First Record of Cooperative Breeding in a Thryothorus Wren

2004·3.7 MB·English

by  Sharon A Gill

Wilson Bulletin 116(4):337—341, 2004
FIRST RECORD OF COOPERATIVE BREEDING IN A
THRYOTHORUS WREN
SHARON A. GILL’ 2
—
ABSTRACT. Although offspring delay dispersal past the age ofindependence in many Thryothorusspecies,
cooperative breeding has not been recorded in this genus. Here, I present the first observation of cooperative
breeding in a Thryothorus wren (Buff-breasted Wren, T. leucotis). Of 41 offspring that delayed dispersal past
the age of independence, 4 individuals stayed on their natal territory until their parent’s next breeding attempt,
indicating a low potential for cooperative breeding. Ofthese four individuals, one male provisioned 1 1- to 15-
day-old nestlings and one female approached her parents’ nest but was repeatedly driven away from it by her
father. The retained female was apparently tolerated when in the vicinity of fledglings, but feeding was never
positively confirmed. Received 10 May 2004, accepted 15 October 2004.
In cooperatively breeding birds, more than spring is often associated with high adult sur-
two adults participate in a single breeding vival, low reproductive rates, and deferred
event, either by defending the nest, incubating maturation (Brown 1987), and may be a form
eggs, or provisioning young (Brown 1987). ofextended parental investment, in which par-
This is the predominant social system in ents permit or encourage offspring (via aecess
18.5% of oscine passerine species with bipa- to resources) to remain on natal territories
rental care (383 of 2,067 species), and occurs (Ekman et al. 2001).
occasionally in another 3.7% of oscines elas- Wrens (Family Troglodytidae) have been
sified as pair breeders (Cockburn 2003). How- among the most studied New World avian
ever, patterns ofnestling care are unknown for taxa in terms of cooperative breeding, with
most passerines {n = 2,385), particularly much of this research focusing on the genus
those in the Neotropics (Cockburn 2003); Campylorhynchus, in which 9 of 13 species
thus, cooperative breeding may occur in still are known to breed cooperatively (e.g., Aus-
other species. Intensive studies of populations tad and Rabenold 1986, Rabenold 1990). Co-
of banded birds are needed to determine the operative breeding occurs regularly in Black-
extent of pair versus cooperative breeding in capped Donacobius (Donacohius atricapillus)
these species. and has been reported in House Wrens {Trog-
Cooperative breeding may arise in several lodytes aedon\ reviewed in Cockburn 2003).
different ways, including when (1) indepen- However, the breeding systems of most spe-
dent offspring remain on natal territories until cies in this family, especially those in tropical
their parents’ next breeding period (Cockburn areas, have not been described and coopera-
1998), (2) individuals initially disperse and tive breeding may be more common than re-
then preferentially immigrate into social ported. For example, many species in other
groups composed of related individuals (Bag- genera, in particular Thyrothorus, are often
lione et al. 2003), and (3) groups of unrelated found in family groups (e.g., Skutch 1960,
individuals reproduce together (Davies 1992). 2001; Skid 1964; Hilly and Brown 1986;
In the first case, delayed dispersal by offspring Brewer 2001), suggesting that independent
is a necessary prerequisite for cooperation. offspring may stay on natal territories for ex-
However, in some species in which offspring tended periods, potentially setting the stage
remain on their natal territories for extended for cooperative breeding. Here, describe the
I
periods, helping never occurs (reviewed in lirst record of cooperative breeding in the ge-
Ekman et al. 2001). Delayed dispersal by olT- nus Thryothorus made during a study of pat-
terns of nestling provisioning in Buff-breasted
' Dept, of Biology, York Univ., Toronto. ON M3.I Wviroeunssst(u/d.ielseuhcaotvies:reSpoArGteduntphautblB.ufdfa-tbar).easPtreed-
P3, Canada.
