Grass Wrens Cistothorus platensis build two types of non‐breeding nest structures: platforms and dummy nests. Platforms are rudimentary accumulations of grasses concealed between vegetation. Dummy and breeding nests are dome‐shaped with a similar structural layer. We used a nest‐removal experiment and observational data to evaluate several hypotheses regarding the adaptive significance of building multiple nests in a south temperate population of Grass Wrens. Building non‐breeding nests was not a strategy of males to attract additional females, as most of these nests were built after pair formation and both sexes collaborated during building. Building non‐breeding nests was not a post‐pairing display as the presence of multiple nests did not increase female investment in the breeding attempt: clutch size and female provisioning to nestlings did not differ between experimental and control territories where no non‐breeding nests were removed. Similarly, in non‐manipulated territories, clutch size and female provisioning were not correlated with the number of non‐breeding nests or with males’ nest‐building effort. Contrary to this hypothesis, the number of non‐breeding nests was associated with delayed clutch initiation and reduced hatching success. The presence of non‐breeding nests did not reduce nest predation and brood parasitism, which did not differ between experimental and control territories. We did not detect differences in concealment between non‐breeding and breeding nests, suggesting that non‐breeding nests were not the result of abandonment before egg‐laying to reduce subsequent nest predation. Dummy nests did not provide shelter; they were not used frequently for roosting over the breeding season and were not maintained during the non‐breeding season. We suggest that building non‐breeding nests may be an attempt by males to manipulate the decision of females to breed with a mate they might otherwise reject or to start reproduction earlier than optimal for the females.

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