonoica maluroides)
Paulo E. Llamb??as,1,6 Valentina Ferretti,1,2 Daniel A. Cardoni,3,4,5 and Jesu?s E. Maldonado5
ABSTRACT.?We studied breeding biology, paren-
tal roles, and social mating system of the Bay-capped
Wren-Spinetail (Spartonoica maluroides), a habitat
specialist furnariid, in the Argentinean pampas. We
found 42 nests during 2004?2007, two during egg
laying, 28 during incubation, and 12 with nestlings.
Mean clutch size was 3.17 eggs (n 5 29), the incubation
period was 13 days, and nestlings remained in the nest
for 12 days before fledgling. Bay-capped Wren-Spine-
tails are socially monogamous; both males and females
develop a brood patch and contribute to incubation,
brooding, and provisioning of nestlings. Wren-Spine-
tails are unique among furnariids as they build an open
cup nest with a few presenting a loose domed roof.
Breeding success of Bay-capped Wren-Spinetails was
higher (total probability of nesting success 5 0.508)
than other species of sympatric passerines because of
low nest predation and high nest survival rates during
incubation and nestling rearing stages. Received 20
January 2009. Accepted 17 June 2009.
The members of the Furnariidae (236 described
species) inhabit a great diversity of biomes in the
Neotropics, including lowland and montane
1 Department of Ecology and Evolutionary Biology,
Cornell University, Ithaca, NY 14853, USA.
2 Fuller Evolutionary Biology Program, Cornell Labora-
tory of Ornithology, 159 Sapsucker Woods Road, Ithaca,
NY 14850, USA.
3 Departamento de Biolog??a, Facultad de Ciencias
Exactas y Naturales, Universidad Nacional de Mar del
Plata, Funes 3250 (CP 7600), Mar del Plata, Argentina.
4 Consejo Nacional de Investigaciones Cient??ficas y
Te?cnicas (CONICET), Avenida Rivadavia 1917 (CP
C1033AAJ), Ciudad de Buenos Aires, Argentina.
5 Center for Conservation and Evolutionary Genetics,
NZP/NMNH, Smithsonian Institution, Washington, D.C.
20008, USA.
6 Corresponding author; e-mail: pel24@cornell.edu
SHORT COMMUNICATIONS 803
forests, grasslands, salt and fresh water marshes,
and rocky areas and meadows in the High Andes
(Ridgely and Tudor 1994, Remsen 2003). Furnar-
iids show no variation in mating systems and
parental roles, and all species studied have been
described as socially monogamous with males and
females having similar parental roles (shared
incubation, brooding, and feeding nestlings)
(Remsen 2003). However, studies of color-banded
populations are rare and current knowledge on
parental roles and breeding biology is based on a
small number of species.
Bay-capped Wren-Spinetails (Spartonoica mal-
uroides) are small (10?12 g), insectivorous,
habitat specialists that inhabit fresh water and
brackish marshes in the Pampas region in north-
central Argentina, southeastern Brazil, and Ur-
uguay (Ridgely and Tudor 1994). The reproduc-
tive biology and social mating system of the Bay-
capped Wren-Spinetail have not been previously
described due to its elusive breeding habits. Wren-
Spinetails, as habitat specialists, are vulnerable to
habitat transformation and are considered near
threatened (Birdlife International 2008). The
objective of this paper is to provide basic
information on the life history, breeding success,
and social mating system of the Bay-capped
Wren-Spinetail.
METHODS
We conducted field work at a private cattle
ranch, Los Zorzales (36u 259 S, 56u 589 W), in
Buenos Aires Province, Argentina between Sep-
tember and December 2004?2007. The habitat is
characterized by small wetlands, salt marshes, and
isolated patches of Celtis tala woods. Wren-
Spinetails breed in this area during the austral
spring (Oct?Jan) in salt marshes where sedges
(Spartina spp.) and rushes (Juncus spp.) predom-
inate.
We located nests of Wren-Spinetails using both
behavioral cues of adults and systematic search-
ing. We checked nests every 2?4 days to assess
clutch size, hatching success, and fledging
success. Nests were checked every day when
eggs were close to hatch or nestlings close to
fledging to ascertain incubation period (number of
days from laying of the last egg until hatching of
the first young) and nestling period (number of
days from hatching of the first young until
fledging of the last nestling). We used mist nests
to capture adults close to the nest and color-
banded 24 pairs during 2004?2007.
