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

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