Myriam E. Mermoz

Brood parasitism of the shiny cowbird, Molothrus bonariensis, on the brown-and-yellow marshbird, Pseudoleistes virescens

We studied the relationship between a generalist brood parasite, the Shiny Cowbird (Molothrus bonariensis) and one of its hosts, the Brown-and-Yellow Marshbird (Pseudoleistes virescens). Seventy-four percent of the nests found were parasitized. Although the parasite lays both white and spotted eggs, most of the cowbird eggs found in the nests of this host were spotted. Artificial parasitism experiments showed that the host rejected the cowbird white eggs. Shiny cowbird parasitism reduced the nesting success of the Brownand-Yellow Marshbird mainly by punctures or cracks of the host eggs. The reduction of the nesting success of the parasite was due mostly to the loss of eggs in multiple parasitized nests. Shiny cowbird chicks were not outcompeted for food although they are smaller than the host chicks. The Brown-and-Yellow Marshbird appears to be a very good host, capable of rearing up to four cowbird chicks in a nest. We compared the nesting success of the Shiny Cowbird in Brown-and-Yellow Marshbird nests with its nesting success in the other sympatric hosts studied.

Nesting success in Brown-and-yellow Marshbirds : Effects of timing, nest site, and brood parasitism

--The Brown-and-yellow Marshbird (Pseudoleistes virescens) is a nonterritorial blackbird with helpers at the nest that inhabits temperate marshy areas and humid grasslands of Argentina, Uruguay, and Brazil. We estimated nesting success of marshbirds throughout the breeding season (late September to mid-December) and analyzed the effects of brood parasitism and nest-site selection (i.e. nesting substrate) on daily survival rates at different stages of the nesting cycle. Most nests were started in November, and these nests had a higher daily survival rate than those initiated in October or December. Only 13.3% of the nests fledged chicks. Predation accounted for 77% of nest losses, whereas the other failures resulted from nest desertion, in most cases after the loss of several eggs. Sixty-five percent of the nests were parasitized by Shiny Cowbirds (Molothrus bonariensis), but brood parasitism did not reduce nest survival. Daily survival rates increased from egg laying and incubation to the brood-rearing stage. Nesting success was three times higher in nests built in native black rushes and pampa grasses than in nests built in two exotic species of thistle. Eighty-nine percent of the nests were built in thistles, which were the most abundant plants in the study area. However, many black rushes and pampa grasses remained unoccupied. Received 6 June 1997, accepted 18 February 1998. FOOD LIMITATION is an important factor that shapes life-history variation in birds (Lack 1968, Drent and Daan 1980). Nesting success, in particular nest predation, also may exert an important influence on life-history variation (Slagsvoid 1982, Martin 1995). Several factors affect nesting success in birds. Most species show a decline in nesting success as the breeding season progresses (Perrins 1970, Daan et al. 1988). This decline is thought to result from seasonal variation in the environment (Hochachka 1990), differences in quality between early and late breeders (Nilsson 1994), or a combination of both factors (Verhulst et al. 1995). The site at which the nest is built affects its detectability and therefore the probability of predation (Best and Stauffer 1980, Martin and Roper 1988). In addition, brood parasitism may influence nesting success in some species. Some hosts abandon the nest when parasitized (Rothstein 1990), whereas others desert the nest as a consequence of egg losses (removals or punctures) caused by the parasite (Zimmerman 1983, Wiley 1985, Sedgwick and Knopf 1988, Hill and Sealy 1994). Moreover, the louder begging calls of some parasite chicks (Briskie et al. 1994, Lichtenstein 1997) could increase E-mail: mermoz@proteus.dna.uba.ar the detection of parasitized nests by predators (Hudson 1874). Predation risk is not constant through the nesting cycle. Most studies of nesting success have found that nest loss is highest during the brood-rearing stage and have attributed these losses to the higher frequency of visits by adults and to the begging activities of nestlings, both of which could make the nest more conspicuous to predators (Skutch 1949, Nice 1957, Redondo and Castro 1992, Schaub et al. 1992). However, a few studies have found that nest loss is highest during the laying and incubation stages (Roseberry and Klimstra 1970, Best and Stauffer 1980). This effect has been attributed to increased nest defense by the parents as the nesting season progresses (Andersson et al. 1980). Several studies have analyzed nest predation in the tropics and in north temperate areas (Skutch 1949, 1985; Martin 1995), but almost no information is available on nesting success in south temperate areas (see Martin 1996). In this paper, we present data on nesting success of the Brown-and-yellow Marshbird (Pseudoleistes virescens) and examine the effects of brood parasitism and nest-site selection on daily survival rates at different stages of the nesting cycle. The Brown-and-yellow Marshbird is an insectivorous blackbird that inhabits marshy areas and

REPRODUCTIVE SUCCESS OF SHINY COWBIRD (MOLOTHRUS BONARIENSIS) PARASITIZING THE LARGER BROWN-AND-YELLOW MARSHBIRD (PSEUDOLEISTES VIRESCENS) IN ARGENTINA

