Parasitism by Shiny Cowbirds of Rufous-Bellied Thrushes

The Condor 100:680-687 0 The Cooper Ornithological Society 1998

PARASITISM BY SHINY COWBIRDS OF RUFOUS-BELLIED THRUSHES

GABRIELA LICHTENSTEIN~ Department of Zoology, Universityof Cambridge,Downing Street, CB2 3EJ Cambridge, U.K., e-mail: [email protected]

Abstract. I studied the interaction between a brood parasite, the Shiny Cowbird (Molo- thrus bonariensis),and a large host, the Rufous-bellied Thrush (Turdus rujventris), in the PampasRegion of Argentina. Shiny Cowbird eggs were observed in 34 out of 70 (48.6%) thrush nests throughout the host’s breeding season. The main damage inflicted by Shiny Cowbirds was the punctureof host eggs, which occurred in 55.9% of the parasitizednests and significantly reducedthe host clutch size. Becauseof the host’s short incubationperiod, cowbird eggs did not hatch more than one day before the host young. This small advantage of earlier hatching was not enoughto outweigh the large size difference between the parasitic chicks and the host young. In 68.7% of experimentally-createdbroods containing one host and one cowbird chick, the cowbird chicks died of starvation. I conclude that the Rufous- bellied Thrush is a particularly poor host for Shiny Cowbirds becauseof its large size and short incubation period. The high rate of parasitism, even of such a poor host, indicates little selectivity by Shiny Cowbird females. Key words: broodparasitism, host selection,Molothrus bonariensis,parasitic chicks,Ru- fous-bellied Thrush,Shiny Cowbird, Turdusrufiventris.

INTRODUCTION prefer to parasitize large hosts rather than small Although it has been suggested that Shiny Cow- ones. birds prefer to parasitize hosts larger than them- selves (Mason 1980), studies of the interaction METHODS of Shiny Cowbird chicks with larger hosts show STUDY SPECIES that on most occasions Shiny Cowbird chicks The Rufous-bellied Thrush is a medium sized starved in nests of larger species. Gochfeld sexually monomorphic thrush that weighs (1979) found that 96% of Meadowlark Stumella around 70-80 g. It is a typical member of its loyca nests were parasitized by Shiny Cowbirds, genus and one of eight Turdus speciesthat breed but none of them produced a cowbird fledgling. in Argentina. The Rufous-bellied Thrush breeds A study by Fraga (1978) showed that 37% of throughout northern Argentina (from the South Shiny Cowbirds starved in Chalk-browed Mock- of Cordoba and Buenos Aires), Uruguay, Para- ingbird Mimus satuminus nests. The only case guay, eastern Bolivia, and eastern Brazil. It is in which the Shiny Cowbirds did well at nests common in open woodland, forest borders, of larger hosts was when parasitizing Brown- clearings, and gardens (Ridgely and Tudor and-Yellow Marshbirds (Mermoz and Reboreda 1989). The breeding season in the area of Bue- 1994). However, this result could be due to the nos Aires starts in late August/September, and combined effect of helpers at the nest and the extends until the end of December. The open shorter incubation period of the cowbird eggs. nest is made of twigs and mud, and the clutch This paper has two aims: the first is to provide natural history information on the interaction be- of bluish eggs with spots varies from 2-6 with tween the Shiny Cowbird and a common large a mode of 3 eggs per nest. Incubation startsafter host with a short incubation period, the Rufous- laying the second egg, and lasts for 13 days. bellied Thrush Turdus rujiventris, and the sec- Both parents feed the nestlings, which fledge af- ond is to discusswhether these findings support ter about 13 or 14 days and which receive pa- Mason’s (1980) hypothesis that Shiny Cowbirds rental care for a further period of about 14 days. Pairs typically raise two broods per season.The adult diet consists mostly of invertebrates (es- ‘Received 30 June 1997. Accepted17 July 1998. *Currentaddress: International Institute for Environ- pecially earthworms) and fruits. Adults feed the ment and Development-AL,Gral Paz 1180, (1429) young with earthworms, insects, and berries. BuenosAires, Argentina, e-mail: [email protected] Rufous-bellied Thrushes are parasitized by PARASITISM BY SHINY COWBIRDS 681

TABLE 1. Incidence and intensity of parasitism of Rufous-bellied Thrushes by Shiny Cowbirds during the season.

