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SHORT COMMUNICATIONS

in body weight and fat and the relation of fatty
acid composition to diet in the Willow Ptarmigan.
Wilson Bull. 80:426-44 1.
WEST,G. C., ANDL. J. PEYTON. 1980. Fatty acids ofdepot lipids in migrating Lapland Longspurs.J.
Field Omithol. 51:138-143.
YOM-Tov, Y., ANDA. TIETZ. 1978. The effectof diet,ambient temperature and day length on the fatty

The Condor96394-198
0 The Cooper Ornithologicalacid composition in the depot fat of the European
Starling(Sturnus vulgaris) and the Rock Partridge(.4kctorischucar). Comp. Biochem. Physiol. 60A:
161-164.
ZAR, J. H. 1977. Fatty acid composition of Emperor
Penguin (Aptenodytes forsterz] lipids. Comp. Biothem. Physiol. 56B:l09-112.

Society 1994

INTERSPECIFIC
MONTEZUMABROODOROPENDOLASPARASITISM

PARASITISM
BY GIANTOF
COWBIRDS:

OR MUTUALISM?’MICHAELS. WEBSTER~
Section of Neurobiology and Behavior, Cornell University, Seeley Mudd Hall, Ithaca, NY 14853

1979).This resultcontrastssharplywith studiesofmany
other brood parasite-host relationships, where hosts
actively defend their nestsagainstbrood parasites(e.g.,
Davies and Brooke 1988, Rothstein 1990), and paraObligate interspecificbrood parasitismhas evolved in- siteshave evolved many behavioral and developmendependently in several avian families (Hamilton and tal tacticsto circumvent the defensesof their hosts(e.g.,
Orians 1965, Payne 1977). Such parasitism generally Davies and Brooke 1988, Briskie and Sealy 1990, Braa
has a marked negative effect on host fitness(e.g., Post et al. 1992, Scott et al. 1992).
To determine the factors that allow the interdeand Wiley 1976, Davies and Brooke 1988, Power etal. 1989, Scott et al. 1992) and the brood parasiteand pendencedescribedby Smith (1968, 1979) to arise, it
host are usually viewed as being engagedin a coevo- is first necessaryto determine the distribution of thelutionary “arms race” (Davies and Brooke 1988, Roth- phenomenon. Giant Cowbirds parasitize several spestein 1990, Braa et al. 1992). However, Smith (1968, cies of oropendolasand caciques(Orians 1985) many1979) described a remarkable and complex interde- of which have a nesting biology similar to that of thehosts studied by Smith (see Chapman 1929, Skutch
pendence between the parasitic Giant Cowbird (Sca1954, Schafer 1957, Tashian 1957, Drury 1962, Fraga
phidura oryzivora) and two of its hostsin Panama, theChestnut-headedOropendola (Psarocolius waglerl] and 1989, Webster 1994). If several of these host populaYellow-rumped Cacique(Cacicus cela). In this system, tions benefit from and allow cowbirds to lay eggsin
the hostscould apparently benefit from cowbird “par- their nest,then ecologicalfactorscommon amongthem
must favor the evolution of this interdependence.Alasitism” in some situations.
Smith found that ectoparasiticbotflies(Philornis spp.) ternatively, giant cowbirds may be parasitesto mostwere a major source of mortality for oropendola and hosts,suchthat some feature of the populationsSmithcacique nestlings. Cowbird nestlings removed botfly studied makes the complex mutualism unique to Panlarvae from their nestmates,suchthat nestswith cow- ama. These possibilities can be distinguished by debirds fledgedmore host young than nestswithout cow- termining whether other hosts allow Giant Cowbirds
birds. Indeed, in colonies without any other defense to enter their colonies,or whether they actively defend
against botfly parasitism (i.e., colonies without hy- themselvesagainstcowbird parasitism. Only one pubmenoptera nests), oropendolas and caciquesallowed lished studyhasdirectly attempted to answerthis quescowbirds to enter their nestsand did not remove the tion (Robinson 1988).
In this note, I examine the possibility that a similar
eggs of cowbirds that were discovered (Smith 1968,
mutualism exists between Giant Cowbirds and one of
their primary hostsin Costa Rica, the Montezuma Oropendola (Psarocolius montezuma). To do so, I deterI Received 20 October 1993.Accepted7 March 1994. mined whether Montezuma Oropendolasactively de* Present address:Department of Ecologyand Evo- fend themselves against Giant Cowbirds and, if so,lution, University of Chicago, 1101 E. 57th Street,Chi- whether cowbirds show behaviors that might circumcago, IL 60637.
vent this defense. Becauseit was difficult to examine
Key words: Montezuma Oropendola; Psarocolius;
Giant Cowbird; Scaphidura; brood parasitism; mutualism; colonial nesting.

