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Home , Giant cowbird, Icterid, Montezuma oropendola, Oropendola

794 SHORT COMMUNICATIONS

in body weight and fat and the relation of fatty acid composition in the depot fat of the European acid composition to diet in the Willow Ptarmigan. Starling (Sturnus vulgaris) and the Rock Partridge Wilson Bull. 80:426-44 1. (.4kctorischucar). Comp. Biochem. Physiol. 60A: WEST,G. C., ANDL. J. PEYTON. 1980. Fatty acids of 161-164. depot lipids in migrating Lapland Longspurs.J. ZAR, J. H. 1977. Fatty acid composition of Emperor Field Omithol. 51:138-143. Penguin (Aptenodytes forsterz] lipids. Comp. Bio- YOM-Tov, Y., ANDA. TIETZ. 1978. The effect of diet, them. Physiol. 56B:l09-112. ambient temperature and day length on the fatty

The Condor96394-198 0 The Cooper Ornithological Society 1994

INTERSPECIFIC BROOD PARASITISM OF MONTEZUMA BY GIANT : PARASITISM OR MUTUALISM? ’

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

Key words: Montezuma ; Psarocolius; 1979).This resultcontrasts sharply with studiesofmany Giant ; Scaphidura; brood parasitism; mutu- other -host relationships, where hosts alism; colonial nesting. actively defend their nestsagainst brood parasites(e.g., Davies and Brooke 1988, Rothstein 1990), and para- Obligate interspecificbrood parasitism has evolved in- sites have evolved many behavioral and developmen- dependently 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). and Wiley 1976, Davies and Brooke 1988, Power et To determine the factors that allow the interde- al. 1989, Scott et al. 1992) and the brood parasiteand pendence described by Smith (1968, 1979) to arise, it host are usually viewed as being engagedin a coevo- is first necessaryto determine the distribution of the lutionary “arms race” (Davies and Brooke 1988, Roth- phenomenon. Giant Cowbirds parasitize several spe- stein 1990, Braa et al. 1992). However, Smith (1968, cies of oropendolasand caciques(Orians 1985) many 1979) described a remarkable and complex interde- of which have a nesting biology similar to that of the pendence between the parasitic (Sca- hosts studied by Smith (see Chapman 1929, Skutch phidura oryzivora) and two of its hosts in , the 1954, Schafer 1957, Tashian 1957, Drury 1962, Fraga Chestnut-headedOropendola (Psarocolius waglerl] and 1989, Webster 1994). If several of these host popula- Yellow-rumped (Cacicus cela). In this system, tions benefit from and allow cowbirds to lay eggsin the hosts could apparently benefit from cowbird “par- their nest,then ecologicalfactors common amongthem asitism” in some situations. must favor the evolution of this interdependence.Al- Smith found that ectoparasiticbotflies (Philornis spp.) ternatively, giant cowbirds may be parasites to most were a major source of mortality for oropendola and hosts, suchthat some feature of the populationsSmith cacique nestlings. Cowbird nestlings removed studied makes the complex mutualism unique to Pan- larvae from their nestmates,such that nestswith cow- ama. These possibilities can be distinguished by de- 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- themselvesagainst cowbird parasitism. Only one pub- menoptera nests), oropendolas and caciques allowed lished studyhas directly attempted to answerthis ques- cowbirds to enter their nests and did not remove the tion (Robinson 1988). eggs of cowbirds that were discovered (Smith 1968, In this note, I examine the possibility that a similar mutualism exists between Giant Cowbirds and one of their primary hostsin , the Montezuma Or- opendola (Psarocolius montezuma). To do so, I deter- I Received 20 October 1993.Accepted 7 March 1994. mined whether Montezuma Oropendolas actively de- * Present address:Department of Ecology and Evo- fend themselves against Giant Cowbirds and, if so, lution, University of Chicago, 1101 E. 57th Street,Chi- whether cowbirds show behaviors that might circum- cago, IL 60637. vent this defense. Becauseit was difficult to examine SHORT COMMUNICATIONS 795 the contentsof oropendola nestsregularly, the effect of construction, and females presumably lay their eggs cowbirds on the host’s reproductive successwas not during this period (Webster 1994). In 1987, the cow- measured. visitation rate (number of cowbird visits oer hour of observation) during each 10 day period was signif- STUDY AND METHODS icantly correlated with the number of females lining Montezuma Oropendolas are large (adult male mass nests during that period (Fig. 1, r = 0.96, df = 8, P >500 g) passerinesthat inhabit lowland rainforests -c 0.0 1). This correlation was marginally significantin through Central America. Like other oropendolasand 1988 (Fig. 1, r = 0.65, df = 7, P = 0.06) and elimi- some caciques, Montezuma Oropendolas nest colo- nation of a single outlier (days 6 l-70) makes it statis- nially, with up to 100 or more females building their tically significant (r = 0.86, df = 6, P < 0.01). The long; pensile nestsin a single tree (Skutch 1954;Fraga correlation between cowbird visits and number of fe- 1989. Webster 1991). The nesting colonies of Mon- males lining nests was not statistically significant in tezuma and Chestnut-headedOropendolas