Nests with Cowbird eggs per parasitized Month Total nests cowbird egg Parasitism ratea No. of cowbird eggs nestb (mean k SE) October 26 12 46.1% 16 1.33 2 0.26 November 33 14 42.4% 18 1.29 + 0.16 December 11 8 72.7% 19 2.37 + 0.65 Total 70 34 48.6% 53 1.56 ? 0.19

a 2 on original numbers = 3.1, P = 0.21. b ,? companng nests with one versus two or more eggs = 3.0, P = 0.21

Shiny Cowbirds (Friedmann et al. 1977), but the hours of video recordings were analyzed per interactions between them have never been stud- nest. ied in detail. I used nonparametric analysis because of the small sample sizes (Siegel and Castellan 1988). STUDY SITE AND NEST VISITS All tests were two-tailed, and results are report- The study was conducted at parks and gardens ed as significant if P I 0.05. Values reported are in the locality of Del Viso (34”29’S, 58”35’W), means ? SE unless otherwise specified. in the Province of Buenos Aires, Argentina, RESULTS from October 1994 to January 1995. Thrush nests were located usually between 1.6 and 4.0 INCIDENCE AND INTENSITY OF PARASITISM m above the ground in the forks of trees, in Shiny Cowbird eggs were observed in 34 out of vines, dense bushes, ornamental vegetation, or 70 (48.6%) Rufous-bellied Thrush nests found in fruit trees. I monitored daily the progress of during the egg stage. The size of host clutches a total of 70 nests, recording the number of par- was reduced due to egg puncture in a further 10 asitic and host eggs present and the order of lay- nests, but no parasitic eggs were found. As nests ing and hatching. Nests were considered para- with missing or punctured thrush eggs were not sitized if they contained cowbird eggs or nest- consideredparasitized unless they had a cowbird lings at any stage.I did not consider nests which egg, the observed incidence of parasitism might had missing host eggs, but no cowbird eggs, to be an underestimate because cowbird eggs may have been parasitized. have been rejected by the hosts before I had the Eggs were examined for punctures and mea- chance to record them (see below). The inci- sured with vernier calipers to the nearest 0.1 dence of parasitism reached a peak in December mm. The incubation period was measured from (Table l), coinciding with a decreasein the num- the day the penultimate egg was laid until each ber of potential host nests available. Multiple egg hatched. Once eggs had hatched, I manip- parasitism (more than one cowbird egg per nest) ulated nest contents to create broods containing occurred in 24.5% of the parasitized nests (Fig. one thrush and one Shiny Cowbird chick of the 1) and also reached a peak in December. How- same age (2 1 day) in order to standardize nest ever, neither of these trends was statistically sig- contents. The remaining host or Shiny Cowbird nificant (Table 1). chicks were cross-fosteredto nearby nests con- EGG MORPHS taining chicks of approximately the same age. Cowbird eggs measured 22.70 ? 0.26 mm in Chicks were weighed daily between 06:OO and length and 18.00 2 0.09 mm in width (n = 34). 0803 with a 50 g Pesola spring balance, until Rufous-bellied Thrush eggs were larger, mea- the age of 12 days. Tibia, gape length, and gape suring 27.10 + 0.24 mm in length and 20.80 5 width were measuredto the nearest0.1 mm with 0.01 mm in width (n = 20). Fifty-two out of 53 vernier calipers. I used Ricklefs’ (1967) method (98.1%) cowbird eggs found in this study were to estimate the growth constant (K) and compare spotted, whereas only one was of the immacu- the growth rate of the cowbird and the thrush late morph. chicks. Information about the type of food delivered EGGPUNCTURE to the chicks was extracted from video taping of Shiny Cowbirds often puncture host eggs (Fraga nine nests when the chicks were 7 days old. Ten 1985). Although there was no difference in the 682 GABRIELA LICHTENSTEIN

12 days for the cowbirds (range 11-13 days, IZ = 12 nests). In clutches of three eggs, female thrushes started incubation after laying the second egg, which resulted in asynchronoushatch- ing. The first two thrush chicks hatched the first day (usually 0.5 day apart), whereas the third chick hatched the following day. Most of the unhatchedeggs were the eggs that were laid last. Twenty-seven percent (7 of 26) of the cowbird eggs present at the end of the incubation period did not hatch. Three of these had developed em- bryos indicating that they were laid too late in