SHORT COMMUNICATIONS
the contentsof oropendola nestsregularly,the effect of
cowbirds on the host’s reproductive successwas notmeasured.
STUDY SPECIES AND METHODS
Montezuma Oropendolas are large (adult male mass>500 g) passerinesthat inhabit lowland rainforests
through Central America. Like other oropendolasandsome caciques, Montezuma Oropendolas nest colonially, with up to 100 or more females building theirlong; pensile nestsin a singletree (Skutch 1954;Fraga
1989. Webster 1991). The nesting colonies of Montezuma and Chestnut-headedOropendolasare similar
in appearance, and they sometimes nest together inadjacentcolony trees(pers.obs.). In addition to nestingin a single colony tree, female Montezuma Oropendolas cluster their nestswithin the colony tree as well:often a dozen or more nests are massed together on
single limb (Fraga 1989, Webster 1994). Unlike someoropendolas and caciques, Montezuma Oropendolas
do not nest in association with social hymenoptera(Fraaa 1989). and none of the nestsI observedduring
this study were built near hymenoptera nests. Chestnut-headed Oropendolas and Yellow-rumped Caciques do nest with hymenoptera, and this behavior
apparently protects the birds from botfly parasitism(Smith 1983) as well as from nest predators(Robinson
1985).
I studied a population of Montezuma Oropendolasat the Estaci6n Biologia La Selva, Costa Rica, during
eachbreedingseason(JanuarythroughMay) from 1986
to 1990, and collected data on cowbirds starting in
1987. Oropendolas were observed at a focal nestingcolony during eachseason.Whenever a Giant Cowbird
entered the vicinity of the observation colony, I and
my assistantsobserved it until it left the colony area.
We recorded the time of the visit, the nests that the
cowbird visited, any nests that it entered, and the reactions of any oropendolas in the colony. Cases inwhich two or more cowbirds visited the colony simultaneouslywere counted as a single visit.
The nesting activities of individual female oropendolasat the focal colony were closelymonitored during
1987, 1988 and 1989. However, the focal nestingcolony was abandonedearly in the 1989 breeding season(Webster 1994), leading to a reduced sample size inthat year. In 1987 and 1988, we counted the numberof females perched in the colony during scan samples
conductedevery 20 minutes. Although this figure underestimatesthe total number of femalespresentin thecolony (becausefemales inside of nests could not beseen or counted), it is likely to be correlated with thenumber of females present and gives a rough measureof the number of females able to defend against cowbirds.
RESULTSWe recorded a total of 88 visits by Giant Cowbirds tooropendolacolonies.Although most visits were by single cowbird females, pairs or small groups (up to fourindividuals)sometimesvisited coloniestogether(17.7%of all cowbird visits). Most cowbird visits were during
periodswhen female oropendolaswere lining their nests
with leaves(Fig. 1). Nest-lining is the final stageof nest

795

construction, and females presumably lay their eggs
during this period (Webster 1994). In 1987, the cowbird visitation rate (number of cowbird visits oer hourof observation) during each 10 day period was significantly correlated with the number of females lining
nests during that period (Fig. 1, r = 0.96, df = 8, P
-c 0.01). This correlation was marginally significantin
1988 (Fig. 1, r = 0.65, df = 7, P = 0.06) and elimination of a single outlier (days 6 l-70) makes it statistically significant (r = 0.86, df = 6, P < 0.01). The
correlation between cowbird visits and number of females lining nests was not statistically significant in1989 (Fig. 1, r = 0.76, df = 2, P > O.lO), although theabbreviated nesting period in that year was too shortfor solid conclusionsto be drawn.
Cowbirds were not toleratednear nestsat any colony.
Cowbirds that approached nests were attacked anddriven from the colony, and we never saw a cowbird
successfullyapproachor enter a nest when an oropendola was perched in the colony tree. Cowbirds werechasedfrom the colony more often by female oropendolas than by males (75 vs. 26 chases,respectively).
This last result may reflect a biased adult sex ratio at
nestingcolonies rather than a tendency for females tobe more aggressivetoward cowbirds,asthe mean number of males present in the colony during scans(0.87,
all yearscombined) wassomewhatlower than the mean
number of females visible outside of their nests(1.27)
and several other females were often present in thecolony but hidden inside of their nests.Cowbirds wereable to enter nests during only seven of 83 visits tocolonies(only visits in which we could determine success of the cowbird were counted, total of 17 nests
entered).
Giant Cowbirds were sometimes tolerated in a colony tree if they did not approach any nests (n = 7
cases).Furthermore, a female usually did not chasea
cowbird until it approached her own nest: of 35 occasionsin which an identified female chaseda cowbird,
28 (80%) were chasesby a female who had a nest in
the cluster where the cowbird was located when thechasebegan, and only 7 (20%) were by a female witha nest in a different part of the colony. Although not
rigorously quantified, females inside of nests usually
did not come out of their nest to chasea cowbird until
it was very near, possibly because females were unaware of the cowbird’s presenceor becausethe cowbird
was not a threat.
Relatively few female oropendolas were present in
the nesting colony during the late morning and early
afternoon hours (Fig. 2) when temperaturesat the relatively exposedcolony sites were high. The daily pattern of cowbird visitation was significantly different
from random (data from all yearscombined,x2 = 35.83,
df = 4, P < 0.001) and showed the opposite pattern:
few visits occurred in the early morning and late afternoon, and most visits occurredduringthe late moming hours when few females were present to defend
their nests(Fig. 2). Although sample sizeswere small,this pattern was also statisticallysignificantin 1987 (n= 25 cowbird visits, x2 = 13.91, df = 4, P < 0.01) and1989 (n = 32 cowbird visits, x2 = 16.46, df = 4, P <O.Ol), and of border-line significancein 1988 (n = 21
cowbird visits, x2 = 8.06, df = 4, 0.05 < P < 0.10).

