NEPHEW, B.C.; ROMERO, L.M.: Behavioral, cardiovascular, and endocrine responses of European starlings (Sturnus vulgaris) to acute crowding

Although numerous studies have investigated the behavioral, cardiovascular, and endocrine responses to acute stressors, it is unknown for most species how these components interact to create an integrated stress response. We simultaneously measured behavior, heart rate (HR), and plasma corticosterone (CORT) concentrations in European starlings following an acute increase in conspecific density. To initiate crowding, two cages were stacked on top of one another and separated by a trap door. Eight resident starlings with subcutaneously-implanted HR transmitters were individually housed in the upper cage. At the start of the experiment, 1, 3, or 5 intruder starlings entered the upper cage through the trap door. We then compared behavioral and HR responses from 30 minutes of pre-intrusion to 30 minutes post-intrusion, as well as pre- and post experimental CORT concentrations. Resident starlings significantly increased pecking, head feather expansion, and body contact, while decreasing preening. Increased pecking and decreased preening were also recorded from the intruder birds. HR showed a significant density-dependent elevation in the five minutes following the intruder introduction. Although CORT in residents did not change with crowding, CORT increased with increasing density in intruders. These data indicate that crowding in starlings results in increased aggression, decreased maintenance behaviors, elevated HR, and a density-specific CORT increase in intruders, and that starlings are capable of modulating HR independently of significant activation of the hypothalamic-pituitary-adrenal axis. These results demonstrate that three pathways are important when responding to crowding, a significant factor in population density-dependent changes following habitat loss.