New information on the interactions among Shiny Cowbirds (Molothrus bonariensis) and 12 host species in Argentina is presented. Observations and experiments suggest that Brown Cacholotes (Pseudoseisura lophotes) remove cowbird eggs from their nests. I present data on care and provisioning of Shiny Cowbird chicks by nine species (including five icterids), all new or rarely reported in this role in host catalogs. Fifteen of 19 (79%) of the nesting Argentinian icterids are reported as Shiny Cowbird hosts, and seven (37%) are known to rear chicks of this parasite to fledging (effective hosts). Saffron-cowled Blackbirds (Xanthopsar flavus) are frequent effective hosts but now have a restricted distribution. The Pale-breasted Thrush (Turdus leucomelas) was an important effective cowbird host in the area of Iguazú falls, and the Diuca Finch (Diuca diuca) is a frequent effective host in the Argentinian Patagonia.