The Condor 89:349-356 0 The Cooper Ornithological Society 1981

PAIR FORMATION IN COWBIRDS: EVIDENCE FOUND FOR SCREAMING BUT NOT SHINY COWBIRDS

PAUL MASON Department of BiologicalSciences, University of California, Santa Barbara, Santa Barbara. CA 93106

Abstract. Sixty ScreamingCowbirds (Molothrus rufoaxillaris) and 150 Shiny Cowbirds (M. bonariensis)were trapped and banded in Buenos Aires Province, Argentina. Analysis of recapturesprovided statistical evidence for pair formation in Screamingbut not Shiny cowbirds, a result supportedby observationsof free-living birds. Pairs of ScreamingCow- birds were stable throughout the breeding season.They are probably monogamous. Shiny Cowbirds showed no pattern of associationbetween the sexes. This is consistent with a promiscuousmating system, although this conclusionis tentative. I also present ancillary data on sexual size dimorphism, sex ratio, and related aspectsof behavior. Both speciesare dimorphic in size to a similar extent, although the Screaming Cowbird is slightly larger. Neither speciesdeparts from an adult (quatemary) sex ratio of unity. Pairs of ScreamingCowbirds are conspicuousnear host nests.Female Shiny Cowbirds are not accompaniedby males at nests, but may be accompanied by other females. Shiny Cowbird females were surreptitious near nests. Differences in host selection behavior may have profound effects on other aspectsof the speciesbreeding biology. Key words: ScreamingCowbird; Shiny Cowbird;pair formation; mating systems;brood parasitism;Argentina.

INTRODUCTION with respect to host selection behavior and this The unusual extent of behavioral and morpho- single difference might be responsible for con- logical variation in Icterinae has served to make trasts in a variety of other traits. The Screaming this subfamily a compelling example of adaptive Cowbird is a host specialist, thought to use the radiation in birds (Lack 1968, Selander 1972). communally breeding Bay-winged Cowbird (M. Study of these birds has contributed substantially badius) almost exclusively (Friedmann and Kiff toward the construction of a general theory of 1985; but see Hudson 1874, 1920; Grant 1911, sexual selection and mating systems (Selander 1912; Pereyra 1938; Hoy and Ottow 1964; Sick 1965, 1972; Orians 1969, 1972). I add further 1985). The Shiny Cowbird lays its eggs in the detail to this panorama by contrasting the pairing nests of many species, and the pattern of host behavior of two brood parasites, the Screaming selection, at least in the Rio de La Plata region Cowbird (Molothrus rufoaxillaris) and the Shiny (Argentina and Uruguay), seems to vary with the Cowbird (M bonariensis) at a site in Buenos Aires structure of the host community (Mason 1986a). Province, Argentina, where the species are sym- A total of 20 1 species of birds are known as hosts panic. I also present ancillary data on sexual (Friedmann and Kiff 1985). dimorphism, sex ratio, and other related aspects Another difference is in the extent of plumage of behavior. dimorphism. Screaming Cowbirds are mono- On the one hand, we might expect the species morphic in plumage (both sexes a dull black), to resemble each other very closely. First, taxo- while Shiny Cowbirds are strongly dimorphic nomic differences are minimal between conge- (females are drab gray, while males are black with ners. Second, habitat differences are minimized a blue gloss). This difference is associated with in areas of sympatry for species with similar feed- a putative difference in mating system: Hudson ing ecologies. Finally, the species are brood par- (1874, 1920) and Friedmann (1929) described asites. Screaming Cowbirds as monogamous since they On the other hand, the species differ greatly typically travelled in pairs, but Shiny Cowbirds as promiscuous since they apparently lacked any regular association between the sexes. Both au- I Received 19 May 1986. Final acceptance 10 De- thors drew these conclusions without the aid of cember 1986. individually marked birds.

