Playback of natural and electroacoustically-generated songs to 49 European (Phylloscopus c. collybita) and 48 Iberian (P. c. brehmii) chiffchaffs gave the following results. In collybita, males respond little to the song of brehmii whereas females show a strong response. In brehmii, males react strongly to collybita's song whereas females respond very little. The following data concern only the males. In general, brehmii are more aggressive than collybita. The two forms are not sensitive to the same song parameters. The temporal biological phases have no effect on the relative responses. Collybita are most sensitive to song broadcasts of the beginning of the breeding cycle; acoustic sensitivity then decreases and once the territories are established, it remains constant. In brehmii, territoriality and aggressiveness remain high throughout the season. Collybita show the same level of reaction to song broadcasts both in allopatry and in sympatry; on the other hand, brehmii react less to the European song when the Iberian chiffchaffs prevail. When acoustic contact of both forms is continuous, collybita becomes more sensitive to a broadcast Iberian song. The relative responses do not depend on the initial excitement level.
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Marc Salomon (1989) studied this question.