1 ‘Current address: Dept, of fxology and Hvolution- Wrens are often found in groups of 3-4 in-
ary Biology, Guyot Hall. Princeton Univ., F’rinceton. dividuals (Skutch 1968, P'arabaugh 1983,
NJ 08544. USA; e-mail; sagillCnfiirinceton.edu Ahumada 2001), but none has recorded dis-
337
)
338 THE WILSON BULLETIN • Vol. 116, No. 4, December 2004
persal delayed into the breeding season or co- gave their first song of the morning (these are
operative breeding. typically given when birds are close to their
METHODS nests), during regular searches of territories,
and by following individuals that were col-
Buff-breasted Wrens are small (16-23 g), lecting or holding nesting materials. I did not
monochromatic birds that inhabit secondary mark nest locations, as they were easy to re-
forest throughout their range from central find. Nests were checked every 2 days until
Panama to northern South America (Ridgely clutch completion (modal clutch size = 3,
and Tudor 1989). They maintain territories range = 2-3, n = 42) and inegularly there-
and partnerships throughout the year, and after until the young fledged or the nest failed.
breed during the wet season (April to October; As part of a broader study examining male
SAG unpubl. data); initiation of breeding is and female effort during nest construction and
closely tied to the onset of rains (Ahumada nestling provisioning, I made 1-hr observa-
2001). Buff-breasted Wrens are socially and tions of nestling provisioning every 2 days
genetically monogamous: extra-pair young from the day after hatching (day 0) until the
were detected in only 1 of 31 broods (Gill et nestlings fledged (usually day 15). All obser-
al. 2005). Throughout the year, pairs produce vations were performed between 06:00 and
antiphonal duets in which males and females 14:00 EST, from a position at least 5 m from
give alternating sex-specific songs (Farabaugh nests, to avoid influencing the behavior of the
1983). parents as they returned to feed or brood (fe-
I studied a color-banded population ofBuff- males only). During these observations, I
breasted Wrens in Gamboa, Republic of Pan- quantified the number of visits per hour by
ama (9° 1' N, 79°42' W) during both dry and male and female parents, as well as visits by
wet seasons (February-May 1997, February- the retained offspring. In addition, I noted the
July 1998, October 1998, February-June singing behavior of offspring, as well as ag-
1999). The study site was a 22-ha secondary gressive interactions between them and their
forest bordered on three sides by Gamboa, the parents.
Rio Chagres, and the Panama Canal, and it
was separated from the nearest forest by a RESULTS
100-m grassy marsh. In 1997, both individuals
in 15 of 18 pairs were banded, whereas in Of 57 offspring banded as nestlings, fledg-
1998 (/? = 24) and 1999 {n = 19), both mem- lings, or independent birds, 35 remained on
bers of all pairs were banded. I banded a total their natal territories for >3 months after
of65 adults, 17 adult philopatric offspring, 27 fledging. Another six unbanded individuals
fledglings, and 13 nestlings. I sexed individ- lived on territories with banded adults. I as-
uals based on body size (females are smaller sumed that these unbanded individuals were
than males; SAG unpubl. data), singing be- the offspring of the banded adults because (1
havior (Farabaugh 1983), and egg laying and their presence was consistent with the Hedging
incubation (Gill 2003). When more than two ofoffspring in those territories in the previous
adult birds resided on a given territory, I dis- breeding season; (2) 17 individuals, banded as
tinguished paired adult birds from their adult adults and residing on territories with 2 other
philopatric offspring by behavior, as paired in- adult birds, were the genetic offspring ofthose
dividuals duetted more frequently, and spent adults (SAG unpubl. data); and (3) I have no
more time in close association (<5 m apart) evidence that groups formed other than by the
than with their offspring. I defined a philo- retention of offspring. Thus, a total of41 off-
patric offspring as one that remained on its spring (65.1% of offspring fledged) remained
natal territory for >3 months. on their natal territories for >3 months. Prior
Buff-breasted Wrens—typically construct to their parents’ next breeding attempt, most
separate d—ormitory nests in which they roost philopatric offspring either dispersed to be-
overnight and breeding nests, although ap- come teiTitory holders in the study population
proximately 10% of dormitory nests are used (// = 13) or disappeared (n = 24). Only four
for breeding (SAG unpubl. data). I located individuals (9.8% of philopatric offspring)
nests by searching areas from which pairs postponed dispersal for >1 year, staying on
.
Gill • HELPING IN THRYOTHORUS LEUCOTIS 339
30 min of this interaction, while the female
visited the nest four times within that period.
Near the end of the 1-hr observation period,
the unbanded male, with food in his bill, tried
to approach the nest once more. The parental
male was in the nest, exited it and then chased
the unbanded male away from the nest. Soon
after, the parental male and the unbanded male
were observed foraging within 2 m of each
other.
