Territorial meadow pipit males (Anthus pratensis; Passeriformes) become more aggressive in female presence.

Although mate guarding as prevention of extra-pair copulation is common among birds, evidence for aggressive behaviour involving physical contact related to mate guarding in passerines is scarce and cases of the presence of one partner directly influencing the aggressiveness of the other are lacking. We investigated the intra-specific territorial behaviour of male meadow pipits (Anthus pratensis; Passeriformes: Motacillidae) at the beginning of the breeding season by placing a pipit model accompanied by an intra-specific song playback in the territory of socially paired males and compared the responses of males whose mates were physically present during trials with those whose females were out of sight. The level of aggression of males was significantly higher in the presence of the female; half of the males in this group physically attacked the model (the most intense and risky aggressive behaviour). Physical attacks did not occur among males whose female was absent during the trial; response to the playback by most of these males was only weak. This pattern may be related to the prevention of extra-pair copulation; if the risks involved in the conflict are outweighed by potential loss of paternity, such aggressive mate guarding may pay off. The apparently overlooked effect on the territorial behaviour of a partner's passive physical presence during conflict should be further evaluated because it may be important for the design and interpretation of results of behavioural experiments.