Territorial Aggressiveness and Its Relation to the Endocrine System in the Pied Flycatcher

Abstract

Behavioral and endocrine responses were studied following simulated territorial intrusions performed at two different stages in the breeding cycle (pre-nest-building and nestling period) of the pied flycatcher (Ficedula hypoleuca). Only 45% of the unpaired territorial males from the pre-nest-building period defended their territories by attacking the decoy. After the experimental intrusion unpaired males attacking the decoy had significantly higher plasma testosterone levels than nonattacking unpaired males. Plasma corticosterone levels were significantly higher in both attacking and nonattacking unpaired males than in control males. There were no differences in body size between attacking and nonattacking males. The attacking paired males and paired control males, however, had significantly lower fat depots than other males. The endocrine system of the male pied flycatcher responded within 1 day to the settlement of a female in the territory in that plasma testosterone levels of paired control males were significantly higher than those in unpaired territorial control males. The proportions of attacking males increased significantly (up to 80%) once a female had settled. Unlike in unpaired territorial males, testosterone levels in paired males that attacked the decoy were similar to those in non-attacking males and to those in paired control males after a simulated territorial intrusion. Data suggest that the high testosterone levels are related to territorial aggressiveness and not to sexually related events such as mate-guarding or singing. Forty-five percent of the males from the nestling period did attack the decoy. Despite the strong behavioral response these attacks had no effects on plasma testosterone or corticosterone levels. As all males during the nestling period only have basal levels of testosterone, it is concluded that in the pied flycatcher, high testosterone levels are not necessary for the expression of an aggressive behavior at this stage in the breeding cycle. In the pied flycatcher aggression at this stage in the breeding cycle is likely only to be associated with the defense of food resources.