On the Origin of Divergence of Learned Signals (Songs) in Isolated Populations

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.