13491 350 PAUL MASON

with the U statistic. Species comparisons were METHODS performed when no intersexual differenceswere The study site was conducted at Estancia El Talar found. (Site II of Mason 1986a, described in Mason Pairwise relationships were recognized using a 1985) near Magdalena, Buenos Aires Province, randomization procedure for testing the associ- Argentina from 21 September 1978 to 10 Feb- ation between two species(Pielou 1977), but sub- ruary 1979. Birds were trapped on 27 days be- stituting individuals for species. On each trap tween 6 October 1978 and 20 January 1979 in day, birds were recorded as either present or ab- a walk-in decoy trap, baited with grain and water, sent, and then 2 x 2 tables were constructed and similar in construction to that described by describing the frequency with which each bird of Carter (1986). The trap was left baited but open every possiblepair was presentor not. The entries on nontrap days, so that birds could habitually in the four cells represent the number of times enter the trap to eat. that: (1) both birds were caught together; (2) and Sixteen of the 27 trap days were in October. (3) one bird was caught but the other was not; The others occurred sporadically in the remain- and (4) neither bird was caught. The random- ing interval. Poor weather and destruction of the ization procedurethen calculatesa 1-tailed prob- trap by cattle and horsesprevented a regular trap- ability that those two particular birds enter the ping schedule. trap independently. In this sense,the procedure To identify possible pairs of birds, I occasion- is similar to Fisher’s exact test (Siegel 1956). ally observedanimals near the trap and removed Positive association is indicated by dispro- pairs immediately after entry. The trap was usu- portionate scoresin cells 1 and 4 of a particular ally left unattended. On first capture, birds were 2 x 2 table. To eliminate the possibility of in- weighed to the nearest 0.5 g with a spring scale flating the absent-absent cell (Cell 4) I made and given a unique color combination of enam- conservative assumptions about survivorship: a elled aluminum leg bands (5 2 each leg). Shiny bird was not assumed to be in the population Cowbirds were sexed by plumage. Screaming until it was trapped, and not assumedto live after Cowbirds were laparotomized (Risser 197 1). it was last trapped. Tests were carried out only Identification of pairs through the analysis of for pairs whose period of joint observation was recapturesis sensitive to any difference between at least six trap days, the minimum sample size the sexesin susceptibility to enter the trap. Two needed to show significance(P = 0.05 when both tests were performed to assesstrapability. First, birds were caught together three times, and nei- distributions describing the frequency of capture ther caught on three other days). were compared using the Heterogeneity G test Other kinds of information are made available (Sokal and Rohlf 1981). Second, probability of if the probability of independent recapture is also recapture was calculated for retrapped birds calculated for intrasexual pairs. Although com- (Darley 1971) and sexeswere compared using parison of inter- with intrasexual pairs cannot be the Mann-Whitney U-test (Siegel 1956). These construed as a control procedure, doubt would techniques have two additional benefits. First, if be cast on the entire approach if intersexual pairs no evidence is found of a sex-specific bias in were identified as frequently as intrasexual pairs capture, then the population sex ratio is esti- and no biological correspondence could be es- mated by the observed sex ratio. Sex-ratio was tablished between them. In some cases, an ap- testedfor skewnesswith the Chi-square test (Sie- propriate explanation might be available. For ex- gel 1956). Second, equivalent rates within sexes ample, if males are polygynous and hold stable allow the sexes to be pooled such that species harems, then significant pairwise associations comparisons can be made. should occur between the harem members as well A differencein capture frequency (which could as between the male and each female. affect the ease with which pairs could be iden- Data collected through the trapping program tified) might be due to differencesin site fidelity. were compared with observations on free-living Site fidelity wascalculated by recordinghow many birds (both marked and unmarked). I comment days separated the first and last date of obser- on the behavior of cowbirds at nests where I vation for each bird that was recaptured or re- made repeated “spot checks” of lessthan 15 min sighted. Differences between sexes were tested duration (Mason 1986b). PAIR FORMATION IN COWBIRDS 351

