Levels of conspecific brood parasitism (CBP) and quasi-parasitism (QP) are important parameters for the general understanding of alternative female mating strategies. We deployed double-digest RAD sequencing to assess CBP and QP in a south temperate population of Grass Wrens (Cistothorus platensis). CBP rate was low and varied annually (0–10%). No cases of QP were identified in our population. Grass Wrens showed similar levels of CBP when compared to other Neotropical songbirds. Given that females could increase their reproductive output by laying eggs in both their nests and in other females' nests, it is striking that CBP is so rare. Future work should evaluate counter-adaptations (egg pecking and rejection, nest desertion, and retaliation) that reduce the success of CBP.