MAY (1949) asserts that as pair-formation behaviour patterns " a r e not related to the sex act, which is the criterion for 'sexual behaviour' " , pair-formation is not sexual behaviour, at any rate in the Willow Warbler (Phylloscopus trochilus) which he studied, but apparently social. The male's resistance to a companion is worn down by the female's persistence in invading his territory and continuing...

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