We analyzed the effect of nest temperatures, fledging date, age at fledging, fledgling mass and size on postfledging survival of Great Tits Parus major in eastern Spain. We manipulated temperature during nestling development in 26 nests (average temperature was 39.8, 34.6 and 26.4 °C for heated, control and cooled nest-boxes, respectively), and used radio-telemetry to monitor the survival of 48 nestlings (16 heated, 18 cooled, 14 controls) during the first 15 days after fledging. Heated chicks were lighter than control and cooled chicks. Estimated survival of heated fledglings was lower than that of controls. Additionally, survival of control fledglings increased with size, but this relationship was reversed for heated fledglings. Our results suggest that high temperatures experienced in the nest could have negative consequences on immediate post-fledging survival, and that smaller nestlings may deal more effectively with temperatures surpassing their optimal thermal range.