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).