Interspecific territoriality by mixed-singing and song-switching Willow Warblers: a possible case of character convergence

Published on 11 June 2018 in Main articles

By Mike Lawn

Abstract Observations of two mixed-singing Willow Warblers Phylloscopus trochilus in Surrey indicated that both birds were interspecifically territorial with Common Chiffchaffs P. collybita, whereas normal Willow Warblers were not obviously so. One of the mixed-singers was already under observation prior to the use of Chiffchaff-type song, enabling the development of mixed singing to be documented. The ‘misdirected song learning’ hypothesis did not explain all the observations, and it is suggested that the birds’ behaviour, considered together with habitat and population changes, is consistent with ‘character convergence’.

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Willow Warbler male B at Milford Common, Surrey, May 2017.  Michael Lawn