You will, no doubt, have seen the impression left by a bird crashing into your patio window. You may be interested to know that only certain birds leave their waxy mark. I had received this response from RSPB:
“These are the natural oils and feather dust which originate from the downy feathers close to the skin. These provide woodpigeons with great insulation and they also have fine barbs which crumble to form this waxy dust, which the bird then uses to preen the outer feathers to make them waterproof. Birds with powder down feathers such as woodpigeons and members of the parrot family generally have a reduced or no preen gland, which is how most other birds look after their feathers. Feather dust is therefore vital in order to keep the woodpigeons feathers waterproof and in good condition. Bathing is an important part of keeping feathers in good condition for all birds but especially important for birds with powder down. Washing the dirt-covered powder off enables the woodpigeons to reapply fresh powder to their outer feathers in order to keep them water proof and well insulated.”
I had noticed a film of powder on my pond every time Woodpigeons bathed….
“The powder deposits on the pond are not harmful to the pond wildlife or other birds that may be using the pond for drinking or bathing and will eventually disperse with the movement of the water.”
I hope this is of as much interest to you as it was to me.