A wood pigeon flies in to perch on a branch of the ivy-covered ash. A few seconds later there a puff of ‘smoke’ two or three feet across which drifts off to the right.
When I remember the powdery silhouettes that wood pigeons make when they fly into our patio windows, the most likely explanation is that it this was part of preening routine. Side-lit by the 
midday sun, the powdery wisp showed up against the dark background of the ivy.
This happened again a couple of times over the next few minutes.