RESULTS TABLE 1. Frequencydistribution of captures.In both ScreamingCowbirds and Shiny Cowbirds, males and TRAPPING RESULTS females a& equally as likely to be trapped and re- traooed (GH = 0.58. df = 3. P > 0.75. GH = 3.78. I trapped 60 Screaming Cowbirds (27 females, df = 3, 6 > 0.25, respectively). Screaming Cowbirds 33 males) between 9 October 1978 and 20 Jan- are trapped more readily than Shiny Cowbirds (GH = uary 1979, and 150 Shiny Cowbirds (70 females, 42.18, df = 3, P < 0.001). 80 males) between 6 October 1978 and 29 De- cember 1978. While Shiny Cowbirds were calm No. captures I 2 3 >3 in the trap, ScreamingCowbirds (especiallymales) were restless. They typically struggled, and Screaming Females 12 6 3 6 crashed against the walls. The lores were gen- Cowbirds: Males 17 5 4 7 Total 29 11 7 13 erally bloodied and recaptured birds often pos- Shiny Females 74 5 0 1 sessedinfected lesions, a source of injury that Cowbirds: Males 59 7 1 3 may have increased mortality and made the de- Total 133 12 1 4 tection of pairs more difficult. Infections were not observed around laparotomy wounds. The ratio of male weight to female weight, an from a sex ratio of unity in either species index of sexual dimorphism, is approximately (Screaming Cowbird: x2 = 0.60 for 1 df, ns; Shiny equal between the two species, despite differ- Cowbird: x2 = 0.67 for 1 df, ns, 0.5 < P < 0.7 ences in body weight: ratio = 1.20 for the in both cases). Screaming Cowbird (mean male weight = 58.0 Among Screaming Cowbirds, sufficient data g, k4.0 g SD, n = 33; mean female weight = were collected to calculate the probability of in- 48.4 g, +4.4 g, n = 27); ratio = 1.24 for the Shiny dependentoccurrence of 175 pairs of birds. Three Cowbird (mean male weight = 55.5 g, k7.6 g, of 90 intrasexual comparisons were significant, n = 69; mean female weight = 44.9 g, k4.3 g, while eight of 85 intersexual comparisons were n = 78). significant. Later, 10 additional intersexual com- Trapability was statistically indistinguishable parisonswere performed; one was significant (see for the sexeswithin each species,as judged by below). All significant interactions indicated that both the frequency distribution of captures (Ta- birds tended to occur together more likely than ble 1) and by recapture rates (Screaming Cow- chancewould predict. No evidence suggestedthat birds: U = 114, ns, for 15 females, 16 males; any birds avoided each other (that is that they Shiny Cowbirds: U = 21.5, ns, for 6 females, 11 co-occurredin the trap lesslikely than by chance), males). Screaming Cowbirds repeated more than although the technique would identify such a re- Shiny Cowbirds (Table 1). This difference is re- sponse. The intersexual comparisons involved flected in the greater site fidelity of Screaming 10 males and 12 females; 13 individuals ac- Cowbirds relative to Shiny Cowbirds, although counted for all eight significant relations (Table the difference does not quite reach the conven- 2). These data identified six pairs (Fl-Ml, F2- tional level of significance (z = 1.95, P = 0.052, M2, F3-M3, F4-M4, F5-M5, F6-M6). Females 2-tailed, for 3 1 Screaming Cowbirds, 16 Shiny Fl and F5 were also significantly associatedwith Cowbirds). Sexeswithin each speciesshow sim- male M7, although to a lesserextent than to their ilar site fidelity (Screaming Cowbirds: U = 117, putative mates. This unusual male (M7) account- ns, for 15 females, 17 males; for Shiny Cowbirds, ed for four significantassociations, two with these U= 60.5, ns, for 9 females, 17 males). The equiv- females and two with other males, one of them alent susceptibility to trapping between the sexes (Ml) the mate of female Fl (Table 3). The role (in both species)establishes a precondition for of male M7 is discussedmore precisely below. the successfulidentification of intersexual pairs. On 10 occasions I observed two birds enter However, the lower site fidelity of Shiny Cow- the trap together. In each case, one was a male birds means that pairs will be more difficult to and the other a female. Four of the six pairs identify in this species.No Shiny Cowbird was identified above accounted for eight of the 10 observed in the study area for more than 54 days observations.Pairs Fl-M 1 and F3-M3 were each (a female) while 14 Screaming Cowbirds exceed- observed to enter once each. Pairs F5-M5 and ed this total. No evidence suggestsa departure F6-M6 were each observed to enter three times. 352 PAUL MASON

TABLE 2. Probability of independent recapture of male and female Screaming Cowbirds. Significant associ- ations underlined. Dashes indicate pairwise interactions lacking a sufficient period of joint observation.