are similar 1989 (Fig. 1, r = 0.76, df = 2, P > O.lO), although the in appearance, and they sometimes nest together in abbreviated nesting period in that year was too short adjacentcolony trees(pers. obs.). In addition to nesting for solid conclusionsto be drawn. in a single colony tree, female Montezuma Oropen- Cowbirds were not tolerated near nestsat any colony. dolas cluster their nests within the colony tree as well: Cowbirds that approached nests were attacked and often a dozen or more nests are massed together on driven from the colony, and we never saw a cowbird single limb (Fraga 1989, Webster 1994). Unlike some successfullyapproach or enter a nest when an oropen- oropendolas and caciques, Montezuma Oropendolas dola was perched in the colony tree. Cowbirds were do not nest in association with social hymenoptera chasedfrom the colony more often by female oropen- (Fraaa 1989). and none of the nests I observed during dolas than by males (75 vs. 26 chases,respectively). this study were built near hymenoptera nests. Chest- This last result may reflect a biased adult sex ratio at nut-headed Oropendolas and Yellow-rumped Ca- nesting colonies rather than a tendency for females to ciques do nest with hymenoptera, and this behavior be more aggressivetoward cowbirds,as the mean num- apparently protects the birds from botfly parasitism ber of males present in the colony during scans(0.87, (Smith 1983) as well as from nest predators(Robinson all yearscombined) was somewhatlower than the mean 1985). number of females visible outside of their nests (1.27) I studied a population of Montezuma Oropendolas and several other females were often present in the at the Estaci6n Biologia La Selva, Costa Rica, during colony but hidden inside of their nests.Cowbirds were eachbreeding season (January through May) from 1986 able to enter nests during only seven of 83 visits to to 1990, and collected data on cowbirds starting in colonies (only visits in which we could determine suc- 1987. Oropendolas were observed at a focal nesting cess of the cowbird were counted, total of 17 nests colony during each season.Whenever a Giant Cowbird entered). entered the vicinity of the observation colony, I and Giant Cowbirds were sometimes tolerated in a col- my assistantsobserved it until it left the colony area. ony tree if they did not approach any nests (n = 7 We recorded the time of the visit, the nests that the cases).Furthermore, a female usually did not chase a cowbird visited, any nests that it entered, and the re- cowbird until it approached her own nest: of 35 oc- actions of any oropendolas in the colony. Cases in casionsin which an identified female chaseda cowbird, which two or more cowbirds visited the colony si- 28 (80%) were chasesby a female who had a nest in multaneously were counted as a single visit. the cluster where the cowbird was located when the The nesting activities of individual female oropen- chase began, and only 7 (20%) were by a female with dolas at the focal colony were closelymonitored during a nest in a different part of the colony. Although not 1987, 1988 and 1989. However, the focal nesting col- rigorously quantified, females inside of nests usually ony was abandoned early in the 1989 breeding season did not come out of their nest to chasea cowbird until (Webster 1994), leading to a reduced sample size in it was very near, possibly because females were un- that year. In 1987 and 1988, we counted the number aware of the cowbird’s presenceor becausethe cowbird of females perched in the colony during scan samples was not a threat. conducted every 20 minutes. Although this figure un- Relatively few female oropendolas were present in derestimatesthe total number of females presentin the the nesting colony during the late morning and early colony (becausefemales inside of nests could not be afternoon hours (Fig. 2) when temperaturesat the rel- seen or counted), it is likely to be correlated with the atively exposed colony sites were high. The daily pat- number of females present and gives a rough measure tern of cowbird visitation was significantly different of the number of females able to defend against cow- from random (data from all yearscombined, x2 = 35.83, birds. df = 4, P < 0.001) and showed the opposite pattern: few visits occurred in the early morning and late af- RESULTS ternoon, and most visits occurredduring the late mom- We recorded a total of 88 visits by Giant Cowbirds to ing hours when few females were present to defend oropendolacolonies. Although most visits were by sin- their nests (Fig. 2). Although sample sizes were small, gle cowbird females, pairs or small groups (up to four this pattern was also statisticallysignificant in 1987 (n individuals)sometimes visited coloniestogether (17.7% = 25 cowbird visits, x2 = 13.91, df = 4, P < 0.01) and of all cowbird visits). Most cowbird visits were during 1989 (n = 32 cowbird visits, x2 = 16.46, df = 4, P < periodswhen female oropendolaswere lining their nests O.Ol), and of border-line significancein 1988 (n = 21 with leaves(Fig. 1). Nest-lining is the final stageof nest cowbird visits, x2 = 8.06, df = 4, 0.05 < P < 0.10). 796 SHORT COMMUNICATIONS

