Zon-gedrag van een Wespendief Pernis apivorus

The sunning behaviour of a semi-captive juvenile (71-77 days old) Honey Buzzard was studied in September 1997 in The Netherlands under excellent weather conditions (17-24°C, often sunny). It was a free-flying, very tame bird that recovered from a long period of undernourishment during its nestling stage. The typical sunning posture was the ‘delta-wing’: wings held half-open, bent at the carpal joint in such a way that the primaries were aligned with the legs, tail fully spread but partly covered by the secondaries (often hiding the rump), back always turned towards the sun, bill closed, neck stretched, head facing away from the sun and eyes normally half-closed, giving the bird an ‘enjoying’ look (Photo 1). Sometimes, the bird looked sideways, with half-opened eyes and bill still closed (Photo 2). Sunning was never followed or accompanied by preening. No other sunning postures were noticed. The wingspread posture was seen after the bird had taken a bath, but this posture was very different from sunning: wings drooping, tail spread, position changing regularly (intermittently facing the sun, and turning around), occasionally wing flapping, eyes fully open, intently fixing passing insects and birds, and preening. Sunning was clearly stimulated by direct, intense sunlight, especially on partly overcast days when the sun re-appeared at full strength from behind the clouds. Sunning was particularly evident in the morning; not only adopted the bird a sunning posture more often, sunning bouts were also more prolonged in the morning (Table 1). Sunning was ended either spontaneously, or abruptly as soon as the sun disappeared behind a cloud. Even the slightest cloud in front of the sun was sufficient to let the Honey Buzzard stop with sunning. The average sunning bout lasted 303 seconds (SD=209, N=13), ranging from 100 to 900 sec. The interval between successive sunning bouts was clocked at 91 minutes, ranging from 25 to 190 min (SD=69, N=B). Raptors for which sunning behaviour has been recorded (Simmons 1986) are spending (a part of) their life in relatively hot, sunny regions. Despite thousands of observation hours spent in the field, sunning Goshawks. Accipiter gentilis, Sparrowhawks A. nisus and Common Buzzards Buteo buteo were never observed by three raptor specialists in The Netherlands, whereas it is a common sight in Honey Buzzards. The authentity of observations of sunning raptors spending their complete life cycle in temperate and boreal conditions should be checked more carefully.