Spread‐winged behaviour of Double‐crested and Flightless Cormorants Phalacrocorax auritus and P. harrisi: wing drying or thermoregulation?

The behaviour of free‐living Double‐crested and Flightless Cormorants, and the thermoregulation of Flightless Cormorants, were studied in the field to determine the influence of weather and the function of wing‐spreading. The behaviour of both species was sensitive to changes in ambient temperature and insolation intensity. Spread‐winged behaviour in both species w‐as displayed only by wet individuals and was never accompanied by gular flutter. The frequency of spread‐winged behaviour in Double‐crested Cormorants was not correlated with ambient (shade) temperature or solar intensity. In Flightless Cormorants, however, the frequency of wing‐spreading was positively correlated with ambient temperature, and negatively correlated with insolation intensity. Body temperatures in Flightless Cormorants were statistically higher during the day than during the night, while gular fluttering than while not gular fluttering, and before entering the water than after leaving the water. There were no significant differences in the body temperatures of Flightless Cormorants after wing‐spreading as compared to before ‘sunning’. Spread‐winged behaviour in these cormorants appears to function primarily in wing drying and not in gaining or losing heat, although the ultimate goal of wing drying may be to conserve metabolic energy.