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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 nests with leaves (solid bars) and number of cowbird visits per hour of observation (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 the first day of observations and approximately equals the initiation of nest building in that season.Numbers in parenthesesare 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) P < 0.05). Pooling data from all years and excluding for a similar analysis. The pattern of cowbird visits time blockswith fewer than 5 hr observation,the num- also differed significantly from random if only the ber of cowbird visits per hour was significantly nega- morning visitations are considered(x2 = 7.14, df = 2, tively correlated with the average number of females SHORT COMMUNICATIONS 791

(198) (269) (159) (161) r 2.5 T 1 0.25

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 (hatched bars) versustime of day. For number of females present, error bars show 1 standarderror. The X-axis gives the mid- point of each 2-hr time block, and numbers in parenthesesare the total number of hours of observation in each time 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 (Spear- rare. There are at least two possiblereasons why such man’s p = -0.683, n = 9 one-hour time blocks, P = a mutualism might have evolved with one host pop- 0.05). ulation but not another. First, the benefits to the host Cowbird visits late in the morning appeared to be might be lower in Costa Rica and Peru than in Panama. more successfulthan visits early in the morning: cow- This explanation seems unlikely for Montezuma Or- birds entered nestson only 6.8% of 44 visits before 09: opendolas, as appear to frequently parasitize 00, but were able to enter nests on 20.0% of 20 visits nestlings;botfly larvae were between 09:OOand 12:OO.However, this last result is evident on four of nine young (featherless)nestlings not statisticallysignificant (x2 = 2.45, df= 1, P > 0.05) examined during this study (range = l-16 bots per and a larger sample size is needed before firm conclu- nestling). Furthermore, Montezuma Oropendolas do sions can be drawn regardingthe successof cowbirds not nest in associationwith social hymenoptera (Fraga at different times of the day. 1989), and therefore have no apparentdefenses against botfly parasitism (see Smith 1968, 1979, 1983). Alter- DISCUSSION natively, the costs of interspecific brood parasitism to Unlike some populationsof Chestnut-headedOropen- the host might be high for Montezuma Oropendolas dolas and Yellow-rumped Caciques, Montezuma Or- in Costa Rica. Although it requiresfurther testing, this opendolas in Costa Rica did not tolerate Giant Cow- explanation seems more likely: Montezuma Oropen- birds near their nests. Similarly, Robinson (1988) dolas have low nesting success(less than one-third of working in Peru, found that Yellow-rumped Caciques all nests built fledge young) and, although the clutch and Russet-backed Oropendolas (Psarocoliusangus- size is two eggs,females rarely 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 success- strategiesto circumvent nest defenseby their hosts;in fully fledge her own young if she also had to raise a this study, cowbirdsvisited coloniesduring thosetimes cowbird nestling (see also Robinson 1988). of the day when hosts were most likely to be absent, a Montezuma Oropendolas appear to have few de- strategy that has been reported for other brood para- fensesagainst cowbirds other than aggressionand, pos- sites (Davies and Brooke 1988). Cowbirds also raid sibly, removal of cowbird eggs.However, the clustered colonies in groups (Robinson 1988, this study), and dispersionofnests within Montezuma Oropendola col- appear 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 pass and caciquesactively defend against cowbird parasit- near occupied nests (and be chased out) when ap- ism, and that the complex host-parasite mutualism proachingnests in clustersthan when approachingiso- reported 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. to chasea cowbird unlessit approachedher own nest, Univ. Washington Press, Seattle. and cowbirds sometimes were not chased from colo- PAYNE, R. B. 1977. The ecologyof brood parasitism nies until they approached a nesting cluster. Other in birds. Ann. Rev. Ecol. Syst. 8: l-28. studieshave also suggestedthat aggregatednesting and POST,W., ANDJ. W. WILEY. 