Ml M2 M3 M4 M5 M6 M7 MS M9 Ml0

Fl

On the only occasion when F6 was trapped and females for whom sufficient data were collected M6 was not, I saw M6 perch on top of the trap to test his association.However, he was observed and call to F6. She responded by trying to lly up twice in the field with female FlO, and the two to him. The remaining two casesof seeing birds of them were observed to enter the trap together enter the trap together are discussedbelow. on a single occasion. Pairs were simultaneously and unequivocally Three additional pairs were visually but not identified in the field and away from the trap on statistically identified. No individual of these 18 occasions.In each case, one was a male and three putative pairs appears in Table 2. In the the other a female. Pairs F2-M2 and F6-M6 first case, a pair was observed to enter the trap were each seen twice, whereaspairs F 1-M 1, F3- together. The observation and the trap data are M3, and F5-MS were identified three times each. consistent with pair formation, but the sample The remaining five casesof visually identifying is too small to find statistical significance. (P = pair members in the field are discussedbelow. 0.167 for four trap days, two with both present Four pairs (F2-M2, F4-M4, F5-M5, F6-M6) and two with both absent.) In the secondcase, a were apparently stable: three remained intact un- pair was identified in the field. Each of thesebirds til the end of the observation period (when all had been trapped only once (different days). The three pairs were trapped), and neither member final caseconsists of two birds which were trapped of a fourth pair was recaptured or resightedafter together once (but not watched while they en- 29 December 1978. The first three pairs were tered), but then identified together once in the observed over a period extending nearly 4 field. If the field identification is counted as a months. This is a minimal estimate for the du- trap day, and the intervening trap days are scored ration of the pair bond, since all birds were prob- as entries in the absent-absentcell, this pair would ably paired before they were captured. In the be identified with a probability of 0.038. remaining two pairs, the termination of the pe- Three lines of evidence suggeststrongly that riod ofjoint observation (in the trap) was defined male M7 was an interloper or “satellite” rather by the disappearanceof only one bird (male Ml than a secondary mate of females Fl and F5 in the first case, female F3 in the second). By (Table 3). First, male M7 was not recorded in analyzing the data after the disappearanceof the any of the encounters when pairs Fl-Ml and mate, I detected that Fl remated with M8 (P = FS-M5 were seentogether. Second,he was never 0.0 18), a previously unmated male. Three other observed with either female. Third, he failed to males showed nonsignificant patterns of associ- maintain a significant associationwith female Fl ation. Female Fl and male M8 were observed (P= 0.286) when male Ml disappeared and fe- once in the field during this period, whereas she male Fl apparently paired with male M8. had been identified only with male Ml previ- The only other pairwise interaction recognized ously (see above). Male M3 apparently failed to among Screaming Cowbirds was between fe- find a mate (after F3 disappeared)among the six males Fl and Fl 1. With the exception of this PAIR FORMATION IN COWBIRDS 353 interaction and those of male M7, all other sig- TABLE 3. Association of male ScreamingCowbird, nificant interactions were exclusive and occurred M7, with other birds. Significant associationsunder- lined. Birds listed at ends of columns are mated pairs. between members of the opposite sex. Forty-five pairwise interactions, 23 intersex- ual, were examined between Shiny Cowbirds. Ml M5 M3 None was significant. 0.020 0.113 0.047