We considered a nest to be depredated if eggs
or nestlings that were younger than 10 days
disappeared from the nest. We considered a nest
to be successful if nestlings fledged and were
observed being fed by adults.
We monitored nests during incubation and
when nestlings were 2?3, 7?8, and 11?12 days
of age to assess parental roles. We observed nests
from a distance of 50 m with 10 3 42 binoculars
and identified the individuals attending the nest.
We assumed the adults were incubating eggs or
brooding nestlings when they remained on the
nest for more than 5 min. Wren-Spinetails are
elusive close to the nest, and we limited
observations to a minimum to avoid disturbance,
remaining in the area sufficiently long to confirm
that both adults were attending the nest. Thus, we
did not assess parental provisioning rates.
We calculated clutch size as the number of eggs
observed in the nest. This method may underes-
timate clutch size as partial predation can reduce
the number of eggs in the nest. We did not
observe partial nest losses during 162 days of
observation of the 29 nests we monitored during
incubation. We took particular care not to disturb
the vegetation in the area or leave any marking
that could attract the attention of potential nest
predators during nest checks and observation
bouts.
We estimated nest survival and daily mortality
rates of nests following Mayfield (1975). We
estimated the time the nest survived as 40% of the
length of the period between visits (Johnson
1979). We estimated the overall probability that
one chick would fledge from a nest as the sum of
daily survival rates for each nesting stage elevated
to the exposure time (number of days the nest
remained in that stage). We estimated exposure
time at 13 days for incubation and 12 days for the
nestling rearing stage based on nests monitored in
the area. We performed all statistical analyses
using SPSS Version 14.0 software (SPSS 2005).
RESULTS
We found 42 nests during 2004?2007, two
during the egg laying period, 28 during incuba-
tion, and 12 with nestlings. Nests were both open
cup or presented a simple roof, built deep inside
sedges and rushes. Success of 34 nests (8 nests
were not monitored through the entire nestling
cycle) was ascertained of which 21 produced
fledglings, 12 were predated, (2 during incubation
and 10 with nestlings), and one was abandoned
804 THE WILSON JOURNAL OF ORNITHOLOGY N Vol. 121, No. 4, December 2009
after a rain storm. Both adults had brood patches
and were involved in incubation (14/14 nests),
brooding (16/16 nests), and feeding nestlings at
day 2?3 (16/16 nests), day 7?8 (20/20 nests), and
day 11?12 (15/15 nests).
Mean (6 SD) clutch size was 3.17 6 0.47, (n 5
29 nests); 22 nests contained three eggs, six
contained four eggs, and one contained two eggs.
We did not observe Shiny Cowbird (Molothrus
bonariensis) eggs, although this species was
abundant in the area. The mean (6 SD) number
of fledglings for successful nests was 2.62 6 0.80
(n 5 21 nests).
We were only able to calculate the incubation
period for two nests (13 days). The mean (6 SD)
nestling period was 12.38 6 0.92 (n 5 8 nests), and
ranged from 11 to 14 days. Daily survival rate
based on 162 nest days during incubation was
0.987 with a total survival rate for the incubation
period of 0.846. Daily survival rate based on 277
nest days during the nestling period was 0.958 with
a survival rate for the nestling period of 0.601. The
total probability of nesting success was 0.508.
DISCUSSION
Furnariids are present in all terrestrial habitats
in the Neotropics and exhibit extreme variation in
nest architecture (Remsen 2003). Nest structure
has been used to study phylogenetic relationships
in the family (Vaurie 1980, Zyskowski and Prum
1999) and nest building evolution in passerines
(Collias 1997). Bay-capped Wren-Spinetails have
a central role in these studies since they are the
only furnariid to construct a rudimentary nest
(Vaurie 1980). Narosky (1973) noted there were
strong discrepancies in the literature concerning
the nest structure of Wren-Spinetails. Some
authors have considered it an open structure,
while others have described it as an enclosed nest.
We observed that some nests had a rudimentary
roof, but others were built as an open cup with
few Juncus spp. stems barely covering the top. It
is likely that open cup nests were found later in
the breeding stage when the cover was lost as a
result of the adults coming in and out of the nest.