796

SHORT COMMUNICATIONS
16

0.3

0.2

0.1

0.0

6

0.2

s
s
k
a
WI

0.1

l

5
.H
+
2
.I

0.0

s
s

8

l-10

II-20

21-30

31-40

41-50

51-60 61-70

71-80

81-90

Date
FIGURE 1. Number of females lining nestswith leaves (solid bars) and number of cowbird visits per hour ofobservation (hatched bars) versus date (shown in 10 day blocks) in 1987 (26 active nests, day 1 = Jan. 30),
1988 (12 active nests,day 1 = Jan. 12) and 1989 (22 active nests,day 1 = Jan. 15). In each year, day 1 is thefirst day of observationsand approximately equals the initiation of nest building in that season.Numbers inparenthesesare the total number of hours of observation during each time period. In 1989, the focal colony
was abandoned on day 42 (verticle arrow).
Too few cowbird visits were observed in 1990 (n = 7)
for a similar analysis. The pattern of cowbird visitsalso differed significantly from random if only themorning visitations are considered(x2 = 7.14, df = 2,

P < 0.05). Pooling data from all years and excludingtime blockswith fewer than 5 hr observation,the number of cowbird visits per hour was significantlynegatively correlated with the average number of females

SHORT COMMUNICATIONS
(198)

(269)

(159)

2.5

(161)

r

T

1 0.25791

Time of Day
FIGURE 2. Mean number of females perched in the colony during scan samples(solid bars; n = 576 scans[19871and 829 scans[ 19881)and number of cowbird visits to the colony per hour of observation(hatchedbars)
versustime of day. For number of females present,error bars show 1 standarderror. The X-axis gives the midpoint of each 2-hr time block, and numbers in parenthesesare the total number of hours of observationin eachtime block during which cowbird data were collected. The 12:00-14:OOtime block was excluded from analyses
due to a low number of observation hours (~5).
present in the colony during each time block (Spearman’s p = -0.683, n = 9 one-hour time blocks, P =0.05).
Cowbird visits late in the morning appeared to be
more successfulthan visits early in the morning: cowbirds entered nestson only 6.8% of 44 visits before 09:00, but were able to enter nestson 20.0% of 20 visits
between 09:OOand 12:OO.However, this last result is
not statisticallysignificant(x2 = 2.45, df= 1, P > 0.05)
and a larger sample size is needed before firm conclusions can be drawn regardingthe successof cowbirds
at different times of the day.

