The efficiency of the Wren's song may be impaired by certain background noises. Natural noises, such as those emitted by sympatric birds decrease the receiving Wren's alertness (Table 1). This phenomenon has no compensating process and is probably rather detrimental to communication between Wrens in an acoustically rich natural environment. When the Wren's song is masked with artificially made signals there are no serious ethological consequences (Table 2) as masking is seldom complete, and decrease in the received information is largely compensated by the song's inherent redundancy. The precise requirements for a mask to hinder communication efficiency are described. They are so stringent that the odds are overwhelmingly against their being ever met with in nature.