Individuals' behavioral responses to predator stimuli may be constrained due to the needs of courtship and reproductive behavior. We tested whether alarm and mobbing behavior in response to predator stimuli was dependent on changes in reproductive behavior in chaffinches (Fringilla coelebs). Male chaffinches may suffer cuckoldry if they become separated from their mates during the fertile period, as would happen in cases of mobbing a predator. After the fertile period has ended, however, as their eggs and subsequent young become increasingly valuable, male chaffinches should devote more effort toward mobbing. We found that the time spent near the female and copulation rates was higher during the preincubation fertile period compared with the incubation period. However, the intensity of mobbing responses to a stuffed avian predator was higher during the incubation period. Furthermore, an experiment that manipulated calling behavior related to mobbing intensity showed that rates of extrapair copulations were higher in treatments of higher intensity mobbing. Taken together, these results indicate that during the breeding season, male chaffinches only engage in antipredator mobbing behavior after the fertile period has ended for their female mate. These data point to an important trade-off between current reproductive effort and antipredator responses.