Well now, dark is a dearth of light, or reflectivity and that is subject to conditions. I agree that your goth kestrel is not as colourful as those few that I have been fortunate enough to have seen but if you had a big collection of shots of this bird you might find that it is not fashioning goth in every shot. The sensor in your camera, and also in my camera, is not quite capable of distinguishing subtle gradations of tone as an Fx-sensor equipped camera might achieve. We tend to get more contrast-rich images thereby missing out on the mid-tone data. Maybe the kestel got wet, which could make it appear darker. Case-in-point: I spotted a duck in a steep-banked creek that looked dark and sleek, with a long slender neck and I immediately thought of a female Blue-winged teal. Oh great, I thought, finally a BWT that I can stalk. It sounded better in my head. Anyway, the duck was dark and sleek because it was wet. It turned out to be a female Mallard. Had it been dry the duck might not have looked quite so sleek and its brighter tan colour would not have looked like burnt umber with streaks. It was just an illusion. I'm sure that kestrels like a good illusion now and then too.