rare. There are at least two possiblereasonswhy sucha mutualism might have evolved with one host population but not another. First, the benefits to the hostmight be lower in CostaRica and Peru than in Panama.
This explanation seems unlikely for Montezuma Oropendolas, as botflies appear to frequently parasitize
Montezuma Oropendola nestlings;botfly larvae were
evident on four of nine young (featherless)nestlings
examined during this study (range = l-16 bots per
nestling). Furthermore, Montezuma Oropendolas do
not nest in associationwith socialhymenoptera(Fraga
1989), and therefore have no apparentdefensesagainst
botfly parasitism (see Smith 1968, 1979, 1983). AlterDISCUSSION
natively, the costsof interspecificbrood parasitism to
Unlike some populationsof Chestnut-headedOropen- the host might be high for Montezuma Oropendolasin Costa Rica. Although it requiresfurther testing, thisdolas and Yellow-rumped Caciques, Montezuma Oropendolas in Costa Rica did not tolerate Giant Cow- explanation seems more likely: Montezuma Oropendolas have low nesting success(less than one-third ofbirds near their nests. Similarly, Robinson (1988)
working in Peru, found that Yellow-rumped Caciques all nests built fledge young) and, although the clutch
and Russet-backedOropendolas (Psarocoliusangus- size is two eggs,femalesrarely fledgemore than a single
tifrons) actively defended their nests against Giant young (Webster 1994). Therefore, it seems unlikely
Cowbirds. Giant Cowbirds have apparently evolved that a Montezuma Oropendola female could successstrategiesto circumvent nest defenseby their hosts;in fully fledge her own young if she also had to raise athis study,cowbirdsvisited coloniesduringthosetimes cowbird nestling (see also Robinson 1988).
Montezuma Oropendolas appear to have few deof the day when hostswere most likely to be absent, astrategythat has been reported for other brood para- fensesagainstcowbirdsother than aggressionand, possites (Davies and Brooke 1988). Cowbirds also raid sibly, removal of cowbird eggs.However, the clusteredcolonies in groups (Robinson 1988, this study), and dispersionofnests within Montezuma Oropendolacolappear to lay mimetic eggs (Smith 1968, but see onies (Fraga 1989. Webster 1994) miaht function. in
Fleischer and Smith 1992).
part, to protect nests from cowbird brood parasitism.
These results, combined with those of Robinson Nests in clustersshould be better guarded from cow(1988), suggestthat many populations of oropendolas bird attack, becausecowbirds are more likely to passand caciquesactively defend against cowbird parasit- near occupied nests (and be chased out) when apism, and that the complex host-parasite mutualism proachingnestsin clustersthan when approachingisoreported by Smith (1968, 1979) is likely to be quite lated nests.The data reported here give some support

198

SHORT COMMUNICATIONS

to this hypothesis;a female oropendola was unlikely ORIANS,G. H. 1985. Blackbirds of the Americas.
Univ. Washington Press,Seattle.
to chasea cowbird unlessit approachedher own nest,
and cowbirds sometimes were not chased from colo- PAYNE, R. B. 1977. The ecologyof brood parasitism
in birds. Ann. Rev. Ecol. Syst. 8: l-28.
nies until they approached a nesting cluster. Other
studieshave also suggestedthat aggregatednestingand POST,W., ANDJ. W. WILEY. 1976. The Yellow-shouldered Blackbird-present and future. Amer. Birds
reproductive synchronycould reducethe costsof cow30:13-20.
bird parasitism(Clark and Robertson 1979, Robertson
and Norman 1977, Wiley and Wiley 1980). If so, then POWER,H. W., E. D. KENNEDY,L. C. ROMAGNANO,
M. P. LOMBARW,A. S. HOFFEN~ERO,
P. C. STOUFvariation in the degreeof nest clusteringamong species
FER,ANDT. R. McGumn. 1989. The parasitism
of oropendola and cacique might reflect variation ininsurancehypothesis:why starlingsleave spaceforthe degreeof cowbird parasitism.
parasitic eggs.Condor 91:753-765.
I thank S. T. Emlen, P. W. Sherman and L. K. Sterk ROBERTSON,
R. J., AND R. F. NORMAN. 1977. The
for advice and commentsduringthe study,and I. Ayub,
function and evolution ofaggressivehost behavior
J. Guild, K. Merg, E. Phillips and L. Sterk for invalutowards the Brown-headed Cowbird (Molothrus
able assistancein the field. The manuscript was imater). Can. J. Zool. 55:508-518.
proved by the comments of S. T. Emlen, H. Landel, ROBINSON,S. K. 1985. Coloniality in the YellowJ. Picman, S. G. Pruett-Jones, P. W. Sherman, L. G.
rumped Cacique as a defense against nest predaSorenson, and an anonymous referee. Financial andtors. Auk 102:506-5 19.
logistic supportwere provided by the Organization for ROBINSON,
S. K. 1988. Ecologyand host relationships
Tropical Studies and JessieSmith Noyes Foundation,of Giant Cowbirds in southeasternPeru. Wilson
and by the American Museum ofNatural History (Frank
Bull. 100:224-235.
M. Chapman Fund), Cornell University, National Sci- ROTHSTEIN,S. I. 1990. A model system for coevoence Foundation (NSF Graduate Fellowship), Orgalution: avian brood parasitism. Ann. Rev. Ecol.
nization for American States,and Sigma Xi (Grant-inSyst. 2 I:48 l-508.
Aid-of-Research). Finally, I thank the La Selva staff SCHAFER,E. 1957. Les conotos: etude comparative
and the many Costa Rican residentswho allowed mede Psarocolius angustijrons et Psarocolius decuto work on their land.
manus. Bonner ZoologischeBietrage 1957:l-l 5 1.
Scorr, D. M., P. J. WEATHERHEAD,
ANDC. D. ANKNEY.
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