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