Previous theory to explain pairing behavior in waterfowl suggested that timing of pairing was constrained by costs to males of being paired and assumed that males incur most of the cost of defense after a pair bond is formed. An alternative hypothesis predicts that male and female partners will mutually defend their pair bond and that an individual will assume a greater share of defense when paired to a relatively high than low quality partner. Behavior of wintering Harlequin Duck (Histrionicus histrionicus) pairs was consistent with the latter hypothesis. Females and males shared equally in pair-bond defense in new pairs involving young females, while males assumed a greater share of defense when paired to an older female. Overall, males performed more aggressive displays in defense of the pair bond than females, but displays by females were more frequently of higher intensity than those of their mate. The relative share of pair-bond defense also varied between females and males depending on the target of the aggressive display. In some pairs, females performed virtually all defensive displays and bore the primary cost of pair-bond defense. Even when sex ratios are male-biased, differences in male quality probably make females willing to protect a pair bond with a high-quality male. Mutual mate choice and shared defense of a pair bond indicated that “pair-bond defense” would be a more appropriate label than “mate-defense” for the mating system of Harlequin Ducks and likely most monogamous avian species.