1976. The Yellow-shoul- reproductive synchronycould reduce the costsof cow- dered Blackbird-present and future. Amer. Birds bird parasitism (Clark and Robertson 1979, Robertson 30:13-20. and Norman 1977, Wiley and Wiley 1980). If so, then POWER,H. W., E. D. KENNEDY,L. C. ROMAGNANO, variation in the degreeof nest clusteringamong species M. P. LOMBARW,A. S. HOFFEN~ERO,P. C. STOUF- of oropendola and cacique might reflect variation in FER,AND T. R. McGumn. 1989. The parasitism the degree of cowbird parasitism. insurancehypothesis: why starlingsleave spacefor 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 commentsduring the study,and I. Ayub, function and evolution ofaggressivehost behavior J. Guild, K. Merg, E. Phillips and L. Sterk for invalu- towards the Brown-headed Cowbird (Molothrus able assistancein the field. The manuscript was im- ater). Can. J. Zool. 55:508-518. proved by the comments of S. T. Emlen, H. Landel, ROBINSON,S. K. 1985. Coloniality in the Yellow- J. Picman, S. G. Pruett-Jones, P. W. Sherman, L. G. rumped Cacique as a defense against nest preda- Sorenson, and an anonymous referee. Financial and tors. Auk 102:506-5 19. logistic support were 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 coevo- ence Foundation (NSF Graduate Fellowship), Orga- lution: avian brood parasitism. Ann. Rev. Ecol. nization for American States,and Sigma Xi (Grant-in- Syst. 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 residents who allowed me de Psarocolius angustijrons et Psarocolius decu- to work on their land. manus. Bonner ZoologischeBietrage 1957:l-l 5 1. LITERATURE CITED Scorr, D. M., P. J. WEATHERHEAD,ANDC. D. ANKNEY. 1992. Egg-eatingby female Brown-headed Cow- BRAA, A. T., A. MOKSNES,AND E. RBS~. 1992. birds. Condor 94579-584. Adaptations of Bramblings and Chaffinches to- SKUTCH,A. F. 1954. Life histories of Central Amer- wards parasitism by the Common . Anim. ican birds. Pacific Coast Avifauna #31. Cooper Behav. 43:67-78. Omithol. Sot.. Berkeley, CA. BRISKIE,J. V., AND S. G. SEALY. 1990. Evolution of SMITH,N. G. 1968. The advantageof being parasit- short incubation periods in the parasiticcowbirds, ized. Nature 219:690-694. Molothrus SVV. Auk 101:789-194. SMITH, N. G. 1979. Alternate responsesby hosts to CHAPMAN,F. M.‘ I929. The nestinghabits of Wagler’s parasiteswhich may be helpful or harmful, p. 7- Oropendola (Zurhynchus wugkri) on Barre Col- 15. In B. B. Nickel [ed.], Host-parasite interfaces. orado Island. Bull. Amer. Mus. Nat. Hist. 58: 123- Academic Press, New York. 166. SMITH,N. G. 1983. Zarhynchus wugkri, p. 6 14-6 16. CLARK, K. L., AND R. J. ROBERTSON.1979. Spatial In D. H. Janzenred.], Costa Rican natural history. and temporal multi-species nesting aggregations Univ. of Chicaao Press. Chicaao. in birds as anti-parasite and anti-predator defens- TASHIAN,R. E. 1957. Nesting behavior of the Crested es. Behav. Ecol. Sociobiol. 5:359-371. Oropendola (Psarocolius decumanus) in northern DAVIES, N. B., ANDM. D. L. BROOKE.1988. Trinidad, B.W.I. Zoologica 42:87-98. versus reed warblers: adaptations and counter- WEBSTER,M. S. 1991. The dynamics and conse- adaptations. Anim. Behav. 36:262-284. quences of intrasexual competition in the Mon- DRURY, W. H. 1962. Breeding activities, especially tezuma Oropendola: harem-polygyny in a Neo- nest building of the yellowtail (Ostinopi decu- tropical bird. Ph.D.diss., Cornell University, manusl in Trmidad. Zooloeica 47139-58. Ithaca, NY. FLEISCHER,‘R.C., AND N. G. SMITH. 1992. Giant WEBSTER,M. S. 1994. The spatial and temporal dis- Cowbird eggsin the nests of two hosts:the tribution of breeding female Montezuma Oropen- use of morphology and electrophoreticvariants to dolas: effects on male mating strategies.Condor identify individuals and species.Condor 94:572- 96~722-733. 518. WILEY, R. H., AND M. S. WILEY. 1980. Spacingand FRAGA,R. M. 1989. CoIony sizes and nest trees of timing in the nesting ecology of a tropical black- Montezuma Oropendolas in Costa Rica. J. Field bird: comparison of populations in different en- Omithol. 60:289-295. vironments. Ecol. Monogr. 50: 153-178. HAMILTON,W. J., III, ANDG. H. 0-s. 1965. Evo- lution of brood parasitism in altricial birds. Con- dor 67:361-382.