DICKENS, M. J.*; ROMERO, L. M.; Tufts University: Crowding induces a greater cardiovascular stress response in male European starlings (Sturnus vulgaris) when in breeding condition

European starlings alter their physiology and behavior between seasons. As daylight lengthens, starling males enter breeding condition and become territorial, while shortening daylength induces flocking behavior. This changing physiology and behavior may also alter how these animals react to crowding. We compared the potential stressor of crowding on captive starlings in both breeding (shortly after photostimulation to 18L:6D when testosterone titers were highest) and non-breeding (birds maintained on 10L:14D) conditions. In both conditions, one or five intruders entered a resident�s cage without any human aid or disturbance. A subcutaneous heart rate transmitter recorded cardiovascular output in residents. Corticosterone and testosterone were measured in plasma samples taken before and after the intrusion. Although resident�s corticosterone concentrations were not affected by crowding, the cardiovascular response differed significantly. Residents on short days showed a graded response in peak heart rate following the intrusion between one and five intruders, with the strongest response being elicited after five intruders. In contrast, whereas the five intruders elicited an identical peak heart rate response in birds held on both short and long days, the long day males also demonstrated an equivalent response to one intruder. Furthermore, one intruder induced a more sustained elevated heart rate in the long day group than it did for the short day group. These data indicate that male starlings in breeding condition are more sensitive to crowding than males that are not in breeding condition and suggest that a higher tolerance for crowding (i.e. considering crowding as less stressful) during the short daylight months may facilitate flocking behavior.