On day 13, the unbanded male visited the
nest six times within an hour, equaling the
contribution ofthe parents combined. His first
FIG. 1. Pattern ofprovisioning by a single pair of visit was apparently undetected by the parents
Buff-breasted Wrens and a philopatric, unbanded male
when feeding three nestlings. Eggs hatched on day 0 as they were not in the immediate nest area at
and were observed every second day from days 1 to the time. While approaching the nest for his
15, at which time the young fledged. The philopatric second visit, the unbanded male was supplant-
male was not observed within 5 m of the nest area ed by the male parent, who gave an aggressive
prior to day 1 1 churr, but the unbanded male was not chased
out of the area. Rather, he continued to move
around the nest, with the male parent follow-
their natal territories through their parents’ ing 1-2 m behind. The unbanded male then
subsequent breeding season. entered the nest and fed the young. On the
Between 6 and 10 June 1999, I observed next four visits, the unbanded male was fol-
helping at the nest by an unbanded male dur- lowed several times by the male parent, but
ing the second breeding attempt of a banded no aggression was exhibited. On the day of
pair that had been together for >2 years. This fledgling (day 15), the unbanded male made
pair had fledged two males in the previous four visits to the nest, more than the individual
breeding season, one of which I captured and effort of either parent (Fig. 1). No aggression
banded. Both male offspring were present on between the unbanded male and the parental
the territory at the start of my field season in male was observed at this time, and no ag-
February 1999, with the banded male dispers- gression between the unhanded male and the
ing to another territory before breeding com- female was ever recorded. Three young
menced. The unhanded male was last ob- fledged from the nest on 10 June. 1 made three
served on his natal territory on 30 June, a date additional untimed observations of this pair
that corresponded to the end of my held sea- after the young fledged, during which the un-
son. handed male perched within 2 m of the fledg-
Visitations by the parents and the unhanded lings and their parents, and once fed the fledg-
male while provisioning a brood ofthree nest- lings.
lings are summarized in Figure 1. Between During observations at a second nest in
days 1 and 9 after hatch, the unhanded male June 1999. I witnessed aggressive encounters
was not observed within 10 m of the nest, but between a banded female offspring (hatch
often sang within the territory. On day near year 1998) and her banded father around the
1 1
the start of ob.servations, the unhanded male breeding nest, but never ob.scrvcd the female
entered the nest and stayed in it for approxi- enter the nest to feed the young. On day 5,
mately 10 .sec. vShortly after the unhanded the philopatric female approached within 5 m
male left the nest there was a chase involving of the nest and was chased by the male 3-4
two unidentified wrens, and “aggressive" times. She moved to 2 m from the nest and
churring (such calls are heard only during ag- was chased again by the male parent, who
gressivc encounters; SACI pers. obs.). Less gave several aggressive churrs. When the
;
! than 30 sec later, a third unidentified wren ap- nestlings were 13 days old. the philopatrie fe-
proached this pair, and another chase ensued. male perched 4-5 m from the nest and was
I The parental male did not visit the nest within immediately chased by the male, who utteretl
340 THE WILSON BULLETIN • Vol. 116, No. 4, December 2004
aggressive churrs (the philopatric female did care to prevent non-breeders from accessing
not approach the nest during observations nests, possibly to decrease activity around the
made before day 5, between days 7 and 11, nest and thereby minimize the risk ofnest pre-
or on day 15). Two nestlings fledged on 6 dation (Strickland and Waite 2001). Consid-
June, and I observed the philopatric female ering pair-breeding species in the Neotropics,
m
within 3 of them on the day of fledging as Skutch (1949) proposed that nest predation in-
well as during observations 2 weeks later. At creases with the rate at which parents feed the
both times, the family was in thick vegetation nestlings (see Martin et al. 2000). Thus, male
and I made few definitive observations of parents may have behaved aggressively to-
feeding by any adult. No aggression was not- ward their offspring to stop them from visiting
ed between the male and the philopatric fe- nests. Interestingly, in the one case of coop-
male after the young fledged, nor was aggres- erative breeding I observed, parents appeared
sion observed at any time between the paren- to visit their nest less once their male off-
tal and philopatric females. spring started to provision the young (Fig. 1).