ADDITIONAL OBSERVATIONS OF 7 FREE-LIVING BIRDS I I I The two speciesbehaved differently at host nests. 0.002 0.02 1 0.290 Pair formation in Screaming Cowbirds is more Fl F5 F3 apparent than in many nesting species. Males remain closeto females, usually following within 2 m of them in flight. Nest inspections occur throughout the day. Pairs and nest inspections They were silent, and no eggwas laid during this were observed at the beginning of the study pe- sequence.I never saw males at nests. riod, before the trap was constructed.As females inspect host nests, males typically perch above DISCUSSION them, vocalizing and performing “song-spread” The Screaming Cowbird forms pairs (Table 2) displays(Orians and Christman 1968). I saw pairs which are stable throughout the season. Of six of Screaming Cowbirds at host nests on 29 of statistically identified pairs, three remained in- 128 (23%) spot checks at nests. In 30 hr of in- tact for nearly four months, and a fourth may tensive observations, I recorded 42 visits, or one have left the study area after about three months. every 43 min. Nest visits were often performed Two pairs broke up, probably becauseof the death by several pairs at a time; once five pairs in- of a mate. At least one bird, a female, remated. specteda nest simultaneously. Since several eggs Visual identification offered corroborating evi- may appear in a nest in a single day, several dence for five of the six original pairs, and for females will lay in the same nest. the female that remated. Shiny Cowbirds, on the other hand, displayed The statistical technique demonstrated here to no obvious pair formation. Males often perched identify pairs appears to be successful.Direct at tree tops, but also followed females and court- observational data are consistent with the sta- ed them almost anywhere. Females were also tistical analysis. This is encouraging, since ob- seen alone and in the company of other females. servation of marked birds in the wild is often Several female Shiny Cowbirds will lay in the difficult and time consuming. Despite their val- same nest of preferred hosts since several eggs ue, observational data are often vague in the sense may appear in nestsin the same day (Fraga 1985, that their explanatory power frequently depends Mason 1986a). The approach of females to nests on no criterion stronger than intuition or plau- and their behavior at nests is quiet and surrep- sibility. Consider the example of the male and titious, contrastinggreatly with that of Screaming female captured together on two trap days (and Cowbirds. Females will often perch within 5 to seen to enter as a pair once) but both absent on 10 m of host nests, apparently watching the ac- two other trap days. Some observersmight wish tivity there. On several occasions,females emit- to label these individuals a pair, whereas others ted “chatter” calls from their perches as I ex- might not. The technique usedhere indicates that amined host nests. I have only a single the data are too few to make a statistical judg- observation of nest inspection by thesecowbirds: ment. two female Shiny Cowbirds examining a nest of Several phenomena explain why only one the Chalk-browed Mockingbird (Mimes satur- member of a pair-forming speciesmight be re- ninus,a specieswhose data base consistsof over captured. First, pairs may not remain together 350 spot checks).Two females entered the nest, all day long. Their tendency to travel indepen- each giving “bill-up” displays to the other (a dently will naturally be reflectedby a greaterlike- common icterine aggressivedisplay, Orians and lihood of capturing one but not the other. Sec- Christman 1968). The females alternated jump- ond, birds may enter the trap sequentially rather ing in and out of the nest and settling on the eggs. than simultaneously. The secondmember of the 354 PAUL MASON pair might have been trapped had the trap been This would be consistent with a promiscuous left in operation more time. The observation of mating system, as claimed by Hudson (1874, male M6 calling to female F6 while the latter was 1920) and Friedmann (1929). This conclusion in the trap is germane in this context. Third, should remain tentative, however, since Shiny some birds managed to escape before I could Cowbirds are