I 2 3 4 the nesting cycle of the thrush, whereas the rest Number of cowbird eggs per nest showed no sign of development. Of the hatched cowbird eggs with known histories, 89.5% of FIGURE 1. Frequencydistribution of numberof ob- the chicks were born in the 3-day interval be- servedShiny Cowbirdeggs per nestof Rufous-bellied Thrush. tween 1 day before and 1 day after the date on which the first thrush chicks hatched (Fig. 2). Taking into account that the incubation period number of eggs laid by thrushes in parasitized of the cowbird is only 1 day shorter than that of and nonparasitized nests (Table 2), the number the thrush chicks, and that thrushes start incu- of host eggs present at the time of hatching was bation after laying the second egg, cowbird fe- significantly smaller in parasitized nests. This males have only 2 days to lay eggs that will was due to egg puncture by the Shiny Cowbirds. hatch before or on the same day as the host From the 34 parasitized nests, 19 (55.9%) lost young. at least one host egg due to puncture. Host eggs disappearedor were punctured in 10 (27.8%) of NEST SUCCESS the nonparasitized nests (x2, = 5.69, P = 0.02). Five parasitized and one nonparasitized nest In five parasitized and one nonparasitized nest, were deserted following egg puncture (Fisher egg puncture was followed by desertion. Two exact test P = 0.1). Fourteen out of 35 (40%) cowbird eggs were found punctured in multiply nonparasitized and 10 out of 29 (34.5%) para- parasitized nests. Shiny Cowbird eggs disap- sitized nests suffered from predation (x2, = 0.2, peared in four of the parasitized nests. These P = 0.65). Nests were considered depredatedif eggs may have been rejected outright, or else the entire clutch or brood disappeared. Likely have been punctured by female cowbirds before mammalian predators include white-eared pos- being ejected by the hosts. sums Didelphis albiventris and domestic cats. Chimangos Milvago chimango were the most EGG LAYING likely avian predators. There was no difference The incubation period was 13 days for the in the predation rate at the egg stage of parasit- thrushes (range 12-13 days, IZ = 16 nests) and ized (6 out of 29) and unparasitized nests (9 out

TABLE 2. Resultsof Mann-Whitney U-testscomparing the numberof hosteggs present per parasitizedand nonparasitizednest. The decreasein the numberof eggspresent at the end of the incubationperiod was due to egg puncture by female cowbirds.”

Nonparasiuzed Parasitized (n = 36) (n = 34) z P Mean No. of hosteggs laid 3.34 * 0.12 3.14 ? 0.07 -1.35 0.18 Mean No. of hosteggs presentlate in the incubationperiod 3.27 IT 0.16 2.31 ? 0.17 -4.52

aThere was no difference between the number of host eggs laid and the number of host eggs present late in the incubation period m nonparasitized nests (Wilcoxon sign ranks test, z = -1.3, P = 0.18). In parasitized nests the difference was significant (z = -4.3, P < 0.001).

