When facing life-threatening perturbation the following stress reaction will usually turn current behavior towards survival behaviors. However, this can have a potential fitness trade-off if it turns behavior away from reproduction towards survival. Animals will therefore try to modify their sensitivity to stressors so that they will turn current behavior towards survival when this gives the highest fitness. Animals that, for varies reasons, are more reactive to stressors should therefore avoid behavior that can more quickly resemble an emergency stress response. In a pied flycatcher (Ficedula hypoleuca) population, only a portion of the males becomes polyterritorial while the rest stays monoterritorial. Polyterritoriality is an energetically demanding strategy, and the behavior could trigger emergency stress responses more quickly than monoterritorial behavior. The hypothesis in this study is therefore that pied flycatcher males that are more sensitive towards stressors should avoid polyterritorial behavior, and males with this behavior should also show a suppressed stress response in order to continue to be polyterritorial. To examine this male pied flycatcher were captured twice, once in the territorial establishment period and once in the nestling feeding period. Breathing rate and corticosterone levels were used as stress response measurements. Three predictions were tested: 1. Pre-breeding males that later become polyterritorial will show a lower acute stress response than males who stay monoterritorial. 2. Males that have become polyterritorial will show a lower acute stress response than males who stay monoterritorial. 3. Males that are polyterritorial will have higher baseline corticosterone levels than monoterritorial, because polyterritoriality is an energetically demanding strategy. None of the predictions regarding stress response tested in this thesis showed any significant difference between mono- and polyterritorial males. This suggests that responsiveness to stressors does not affect the likelihood of becoming polyterritorial. Other proximate explanations for this behavior are necessary, or a combination of many factors together could explain why some males become polyterritorial. Further, the tendency for higher baseline corticosterone levels in polyterritorial males during the nestling feeding period supports previous studies that polyterritorial behavior is in fact an energetically demanding strategy.