probably more mobile than identify them. The analysis has shown itself ro- Screaming Cowbirds. bust to these aspectsof behavior which tend to The behavior of free-living Shiny Cowbirds weaken the sensitivity of the test. similarly gives no hint of pair formation. Males Monogamy is probably the predominant mat- may be more sedentary than females, although ing system in Screaming Cowbirds. Hudson I failed to find a significant effect with a small (1874, 1920) and Friedmann (1929) described sample. Eight males but only two females resided Screaming Cowbirds this way, although their in the study area for more than three weeks. Fra- conclusion was drawn without the advantage of ga (1985) reported that none of five banded fe- marked birds and without seeing copulations. male Shiny Cowbirds in Buenos Aires was pres- Both authors implied that pairs were stable, since ent in his study area for more than three weeks. they felt that this species displayed strong site He felt that this interval was the typical time fidelity (relative to Shiny Cowbirds). This study spent in one area by a female, since unusually supportsthat claim as well, at least within a sea- marked eggs(attributed to single females) were son. More recently, Fraga (1986) banded a single not found for more time than this. The breeding pair in Buenos Aires Province; the birds re- season of Shiny Cowbirds lasts at least three mained paired for the entire breeding season. months (Mason 1986a). Pair associationdoes correspondto the observed Although the Shiny Cowbird is strongly di- pattern of mating in Brown-headed Cowbirds morphic in plumage, both speciesare dimorphic (M a&r) in the Owens Valley of California (Yok- in size to a similar extent. It is incorrect to as- el 1986, the only study of a monogamous brood sume that a particular level of dimorphism is parasite which reports extensive data on both necessarilyassociated with a promiscuous or po- pair formation and copulatory behavior). lygynous mating system (Orians 1972:309). Sex- One male, M7, showed a complex pattern of ual dimorphism in plumage also fails to have interaction with other birds (Table 3). During much predictive value. By comparison, Brown- most of the study period, he apparently followed headed Cowbirds (also strongly dimorphic in two females mated to other males. It is unlikely plumage) are monogamous in some areas (Dar- that this male was bigamously mated to both ley 1982; Dufty 1982a, 1982b; Yokel 1986), while females. He was not observed with them away at least one other population is promiscuous (El- from the trap and when the mate of one of these liott 1980) and another polygynous (three of sev- females disappeared, he was unable to maintain en males mated polygynously, Teather and Rob- his association with her. Fraga (1986) observed ertson 1986). occasional unpaired male Screaming Cowbirds. A difference in mating system between the two His only two records of male-male aggression species cannot be explained by reference to a occurredwhen unpaired males attempted to cop- difference in the sex ratio (Murray 1984), since ulate with mated females. neither speciesdeparted from a sexratio of unity. Two patterns of interaction are puzzling. Male Eventually, contrastsin host selection behav- M7 showed a significant association with male ior may help to explain variation in mating sys- M3, but not his mate, F3 (Table 3). Two females tem, particularly if populations of brood para- also showed a significant interaction with each sites are host limited. Brood parasites are other. I cannot offer any reasonable biological obligately dependent upon the parental care of interpretation of such behavior. Perhaps Type I host speciesand Screaming and Shiny cowbirds error (false rejection of the null hypothesis)is the contrast greatly with respect to their use of the most appropriateexplanation, given that 185 tests “host niche.” were run. Lacking more detailed observations of Behavior near the nest is extremely different individual behavior, I suspendjudgment on the in the two species.Pairs of Screaming