Wren-Spinetails may be the only furnariid to
build a simple nest, but several aspects of its
breeding biology are similar to other members of
the Furnariidae. The mating system of Wren-
Spinetails is social monogamy, similar to other
members of the family (Remsen 2003). Cooper-
ative breeding has been documented in the
Rufous-fronted Thornbird (Phacellodomus rufi-
frons) (Rodriguez and Carrara 2004), Lark-like
Brushrunners (Coryphistera alaudina) (Areta and
Bodrati 2007), and is suspected in the Firewood-
gatherer (Anumbius annumbi) (Rodriguez and Car-
rara 2004), Pink-legged Graveteiro (Acrobatornis
fonsecai) (Whitney et al. 1996), Caatinga and
Brown cacholotes (Pseudoseisura cristata and P.
lophotes) (Zimmer and Whittaker 2000), and
Orange-fronted Plushcrown (Metopothrix auran-
tiaca) (Remsen 2003). We did not observe more
than two birds attending a nest; thus, cooperative
breeding is unlikely in Wren-Spinetails in our
study area.
Both male and female Wren-Spinetails have
brood patches and share incubation, brooding, and
feeding of nestlings. Similar results have been
reported for Rufous Hornero (Furnarius rufus)
(Fraga 1980), Brown Cacholote (Nores and Nores
1994), and Thorn-tailed Rayadito (Aphrastura
spinicauda) (Moreno et al. 2007); it is likely this
pattern is typical of all furnariids (Remsen 2003).
No variation in parental roles and social mating
system has been reported for furnariids. Only
three other suboscine passerine families have a
similar broad neotropical distribution: tapaculos
(Rhinocryptidae), cotingas (Cotingidae), and ty-
rant flycatchers (Tyrannidae). Tapaculos exhibit
similar parental roles and are socially monoga-
mous (Krabbe and Schulenberg 2003). However,
social polygyny has been observed in flycatchers
and cotingas, but only the female develops a
brood patch, incubates, and broods the young
(Fitzpatrick 2004, Snow 2004). Male furnariids
and tapaculos appear to be constrained to
monogamy because parental care may be essential
during incubation.
Polygyny and polyandry are present in other
passerine species, such as Eurasian Penduline Tit
(Remiz pendulinus), where both males and fe-
males incubate eggs and brood nestlings (Valera
et al. 1997). An alternative view would be that in
species with similar parental roles either gender
can desert, and polyandry and polygyny could be
expected. Bennet and Owens (2002) proposed that
entire bird lineages are predisposed to a particular
mating system (as in the case of furnariids and
tapaculos) while in other groups, ecological
variables may facilitate differences within the
lineages.
The clutch size of Wren-Spinetails is small and
similar to other members of the family. Shiny
Cowbirds parasitize several sympatric marsh
nesting passerines (Massoni and Reboreda 1998,
SHORT COMMUNICATIONS 805
Ferna?ndez and Mermoz 2000), but we did not
observe cowbird eggs in nests of Wren-Spinetails.
Nest survival was also higher than other sympatric
passerines. The probability of a Wren-Spinetail
nest surviving the entire nesting cycle was
relatively high (0.51) when compared to two
sympatric marsh-nesting birds, Scarlet?headed
Blackbird (Amblyramphus holosericeus) and
Brown-and-yellow Marshbird (Pseudoleistes vir-
escens) with a probability of 0.25 and 0.11,
respectively (Ferna?ndez and Mermoz 2000) and a
cavity nester, House Wren (Troglodytes aedon),
breeding in adjacent woods with a probability of
0.25 (Llamb??as and Ferna?ndez 2007). Differences
in brood parasitism and predation rates of
sympatric passerines may be due to microhabitat
characteristics, nest location, or behavior. Future
research should focus on examining the environ-
mental and behavioral factors responsible for the
low predation rates observed.
ACKNOWLEDGMENTS
We thank D. J. Cerasale for helpful comments on the
manuscript, G. J. Ferna?ndez, M. E. Mermoz, and P. S.
Rodr??guez for help in the field, the Wisky-Michelli family
for allowing us to work on their ranches, and Mario Beade
for logistic support. We are grateful to two anonymous
referees, and C. E. Braun for stimulating comments and
suggestions on the manuscript. Financial support was
provided to P. E. Llamb??as and Valentina Ferretti by the
Department of Ecology and Evolutionary Biology at
Cornell University, OAS graduate fellowships, and the
Cornell Laboratory of Ornithology. Financial support
during field work was provided by a Mario Einaudi Center
for International Studies International Research Travel
Grant, Latin American Studies Program Travel Grant, and
an Andrew W. Mellon Student Research Grant, Cornell
University, Ithaca, New York.
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