Is it a bird? Is it a plane? ... Is it a fly?

Whenever I am trying out a new camera setting, I point my camera out of the window and take an image of the chimney pots across the road. This must place them among the most photographed chimney pots there are.
Below is an image I took in just that way. When I came to look at it I noticed something I hadn't noticed when taking the picture. There was a dark spot next to the cage on the chimney pot to the far left. What it was is not entirely clear. My initial reaction was that it was a fly with its wings in mid-buzz close to the cage. On reflection, it could be a swallow much further away in line of sight of the cage - although it lacks a clear head. I will never know for sure.