684 GABRIELA LICHTENSTEIN

60 tively short incubation period resulted in Shiny . Shiny Cowbird Cowbird chicks starving in 68.7% of the para- 50 - 0 Thrush I$ 02 sitized nests. The high rate of parasitism, even of such a poor host, indicates little selectivity by 40- ,x: . Shiny Cowbird females or low densities of al- oA . l -l-J. -5 ternative hosts (Barber and Martin 1997). 30 - o-F.- ii’*- Whereas Brown-headed Cowbirds manipulate i- . A host clutches by egg removal (Wood and Bollin- 20 - 0=. . ger 1997), Shiny Cowbirds do so principally by puncturing their host’s eggs. The main damage 10 0 2 4 6 8 10 12 inflicted by Shiny Cowbirds in this study was the destruction of host eggs. Egg puncture was reported as the main damage caused by Shiny Cowbirds in nests of other large hosts such as Brown-and-Yellow Marshbirds (Mermoz and Reboreda 1994), and Chalk-browed Mocking- birds (Fraga 1982). In nests of smaller hosts, however, competition between cowbird and host chicks also strongly decreasesthe reproductive success of the host (Fraga 1978, Marvil and Cruz 1989). Egg removal and egg laying are not necessarily temporally coincident in cowbirds (Sealy 1992). Hence nests with punctured eggs 100 but without parasitic eggs could have been due to females puncturing eggs and not being able 0 2 4 6 8 10 12 to return later to lay their eggs, or to females laying eggs of the white morph that were rejected by the hosts. The low frequency of eggs of the white morph in thrush nests and their presence in nests of 8oI other passerinesbreeding in the same area is in- 60- 500 8 0- direct evidence for the hypothesis that Rufous- bellied Thrushes reject cowbird eggs of the im- 2 0 O .g 40- 0 . . maculate morph. Rejection of the white morph by thrusheshas been reported (Mason 1980), but 3 0 +i. 20 - zi l -- experiments are needed to test this hypothesis. E;” Other large hosts that lay spotted eggs, such as ) : l I I I I Brown-and-Yellow Marshbirds (Mermoz and 0 0 2 4 6 8 10 12 Reboreda 1994) and Chalk-browed Mocking- JW birds (Fraga 1982), also reject eggs of the white morph. FIGURE 3. Comparisonof nestling growth. The figure shows mean gape area and mean ? SD of tibia WHY DO SHINY COWBIRD CHICKS DO SO length and weight for Shiny Cowbirds and thrushes. BADLY IN RUFOUS-BELLIED THRUSH NESTS? Gape area is the product of gape length and gape width. Twelve thrusheswere measuredfrom day 0 to Diet incompatibility. An important factor which day 11. The number of cowbirds measuredper day is: sometimes contributes to nestling starvation in 7, 12, 12, 11, 9, 6, 7, 8, 8, 6, 6, 4. Data comes from cowbirds is dietary incompatibility with the 12 nests containing one thrush and one cowbird chick of the same age 2 1 day. host. Brown-headed Cowbirds fail to fledge in nests of hoststhat feed their nestlings with seeds (House Finch, Carpodacus mexicanus, Kozlovic et al. 1996), fruit (Cedar Waxwing, Bombycilla cedrorum, Rothstein 1976) or other plant ma- terial (House Sparrow, Passer domesticus, PARASITISM BY SHINY COWBIRDS 685