Two other banded offspring were observed Alternatively, parental aggression toward phil-
on their natal territories during subsequent opatric offspring may occur when resources
breeding attempts oftheir banded parents, but become depleted and competition for the re-
helping was not observed. I observed a band- maining resources intensifies (Strickland and
ed, second-year female offspring on her natal Waite 2001). This hypothesis seems unlikely
territory 1 day before her parents began con- to apply to Buff-breasted Wrens, as breeding
structing their second breeding nest, but did occurs during the wet season, when arthropod
not see her during 4 hr ofobservation ofnest- prey are abundant (e.g., Wolda 1996). Prey
ling provisioning. However, this female ap- depletion seems more likely to occur during
parently stayed on her natal territory while her
parents bred, as she was observed with them the dry season, yet 1 witnessed little or no pa-
rental aggression toward philopatric offspring
in the post-breeding season. Finally, a banded
at that time.
male was observed repeatedly foraging and
Most offspring that delayed dispersal did
singing with his banded parents over the
not stay on their natal territories long enough
course of their four breeding attempts. The
to help with reproductive activities of their
philopatric male built a dormitory nest with parents. On average, offspring dispersed just
his father (while the female incubated eggs),
prior to the onset of their parents’ reproduc-
but did not participate in the construction of
tive period (SAG unpubl. data). Prior to dis-
two breeding nests during timed observations.
persing, however, some offspring participated
Although I did not observe this pair during
nestling provisioning, the male offspring was in the construction of dormitory nests, and
most sang either as part of territory defense
still present on the territory at that time.
or to advertise themselves to potential mates.
DISCUSSION Further studies are needed to determine the
The potential for observing cooperative role of offspring in these activities, as well as
breeding in Buff-breasted Wrens was low, as the benefits and costs of philopatry for both
only 4 of41 independent offspring postponed parents and their offspring. The phenomenon
dispersal for >1 year. All four offspring that of delayed dispersal in the absence of coop-
delayed dispersal may have participated in ter- erative breeding is underappreciated, yet its
ritorial defense by singing on their own or study is a logical step toward understanding
with their parents. One male helped by feed- the evolution of cooperation (Brown 1987).
ing young, and one female appeared to try to The monophyletic wren family (Barker 2004)
provision nestlings and may have successfully offers an exceptional opportunity to study the
fed fledglings. The latter cases were marked evolution ofdelayed dispersal and cooperative
by aggression by male, but not female, parents breeding due to the diversity of breeding and
toward their adult offspring during the nest- dispersal patterns, ranging from pair breeders
ling period. Similar aggressive interactions with early dispersal byjuveniles, to those with
between parents and non-breeders occur in delayed juvenile dispersal without coopera-
some small-bodied corvids during nestling tion, to truly cooperative breeders.
Gill • HELPING IN THRYOTHORUSLEUCOTIS 341
ACKNOWLEDGMENTS 2001. Delayed dispersal: living under the reign of
nepotistic parents. Auk 118:1-10.
1 thank the Autoridad Ambiente del Nacional for Earabaugh, S. M. 1983. A comparative study ofduet
permission to work in Panama and the Smithsonian song in tropical Thyrothorus wrens. Ph.D. disser-
Tropical Research Institute for logistical support. M. tation, University of Maryland, College Park.
Griesser and two anonymous referees made construc- Gill, S. A. 2003. Timing and duration of egg-laying
tive comments on anearlierversionofthis manuscript. in duetting Buff-breasted Wrens. Journal ofEield
This study was supported by grants and scholarships Ornithology 74:31-36.
from the American Ornithologists’ Union, American Gill, S. A., M. J. Vonhoe, B. J. M. Stutchbury, E.
Museum ofNatural History, Animal BehaviorSociety, S. Morton, and J. S. Quinn. 2005. No evidence
Ontario Graduate Scholarships, Sigma Xi, and York for acoustic mate-guarding in duetting Buff-
University to SAG; Natural Science and Engineering breastedWrens (Thryothorusleiicotis). Behavioral
Research Council of Canada to B. J. M. Stutchbury, Ecology and Sociobiology. In press.
and Smithsonian Institution to E. S. Morton and B. J. Hilty, S. L. and W. L. Browtm. 1986. A guide to the
M. Stutchbury. birds of Colombia. Princeton University Press,
Princeton, New Jersey.
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