Mate guarding by close following is assumed to function as a paternity guard in pair-bonding birds. Although this behaviour has been described for many species, little is yet known about the variation in mate-guarding intensity between males in a population and the reason for such variation. In this study, we wanted to examine how mate-guarding intensity relates to the sexual attractiveness of the male. Male bluethroats, Luscinia s. svecica, have a bright throat patch, an ornament extensively displayed during courtship. Two experiments are reported. In the first, we show that males which had their throat patch blackened (with Nyanzol D) before pairing had a lower pairing success than control males. We conclude that the blackening of the throat patch reduced male attractiveness. In the second experiment, a group of males were blackened after pairing. These males guarded their mates more closely than did control males, and spent less effort advertising for secondary mates. Thus, we conclude that a male's mate-guarding intensity is flexible and negatively related to his sexual attractiveness. The mechanisms by which a male can assess the likelihood of becoming cuckolded are discussed.