Cowbirds importance of such results. visit host nests throughout the day; often several The trapping data offer no evidence that Shiny pairs are simultaneously involved in such nest Cowbirds form any kind of regular association. visits. Male Shiny Cowbirds do not accompany PAIR FORMATION IN COWBIRDS 355 females as they seek host nests, although the lat- to particular patterns of resource availability ter may be accompanied by other females. Fraga (Orians 1969). In the case of brood parasites, (1985) observed gregariousnest inspections dur- differencesin the quality and distribution of host ing seven of 42 (17%) visits of female Shiny Cow- nests could create varying opportunities for sex- birds to Chalk-browed Mockingbird nests.Twice ual selection to act. he saw five females simultaneously inspecting the same nest. ACKNOWLEDGMENTS The adaptive significance of monogamy in Field research,partially funded by the Chapman Fund brood parasites is difficult to explain. In two of the American Museum of Natural History, New Brown-headed Cowbird populations, mate- York, was performed while the author was a graduate student at the University of Texas. Analysis was per- guarding by males appearsto be the appropriate formed while the author was a postdoctoralfellow at explanation (Dufty 1982a, 1982b; Teather and the University of California, Santa Barbara, supported Robertson 1986, for the four males that mated by NSF grant DEB 8214999 to S. I. Rothstein. The monogamously). In another population, the mate manuscript was improved by comments from R. M. Fraga, L. Hunt, S. I. Rothstein, D. Yokel, and two guarding explanation fails (Yokel 1986). The anonymous reviewers. limited amount of data presentedfor the Scream- ing Cowbird is equivocal. Although M7’s signif- LITERATURE CITED icant interaction with Ml (of pair Fl-M 1) could be interpreted as evidence of mate guarding, no ANKNEY, C. D., AND D. M. SCOTT. 1982. On the mating system of Brown-headed Cowbirds. Wil- sucheffect occurred between M7 and MS (of pair son Bull. 94:260-268. F5-M5). Males may plausibly provide service to CARTER,M. D. 1986. The parasitic behavior of the females in the form of detection of host nests, Bronzed Cowbird in south Texas. Condor 88: 1l- distraction of hostsfrom nests (allowing females 25. DARLEY,J. A. 1971. Sex ratio and mortality in the to enter host nests more easily), or protection of Brown-headed Cowbird. Auk 88:560-566. a territory from parasitism by another female. In DARLEY.J. A. 1982. Territorialitv and matine, be- the caseof the Screaming Cowbird, we can elim- havior of the male Brown-headkd Cowbird. con- inate only the territorial hypothesissince the gre- dor 84: 15-2 1. garious nest searchingof these birds is inconsis- DUFTY,A. M., JR. 1982a. Movements and activities of radio-trackedBrown-headed Cowbirds. Auk 99: tent with territoriality. Such male roles, suggested 316-327. for some cuckoos (Liversidge 1970, Riddiford DUFTY,A. M., JR. 1982b. ResponseofBrown-headed 1986), must remain speculativeuntil appropriate Cowbirdsto simulatedconspecific intruders. Anim. testsare designed.Male removal will most likely Behav. 30:1043-1052. ELLIOTT,P. F. 1980. The evolution of promiscuity result in the formation of new pairs, as it did in the Brown-headed Cowbird. Condor 82:138- here. Even if this were not to occur, the female’s 141. performance without male service would prob- FRAGA,R. M. 1985. Host-parasite interactions be- ably be impossible to measure.In addition, males tween Chalk-browed Mockingbirds and Shiny are not uniquely qualified to perform thesetasks: Cowbirds, p. 829-844. In P. A. Buckley, M. S. Foster, E. S. Morton, R. S. Ridgely, and F. G. females may use other females to find nests(Wyl- Buckleyteds.], Neotropical ornithology. Omithol. lie 1981) or to distract hosts (Smith 1968), and Monoer. No. 36. American Ornithologists’ Union. a female herself can defend a territory against Wash>on, DC. other females (Dufty 1982a). Manipulation of FRAGA,R. M. 1986. The Bay-winged Cowbird (MO/- othrus badills) and its brood parasites: interac- the