Eastzer et al. 1980). Similar results were found pers. comm.), but do well at nests of larger hosts for Shiny Cowbirds parasitizing Saffron Finches with a similar gape color, e.g., Yellow-headed SicaZisJlaveoZu(Mason 1980). However, the diet Blackbird Xunthocephulus xunthocephulus (Or- provided in Rufous-bellied Thrush nests con- tega 1991). The idea that parents might discrim- sisted at least of 88% animal protein, which pre- inate on the basis of gape color is not new. In cludes dietary incompatibility as an explanation 1978, Rothstein hypothesized that the variation for the starvation of cowbird nestlings. in the color of the rictal flanges of the cowbirds Large size difference with the host young. Ru- was influenced by the preferential feeding of fous-bellied Thrush chicks weigh significantly those chicks whose gape is most similar to that more than the Shiny Cowbird chicks from hatch- of their own young. This might explain why, ing. There was no evidence that hatching before when parasitizing larger hosts, Shiny Cowbirds the host young could prevent cowbird chicks are more successfulat parasitizing species with from starving. As a result of the short incubation similar looking chicks. Icterids, which all have period of the thrush, Shiny Cowbird chicks red gapes (Ficken 1965) are among the most could not hatch more than 1 day before the host common hosts of the Shiny Cowbird in South young; thus there was insufficient time for cow- America (Friedmann et al. 1977) Trinidad and bird nestlings to attain body masses to negate Tobago (Manolis 1982), and Puerto Rico (Wiley the size difference and faster growth of host 1988). The more specialist species of parasitic chicks. The result of this study is similar to that cowbirds also parasitize Icterids (reviewed in of previous studies on the interaction of Shiny Lichtenstein 1997). Cowbird chicks and large hosts, showing that In summary, the low success of the Shiny cowbird chicks are generally outcompeted by Cowbird chicks was probably due to their small- host young of a similar incubation period er size and/or having a different appearance. A (Chalk-browed Mockingbirds, 13 days, pers. ob- study comparing the feeding successof parasitic serv.). Having a shorter incubation period over- cowbird chicks and similar sized thrush chicks comes the size disadvantage of Shiny Cowbird showed that Shiny Cowbird chicks were fed chicks parasitizing greater Antillean Grackles only if the host chicks were not begging, sug- Quiscalus niger (14 days, Wiley 1986). The lon- gesting that parental choice plays an important ger incubation period of the Brown-and-Yellow role in determining who gets fed in parasitized Marshbird eggs (14-15 days, Mermoz and Re- nests of Rufous-Bellied Thrushes (unpubl. data). boreda 1994) might be an important factor in determining the successof Shiny Cowbird par- DO COWBIRDS PREFER TO PARASITIZE asitizing this host. LARGE HOSTS OVER SMALLER ONES? DifSerent appearance. The difference in appearance between the Shiny Cowbird and host Mason (1980) suggested that Shiny Cowbirds nestlings also might have contributed to the poor prefer to parasitize large hosts over smaller successof the cowbird chicks. When parents ar- hosts. Considering the low reproductive success rive at the nest with food, they are faced with of the Shiny Cowbird chicks in nests of large the open gapes of nestlings. Rufous-bellied hosts, this preference seems maladaptive. I be- Thrush adults might have an innate preference lieve that Mason’s conclusion was due to the for yellow and wide gapes, in which case the fact that he included species with very small big yellow gapes of thrush chicks could be a sample sizes in his analysis. In any case, host stronger stimulus for feeding than the much preference can not be determined unless the smaller red gape of the cowbirds. The combined whole community of potential hosts is sampled. effect of having a much smaller gape with a dif- In just such a study of multiple coexisting host ferent color of rictal flanges and mouth lining species, Barber and Martin (1997) showed that might put cowbird chicks at a disadvantage for parasitism rates on Black-capped Vireos (Vireo eliciting feedings. A difference in appearance utricupillus) were positively correlated with cu- might explain why Brown-headed Cowbird mulative coexisting host density in general, and chicks (gapes with red lining and white rictal with Northern Cardinal (Cardinalis cardinalis) flanges) are outcompeted by the slightly larger density in particular. This study shows the neg- Northern Cardinal (Scott and Lemon 1996; or- ative effect of coexisting species on the parasit- ange-red lining and white rictal flanges, Eckerle, ism rate of a focal species and the importance 686 GABRIELA LICHTENSTEIN of taking into account the entire host commu- cowbirds seem to be following an opportunistic nity. strategy in which factors such as host aggres- If large hosts were preferred in the past, one siveness, attentiveness(Mason 1980), and avail- would expect to find large hostspreferred in new ability of nests to parasitize seem to be impor- areas of colonization of the Shiny Cowbirds. tant. The opportunistic strategy of the cowbirds The invasion of the Caribbean by the Shiny contrasts with the specialist strategy of the Eu- Cowbird provides a natural test of this hypoth- ropean Cuckoo Cuculus canorus (Wyllie 1981). esis. There is no evidence for Shiny Cowbirds Cuckoos have a comparatively lower fecundity preferring large hosts in newly colonized areas, than cowbirds, and probably allocate more en- such as Puerto Rico (Perez-Rivera 1986, Wiley ergy to selecting host nests and laying at a cer- 1988), or Santa Lucia (Post et al. 1990). tain time. High fecundity and cheap egg pro- Additional evidence against a preference for duction (Kattan 1995) might enable female cow- large hosts comes from studies of parasitism of birds to pursue an opportunistic strategy, laying Shiny Cowbirds in other areas. In a previous eggs in nests of poor hosts in the absence of study, the Rufous-collared Sparrow was reported higher quality hosts to parasitize. However, a to be the most frequent host in Argentina (King generalist strategy is sometimes costly for cow- 1973), Uruguay and Brazil (Friedmann et al. birds, as is shown by the high parasitism rate of 1977, Cavalcanti and Pimentel 1988), whereas such a poor host as the Rufous-bellied Thrush. the Diuca Finch Diuca diuca is the most frequent host in Chile (Johnson 1967), and the ACKNOWLEDGMENTS House Wren Troglodytes aedon in the Cauca I thank Juan Carlos Reboreda and Myriam Mermoz Valley in Colombia (Kattan 1993). All of these for their assistanceand helpful discussion.Nick Da- hosts are smaller than the Shiny Cowbird. vies, Mike de L. Brooke, Anna Lindholm, Dave Noble, In summary, I believe that with the evidence Gustav0 Kattan, and Stephen Rothstein gave valuable comments on the manuscript.The researchwas finan- available we are not in a position to conclude cially supportedby grants from Sigma Xi, Chapman that Shiny Cowbirds show an active preference Fund, and King’s College, Cambridge, U.K. to parasitize large hosts over small ones. I agree with Mason that species larger than LITERATURE CITED

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