Abstract Shiny Cowbirds (Molothrus bonariensis) often parasitize larger hosts. It has been proposed that larger hosts are preferred by that parasite because they provide higher reproductive success, but available data are quite variable. We studied the reproductive success of Shiny Cowbirds in nests of Brown-and-yellow Marshbirds (Pseudoleistes virescens), a larger and often multiply parasitized host. To estimate the extent of interspecific competition, we compared the hatching success of parasite eggs in nests with and without reduction of the clutch size of the host as a result of egg punctures inflicted by the parasite, and the survival and growth of parasite chicks reared with and without host chicks. To estimate the extent of intraspecific competition, we compared Shiny Cowbird egg losses, hatching success, and chick survival in singly versus multiply parasitized nests. Reproductive success of Shiny Cowbirds was 8% with depredation causing 80% of losses. Clutch reduction due to egg punctures were higher in multiply than in singly parasitized nests, but it did not improve hatching success of parasite eggs. Neither survival nor growth of parasite chicks was affected by the presence of host chicks. Shiny Cowbird hatching success and chick survival did not differ between singly and multiply parasitized nests. Parasite chicks were smaller than same-age Brown-and-yellow Marshbird chicks. However, because parasite chicks hatched one or two days before host chicks and had a higher growth rate, they were the larger chicks in the nest. Overall Shiny Cowbird reproductive success in Brown-and-yellow Marshbird nests was apparently higher than that reported in other smaller or similar-sized hosts. We think that host life-history traits like large clutch size, a longer incubation period, and slower growth rate of chicks are responsible for the high reproductive success of Shiny Cowbirds with Brown-and-yellow Marshbirds.

Low frequency of Shiny Cowbird parasitism on Scarlet-headed Blackbirds : anti-parasite adaptations or nonspecific host life-history traits?

Interspecific brood parasitism is a strong selective force that has favoured the evolution of anti-parasite strategies. The Shiny Cowbird Molothrus bonariensis is a generalist parasite that uses more than 200 species as hosts. Scarlet-headed Blackbirds Amblyramphus holosericeus are infrequently parasitized by Shiny Cowbirds in contrast to other sympatric marshbird species that are heavily parasitized. In this paper, we attempted to identify specific anti-parasite defences of the Scarlet-headed Blackbird. Parasitism by Shiny Cowbirds of Scarlet-headed Blackbirds averaged 14.2%, whereas in Brown-and-yellow Marshbirds Pseudoleistes virescens, a sympatric marsh-nesting species, it was 52%. Both host species reared Shiny Cowbird chicks. Scarlet-headed Blackbird did not reject artificially added parasite eggs and did not respond more aggressively toward a female Shiny Cowbrid dummy model than to control species, the Yellow-winged Blackbird Agelaius thilius or the Brown-and-yellow Marshbird. However, high levels of nest attentiveness were recorded during nest-building, egg-laying and incubation stages. About 95-98% of the time at least one parent remained less than 20 m from the nest. Our results show that the low frequency of parasitism on the Scarlet-headed Blackbird may be better explained by life-history traits such as territorial defence, rather than by evolved anti-parasite strategies.

Effect of egg type on the estimation of nest predation in passerines

Abstract We compared predation rates of artificial nests baited with different egg types with the predation rates on nests of the Brown-and-yellow Marshbird (Pseudoleistes virescens), a Neotropical passerine common in the same area, in Argentina. We used deserted natural nests as experimental nests, baited with either Japanese Quail (Coturnix coturnix) or Shiny Cowbird (Molothrus bonariensis) eggs. Each experimental nest was baited with one natural and one paraffin-filled egg. Experimental nests baited with quail eggs suffered lower predation and egg losses than those baited with cowbird eggs. The main cause of egg loss and nest mortality in “cowbird nests” was pecking behavior of cowbirds. However, when we removed the effect of cowbirds, differences still held. In this area, the Shiny Cowbird is common and is the main cause of reduced breeding success of several host species. “Quail nests” had higher nest and egg survival than active Brown-and-yellow Marshbird nests. In contrast, “cowbird nests” and active Brown-and-yellow Marshbird nests had similar nest and egg survival. Experimental nests baited with natural passerine eggs such as those of Shiny Cowbirds seem to approximate the natural nest and egg predation risk for passerines in our study area.

EFFECT OF PREDATION AND COWBIRD PARASITISM ON THE NESTING SUCCESS OF TWO SYMPATRIC NEOTROPICAL MARSHBIRDS

Abstract We compared the effect of nest predation and cowbird parasitism on the breeding success of two simultaneously nesting ecologically similar blackbird species that differ in their breeding strategies. The Scarlet-headed Blackbird (Amblyramphus holosericeus) is a monogamous species that performs territorial defense. In contrast, the Brown-and-yellow Marshbird (Pseudoleistes virescens) is a non-territorial monogamous breeder that performs mate guarding and has helpers at the nest. Both species suffered similar nest predation rates throughout their nesting cycle. However, the Brown-and-yellow Marshbird suffered higher parasitism from Shiny (Molothrus bonariensis) and Screaming cowbirds (M. rufoaxillaris) than the Scarlet-headed Blackbird (62.6% vs 15.4%). Brood parasitism accounted for most of the egg losses and hatching failures in Brown-and-yellow Marshbird. Parasitized nests had lower egg survival and hatching success than non-parasitized ones. Mean clutch size was 1.5 eggs larger in Brown-and-yellow Marshbird than in Scarlet-headed Blackbird. However, Scarlet-headed Blackbird had higher hatching success than Brown-and-yellow Marshbird and similar fledging success. Consequently, both species produced similar numbers of fledglings. We did not detect any relationship between the reproductive success of these species and their breeding strategies. The presence of helpers at Brown-and-yellow Marshbird nests did not affect nest defense or chick survival, but helpers might account for reduced parental effort by supplementing food delivery to chicks/fledglings.