resource (host) base may be a more feasible tions, coevolution and comparative efficiency. research method. In Buenos Aires, a change in Ph.D.diss., Univ. Calif., Santa Barbara. host selection in the Shiny Cowbird was attrib- FRIEDMANN.H. 1929. The cowbirds: a studv in the uted to a changein the structure of the host com- biology’ of social parasitism. Charles C Thomas, munity (Mason 1986a), although this compari- Baltimore. FRIEDMANN,H., ANDL. F. KIFF. 1985. The parasitic son was made between two study sites. Local cowbirdsand their hosts.Proc. West. Found. Ver- variation in host selection may be common. tebr. Zool. 2: l-302. Mating systems should not be conceived as GRANT,C.H.B. 19 11. List of birds collected in Ar- fixed properties of species,even in brood para- gentina, Paraguay, Bolivia, and southern Brazil, with field notes. Part I. Passeres.Ibis. 9th Series siteswhich at first glance seem to share the same 5:104-131. mode of reproduction (Ankney and Scott 1982). GRANT,C.H.B. 1912. Notes on some South Ameri- Rather, they appear to be dynamic adaptations can birds. Ibis, 10th Series 1:273-280. 356 PAUL MASON

HOY, G., AND J. OTTOW. 1964. Biological and oolog- parative study of the behavior of Red-winged, Tri- ical studiesof the molothrine cowbirds (Icteridae) colored, and Yellow-headed blackbirds. Univ. of Argentina. Auk 8 1:186-203. Calif. Publ. Zool. 84. HUDSON,W. H. 1874. Notes on the procreant in- PEREYRA,J. A. 1938. Aves de la zona riberefia de1 stincts of the three speciesof Molothrus found in noreste de la provincia de Buenos Aires. Mem. BuenosAyres. Proc. Zool. Sot. Lond. 153-l 74. Jardin Zool. G Plata 9: l-304. HUDSON,W. H. 1920. Birds of La Plata. Vol. 1. E. PIELOU,E. C. 1977. Mathematical ecology.John Wi- P. Dutton, New York. ley and Sons, New York. LACK, D. 1968. Ecological adaptations for breeding RIDDIFORD,N. 1986. Why do Cuckoos Cuculusca- in birds. Methuen, London. norus use so many speciesof hosts?Bird Study LIVERSIDGE,R. 1970. The biology of the Jacobin 33: l-5. Cuckoo,Clamatorjacobinus. Ostrich Suppl.8: 117- RISSER,A. C., JR. I97 1. A technique for performing 137. lanorotomv on small birds. Condor 73:376-379. MASON,P. 1985. The nesting biology of some pas- &LANDER,R. i. 1965. On mating systemsand sexual serines of Buenos Aires, Argentina, p. 954-972. selection.Am. Nat. 99:129-141: In P. A. Buckley, M. S. Foster, E. S. Morton, R. SELANDER.R. K. 1972. Sexual selectionand dimor- S. Ridgely, and F. G. Buckley [eds.], Neotropical phism in birds, p. 180-230. In B. Campbell [ed.], ornithology. Omithol. Monogr. 36, American Or- Sexual selection and the descent of man. Aldine, nithologists’ Union, Washington, DC. Chicago. MASON,P. 1986a. Brood parasitism in a host gen- SICK, H. 1985. Omitologia brasileira, uma introdu- erahst, the Shiny Cowbiid: II. Host selection.r\uk c80. Universidade de Brasilia, Brasilia, D.F., Bra- 103:61-69. zil. MASON,P. 1986b. Brood parasitism in a host gen- SIEGEL,S. 1956. Non-parametric statisticsfor the be- eralist, the Shiny Cowbird: I. The quality of dif- havioral sciences.McGraw Hill, New York. ferent speciesas hosts. Auk 103:52-60. SMITH,N. G. 1968. The advantageof being parasit- MURRAY.B. G.. JR. 1984. A demographictheory on ized. Nature 219:690-694. the &oluti& of mating systems& exemplifieh by SOKAL,R. R., ANDF. J. ROHLF. 1981. Biometry. 2nd birds. n. 71-140. In M. K. Hecht. B. Wallace. and ed. W. H. Freeman. San Francisco. G. T.‘l%ance leds.1.Evolutionary ’biologv. Voi. 17. TEATHER,K. L., AND k. J. ROBERTSON.1986. Pair Plenum Press, N&w York. _ -_ bonds and factorsinfluencing the diversity of mat- ORIANS. G. H. 1969. On the evolution of mating ing systemsin Brown-headed Cowbirds. Condor systems in birds and mammals. Am. Nat. 103: 88:63-69. 589-603. WYLLIE,I. 1981. The cuckoo. Universe Books, New ORIANS, G. H. 1972. The adaptive significance of York. mating systems in the Icteridae. Proc. XV Int. YOKEL,D. A. 1986. Monogamy and brood parasit- Omithol. Congr. (1970):389-398. ism: an unlikely pair. Anim. Behav. 34: 1348-l 358. ORIANS,G. H., ANDG. M. CHRISTMAN.1968. A com-