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Summary

Short-toed Treecreepers Certhia brachydactyla in Morocco sing very differently from European Short-toed Treecreepers.

Short-toed Treecreepers from Central Europe do not react to the playback of songs of North African Short-toed Treecreepers. This result is statistically highly significant.

Treecreepers C. familiaris of Central Europe do not react to the playback of songs of North African Short-toed Treecreepers or (some males) react just as little as they do to songsof Central European Short-toed Treecreepers. This result is statistically highly significant.

Three hypotheses are examined as interpretations of these results: contrast reinforcement, loss of contrast and withdrawal of learning. The facts are best explained by withdrawal of learning. According to this hypothesis, Morocco was colonised by young European Short-toed Treecreepers with a song that had not yet been fixed by learning. The founders of the new population evolved a divergent song, which has been transmitted by tradition ever since.